Abstract
Objectives:
Asthma has substantial morbidity and impact on quality of life. Symptoms can worsen when asthma coexists with allergy. We assessed the prevalence of asthma–allergy comorbidity in a nationally representative sample.
Methods:
We used 2021 National Health Interview Survey data for 7343 children and adolescents aged 2 to 17 years and 29 329 adults aged ≥18 years to estimate the prevalence of asthma, allergy symptoms, and lifetime allergy diagnosis. We assessed associations between asthma–allergy comorbidity and characteristics (sex, age group, race and ethnicity, family income, region, urbanicity) and between asthma attacks and allergy by using logistic regression.
Results:
Almost 8% of people aged ≥2 years had asthma, and 52.3% had allergy symptoms. Among children and adolescents, boys had a higher prevalence of asthma–allergy comorbidity than girls (6.2%; 95% CI, 5.3%-7.2% vs 5.0%; 95% CI, 4.2%-5.9%). Among adults, men had a lower prevalence of asthma–allergy comorbidity than women (4.9%; 95% CI, 4.5%-5.4% vs 8.3%; 95% CI, 7.7%-8.8%). The prevalence of asthma–allergy comorbidity was higher among people with low income (<100% vs ≥200% federal poverty level) and non-Hispanic Black (vs non-Hispanic White) people. Among people with asthma, 82.1% had allergy symptoms and 67.3% had a lifetime allergy diagnosis. The prevalence of allergy symptoms differed by sex among adults with asthma but not among children and adolescents with asthma. Among adults with asthma, asthma attacks were associated with allergy symptoms (adjusted prevalence ratio = 1.23; 95% CI, 1.04-1.46).
Conclusion:
Disparities in asthma–allergy comorbidity exist by sex, family income, and race and ethnicity. These findings support national asthma management guidelines on the importance of identifying and treating comorbid allergies among people with asthma.
Keywords: asthma, allergy, respiratory disease, health disparity
Asthma is a common chronic airway disorder characterized by periods of reversible airflow obstruction known as asthma episodes or attacks. 1 Poorly controlled asthma has substantial morbidity and health care costs2,3 and affects health-related quality of life. 4 Asthma control is achieved when attacks are prevented by using evidence-based strategies involving appropriate medication use and avoiding exposure to triggers. 1 Allergens are a leading exposure associated with asthma attacks, 1 and asthma–allergy comorbidity is a known risk factor for poorly controlled asthma and severe exacerbations.5-14 National asthma guidelines for the diagnosis and management of asthma and guidelines updated in 2020 outline strategies for allergen mitigation, including diagnosing allergies and using multicomponent allergen-specific mitigation interventions.1,15
Although allergic asthma is the most common asthma phenotype, 16 few studies have assessed the prevalence of asthma–allergy comorbidity across the lifespan. Questions on allergic conditions were added to the National Health Interview Survey (NHIS) for adults for the first time in 2021. Analyses of 2021 data showed that respiratory allergies were the most prevalent allergy among people in the United States17,18 and that the prevalence of allergic conditions varied by sex, age, and race and ethnicity. Nationally representative results from the National Health and Nutrition Examination Survey on allergic sensitization to airborne and food allergens, measured with serum immunoglobulin E (IgE), found sensitization among nearly 36.2% of children aged 1 to 5 years and 44.6% of people aged ≥6 years. 19 However, these analyses did not assess patterns of asthma–allergy comorbidity. The objective of our study was to analyze 2021 NHIS data to assess associations between asthma–allergy comorbidity and selected demographic and socioeconomic characteristics and the associations between having asthma attacks and allergy among children, adolescents, and adults with asthma.
Methods
Data Description
The NHIS 20 includes questions on respiratory, skin, and food allergy every third year, most recently in 2021 for children and adolescents and for the first time for adults. The NHIS identifies a probability sample to represent the US civilian, noninstitutionalized population with a complex sampling design based on identifying geographic clusters (usually a county) and then identifying a sample of addresses for face-to-face interviews. The NHIS randomly selects a sample adult and sample child (if children were present) from each household. Adults self-reported health information (except for proxy reports for adults mentally or physically incapable of responding), and a knowledgeable adult proxy responded to sample child questions. The final sample child and adult response rates were 49.9% and 50.9% in 2021, respectively. 20 We analyzed data for children and adolescents aged 2 to 17 years and adults aged ≥18 years. We excluded data for very young children (aged <2 y) because diagnosing asthma is difficult in this population given the high prevalence of transient wheezing 21 and because sensitization to aeroallergens generally does not occur before age 2 years. 16 The National Center for Health Statistics Ethics Review Board approved the NHIS data collection protocol; adults aged ≥18 years provided informed consent, and parents or a responsible adult reported in proxy for children and adolescents aged <18 years. Institutional review board review was not required for this secondary data analysis of deidentified public-use data files.
Survey Questions and Variable Descriptions
Current asthma was determined by a “yes” response to 2 questions 22 : “Have you ever been told by a doctor or other health professional that you had asthma?” and “Do you still have asthma?” Among those with asthma, asthma attacks were determined by a “yes” response to this question: “During the past 12 months, have you had an episode of asthma or an asthma attack?”
The survey ascertained 3 types of allergy symptoms: respiratory, skin, and food allergy symptoms. Respiratory allergy symptoms were determined by the question, “Do you get symptoms such as sneezing, runny nose, or itchy or watery eyes due to hay fever, seasonal, or year-round allergies?” Skin allergy symptoms were determined by the question, “Do you get an itchy rash due to eczema or atopic dermatitis?” Food allergy symptoms were determined by the question, “People with food allergies have reactions such as hives, vomiting, trouble breathing, or throat tightening that occur within two hours of eating a specific food. Do you have an allergy to one or more foods?”
We considered respondents to have any allergy symptoms if they reported 1 or more symptom types (respiratory, skin, and/or food). We created an asthma–allergy comorbidity variable with 4 mutually exclusive categories: asthma only, any allergy symptoms only, both asthma and allergy symptoms, and neither asthma nor allergy symptoms.
Another measure of allergy comorbidity was report of ever receiving an allergy diagnosis (hereinafter, lifetime allergy diagnosis). Diagnosis questions were asked only of respondents who reported having allergy symptoms. Diagnosed respiratory allergy was determined by the question, “Have you ever been told by a doctor or other health professional that you had hay fever, seasonal, or year-round allergies?” Diagnosed skin allergy was determined by the question, “Have you ever been told by a doctor or other health professional that you had eczema or atopic dermatitis?” Diagnosed food allergy was determined by the question, “Have you ever been told by a doctor or other health professional that you had an allergy to one or more foods?” We estimated the prevalence of receiving any allergy diagnosis. We assessed the prevalence of allergy symptoms in the main analysis given that lifetime allergy diagnosis may not correspond with current symptoms (eg, allergic conditions may resolve over time) and recall bias may differentially affect reporting by adults and children or adolescents (proxy report).
Covariates known to be associated with asthma and allergy prevalence are age group, sex, race and ethnicity, family income, US Census region, and urbanicity. We classified urbanicity according to the 2013 National Center for Health Statistics Urban–Rural Classification Scheme for Counties. 23 Family income as a percentage of federal poverty level (FPL) is defined as the ratio of family income to federal poverty threshold (ie, minimum amount of income needed to cover the cost of basic needs). Due to high nonresponse for the survey item on family income, we used 10 publicly available imputed income files to estimate missing values for the ratio of income to FPL. 24 The US Census Bureau’s FPL is based on family income and family size.
Statistical Analysis
We excluded respondents with missing responses to asthma or allergy questions (39 respondents aged 2-17 y [0.5%]; 153 respondents aged ≥18 y [0.5%]). The final sample size for those aged 2 to 17 years was 7343 and for those aged ≥18 years was 29 329. We calculated the prevalence of asthma, allergy symptoms, lifetime allergy diagnoses, and asthma–allergy comorbidity. We made pairwise comparisons across categories of respondent characteristics by using a single referent group for each characteristic rather than testing all possible pairs. To assess associations with covariates, we used a multinomial logistic regression model with 1 dependent variable with 4 mutually exclusive levels for the possible comorbid combinations (asthma only, any allergy symptoms only, both, neither). Based on known differences between children and adolescents and adults in asthma prevalence by sex, 25 we tested an interaction term for sex and age group and found it to be significant (P < .05). Therefore, we stratified all analyses by children and adolescents aged 2 to 17 years and adults aged ≥18 years.
We assessed patterns of asthma–allergy comorbidity among people with asthma by using a variable with 3 mutually exclusive categories: asthma only, asthma with allergy symptoms and no lifetime allergy diagnosis, and asthma with allergy symptoms and lifetime allergy diagnosis. We used 2 multivariable logistic regression models to calculate adjusted prevalence ratios (APRs) for having at least 1 asthma attack in the past 12 months. The first model adjusted for the covariates described previously and the dichotomous variable for having any allergy symptoms. The second model replaced the “any allergy” variable with 3 variables, 1 for each type of allergy symptom: respiratory, skin, and food allergy. The correlations between allergy symptom types were low (the highest Pearson correlation coefficient was 0.31 between skin and food allergy among children and adolescents, and all other correlation coefficients were <0.15). A sensitivity analysis assessed the associations between asthma attacks and lifetime allergy diagnosis among people with asthma.
We used SAS version 9.4 (SAS Institute Inc) and SAS-callable SUDAAN version 11.0.0 (Research Triangle Institute, Inc) to account for the complex survey design. We used sample weights to adjust for nonresponse and unequal probability of selection. To assess whether estimates differed between the referent group and other categories, we used pairwise testing. We used a significance level of <.05 for all statistical tests and calculated 95% CIs for all estimates. We followed the NCHS data presentation standards for proportions 26 ; all estimates presented are statistically reliable using these standards.
Results
In 2021, 7.8% of the US population (children and adolescents aged 2-17 y, 7.1%; adults aged ≥18 y, 8.0%) had current asthma, and 52.3% (children and adolescents, 43.1%; adults, 54.6%) had ≥1 allergy symptom (Table 1). Respiratory allergy was the most prevalent allergy symptom among both age groups, followed by skin allergy. We found a similar pattern for lifetime allergy diagnosis. Overall, 31.0% of people were diagnosed with any allergy (28.6% of children and adolescents and 31.6% of adults).
Table 1.
Prevalence of self-reported current asthma, allergy symptoms, and lifetime allergy diagnosis among children and adolescents aged 2 to 17 years and adults aged ≥18 years, United States, 2021a,b
| Health condition | Total (N = 36 672) | Children and adolescents aged 2-17 years (n = 7343) | Adults aged ≥18 years(n = 29 329) |
|---|---|---|---|
| Current asthma | 7.8 (7.5-8.2) | 7.1 (6.4-7.8) | 8.0 (7.6-8.4) |
| Allergy symptoms | |||
| Any allergy | 52.3 (51.4-53.1) | 43.1 (41.7-44.5) | 54.6 (53.7-55.5) |
| Respiratory allergy | 46.1 (45.2-47.0) | 34.8 (33.4-36.2) | 49.0 (48.1-49.9) |
| Skin allergy | 11.3 (10.9-11.8) | 13.6 (12.7-14.6) | 10.7 (10.3-11.2) |
| Food allergy | 9.5 (9.1-9.9) | 8.3 (7.6-9.0) | 9.8 (9.4-10.2) |
| Lifetime allergy diagnosis | |||
| Any allergy | 31.0 (30.3-31.7) | 28.6 (27.3-29.9) | 31.6 (30.9-32.4) |
| Respiratory allergy | 24.5 (23.9-25.2) | 20.4 (19.3-21.6) | 25.6 (24.9-26.3) |
| Skin allergy | 8.0 (7.7-8.4) | 10.9 (10.1-11.8) | 7.3 (6.9-7.7) |
| Food allergy | 6.2 (5.9-6.5) | 6.2 (5.5-6.9) | 6.2 (5.9-6.6) |
| Comorbidity c | |||
| Asthma and any allergy symptoms | 6.4 (6.1-6.8) | 5.6 (5.0-6.3) | 6.6 (6.3-7.0) |
| Asthma only | 1.4 (1.3-1.6) | 1.5 (1.2-1.9) | 1.4 (1.2-1.5) |
| Any allergy symptoms only | 45.8 (45.1-46.6) | 37.5 (36.2-38.8) | 48.0 (47.1-48.8) |
| Neither asthma nor allergy symptoms | 46.4 (45.5-47.2) | 55.4 (54.0-56.8) | 44.0 (43.2-44.9) |
Data source: National Center for Health Statistics, National Health Interview Survey, 2021. 20
All values are percentage (95% CI).
Categories are mutually exclusive.
Overall, 6.4% of the US population aged ≥2 years had both asthma and allergy symptoms, 1.4% had only asthma, 45.8% had only allergy symptoms, and 46.4% had neither asthma nor allergy symptoms (Table 1). Patterns differed by age group; for example, 55.4% of children and adolescents had no asthma or allergy symptoms versus 44.0% of adults.
Asthma–Allergy Comorbidity Status
Children and adolescents aged 2 to 17 years
In general, the prevalence of asthma and/or allergy symptoms was higher among boys than girls and in older children than in younger children (Table 2). Compared with non-Hispanic White children and adolescents, Hispanic children and adolescents tended to have a lower prevalence and non-Hispanic Black children and adolescents had a higher prevalence of asthma, allergy symptoms, or both. Children and adolescents with a family income <100% FPL were less likely to have allergy symptoms only compared with those with family income ≥200% FPL but were more likely to have asthma–allergy comorbidity. Children and adolescents living in the Midwest and South were more likely than those living in the Northeast to have allergy symptoms only, and those living in less urbanized areas were more likely to have allergy symptoms only than were those in urbanized areas.
Table 2.
Prevalence of current asthma, allergy symptoms, and comorbidity a among children, adolescents, and adults, by characteristic, United States, 2021b,c
| Characteristic | Children and adolescents aged 2-17 years (n = 7343) |
Adults aged ≥18 years (n = 29 329) |
||||||
|---|---|---|---|---|---|---|---|---|
| Current asthma only | Any allergy symptoms only | Both asthma and allergy symptoms | Neither asthma nor allergy symptoms | Current asthma only | Any allergy symptoms only | Both asthma and allergy symptoms | Neither asthma nor allergy symptoms | |
| Sex | ||||||||
| Male | 1.9 (1.4-2.5) | 37.7 (35.8-39.6) | 6.2 (5.3-7.2) | 54.2 (52.3-56.2) | 1.3 (1.1-1.5) | 45.4 (44.2-46.5) | 4.9 (4.5-5.4) | 48.4 (47.3-49.6) |
| Female | 1.1 (0.8-1.6) d | 37.3 (35.5-39.2) | 5.0 (4.2-5.9) d | 56.6 (54.7-58.5) | 1.5 (1.3-1.7) | 50.4 (49.3-51.4) d | 8.3 (7.7-8.8) d | 39.9 (38.8-41.0) d |
| Age group, y | ||||||||
| 2-4 | 0.4 (0.2-1.0) | 33.2 (30.3-36.2) | 2.3 (1.5-3.4) | 64.2 (61.2-67.1) | NA | NA | NA | NA |
| 5-11 | 1.9 (1.3-2.6) d | 37.2 (35.1-39.3) d | 5.6 (4.7-6.7) d | 55.3 (53.1-57.5) d | NA | NA | NA | NA |
| 12-17 | 1.6 (1.2-2.2) d | 39.8 (37.8-41.9) d | 7.1 (6.1-8.4) d | 51.5 (49.3-53.7) d | NA | NA | NA | NA |
| 18-34 | NA | NA | NA | NA | 1.5 (1.2-1.9) | 47.8 (46.3-49.3) | 6.9 (6.1-7.7) | 43.8 (42.3-45.4) |
| 35-54 | NA | NA | NA | NA | 1.2 (1.0-1.5) | 48.5 (47.3-49.8) | 6.6 (6.0-7.2) | 43.7 (42.3-45.0) |
| 55-64 | NA | NA | NA | NA | 1.4 (1.1-1.8) | 48.6 (47.0-50.2) | 7.5 (6.7-8.3) | 42.5 (40.9-44.1) |
| ≥65 | NA | NA | NA | NA | 1.4 (1.2-1.7) | 46.9 (45.6-48.2) | 5.8 (5.2-6.4) d | 45.9 (44.6-47.3) d |
| Race and ethnicity | ||||||||
| Hispanic | 1.3 (0.9-2.0) | 31.7 (29.3-34.2) d | 4.7 (3.8-5.9) | 62.2 (59.6-64.8) d | 1.4 (1.0-1.9) | 39.5 (37.6-41.5) d | 4.4 (3.7-5.2) d | 54.7 (52.7-56.7) d |
| Non-Hispanic Black | 3.7 (2.3-5.7) d | 36.1 (32.2-40.3) | 9.7 (7.4-12.6) d | 50.5 (45.9-55.0) | 1.7 (1.3-2.3) | 42.4 (40.2-44.6) d | 8.9 (7.7-10.2) d | 47.0 (44.8-49.3) d |
| Non-Hispanic Other e | 0.5 (0.2-1.2) d | 38.4 (34.6-42.4) | 5.8 (4.4-7.8) | 55.2 (51.2-59.2) | 1.2 (0.7-1.8) | 44.3 (41.7-46.9) d | 6.4 (5.2-7.7) | 48.2 (45.8-50.7) d |
| Non-Hispanic White | 1.3 (0.9-1.7) | 40.5 (38.7-42.4) | 5.0 (4.2-5.9) | 53.2 (51.3-55.1) | 1.3 (1.2-1.5) | 51.8 (51.0-52.6) | 6.9 (6.5-7.3) | 40.0 (39.2-40.9) |
| Ratio of family income to FPL f | ||||||||
| <100% | 2.0 (1.2-3.2) | 30.8 (27.4-34.4) d | 8.4 (6.5-10.8) d | 58.9 (55.2-62.5) d | 2.4 (1.8-3.1) d | 41.5 (39.1-43.9) d | 8.5 (7.4-9.9) d | 47.6 (45.2-0.0) d |
| 100% to <200% | 1.9 (1.2-2.8) | 34.9 (32.3-37.7) d | 5.0 (3.9-6.3) | 58.3 (55.4-61.0) d | 2.0 (1.6-2.4) d | 42.8 (41.0-44.6) d | 7.3 (6.5-8.2) d | 48.0 (46.2-49.8) d |
| ≥200% | 1.2 (0.9-1.7) | 40.3 (38.7-41.9) | 5.1 (4.4-5.9) | 53.4 (51.7-55.0) | 1.1 (1.0-1.3) | 50.1 (49.2-51.0) | 6.2 (5.9-6.6) | 42.6 (41.7-43.5) |
| US Census region | ||||||||
| Northeast | 1.1 (0.5-2.1) | 33.4 (29.8-37.1) | 4.4 (3.2-6.1) | 61.1 (57.2-64.9) | 1.7 (1.3-2.2) | 46.6 (44.8-48.5) | 6.3 (5.5-7.3) | 45.3 (43.3-47.4) |
| Midwest | 1.7 (1.2-2.5) | 38.5 (35.7-41.3) d | 6.3 (5.0-7.9) | 53.5 (50.6-56.4) d | 1.5 (1.2-1.9) | 48.7 (47.0-50.3) | 7.3 (6.6-8.1) | 42.5 (40.9-44.2) d |
| South | 1.7 (1.2-2.5) | 40.5 (38.3-42.7) d | 5.7 (4.7-6.8) | 52.2 (49.7-54.6) d | 1.2 (1.0-1.4) d | 48.8 (47.4-50.2) | 6.2 (5.7-6.9) | 43.8 (42.3-45.3) |
| West | 1.3 (0.8-2.0) | 34.3 (31.8-37.0) | 5.7 (4.6-7.0) | 58.7 (56.0-61.4) | 1.4 (1.1-1.7) | 47.0 (45.3-48.7) | 6.9 (6.3-7.6) | 44.8 (43.0-46.6) |
| Urban–rural classification g | ||||||||
| Large central metropolitan | 1.6 (1.1-2.5) | 34.6 (32.3-37.1) | 4.8 (3.9-5.8) | 59.0 (56.5-61.4) | 1.2 (1.0-1.5) | 45.0 (43.5-46.6) | 6.1 (5.5-6.8) | 47.6 (46.0-49.2) |
| Large fringe metropolitan | 1.4 (1.0-2.2) | 36.6 (34.2-39.2) | 6.2 (5.0-7.7) | 55.7 (52.9-58.5) | 1.2 (0.9-1.5) | 48.5 (47.0-50.0) d | 6.2 (5.5-7.0) | 44.1 (42.6-45.6) d |
| Medium and small metropolitan | 1.4 (0.9-2.2) | 38.4 (36.1-40.8) d | 6.0 (5.0-7.3) | 54.1 (51.6-56.6) d | 1.6 (1.3-2.0) | 50.0 (48.5-51.5) d | 6.6 (6.1-7.3) | 41.8 (40.2-43.3) d |
| Micropolitan and noncore | 1.5 (0.9-2.5) | 43.8 (39.9-47.8) d | 5.5 (4.0-7.5) | 49.3 (45.0-53.5) d | 1.6 (1.2-2.1) | 49.3 (47.3-51.3) d | 8.6 (7.6-9.8) d | 40.5 (38.4-42.6) d |
Abbreviations: FPL, federal poverty level; NA, not applicable.
The 4 outcomes are mutually exclusive.
Data source: National Center for Health Statistics, National Health Interview Survey, 2021. 20
All values are percentage (95% CI).
Estimates are significantly different (at P < .05) from the reference group in pairwise comparison. Reference groups were the following: by sex, male; by age, 2 to 4 years for children, 18 to 34 years for adults; by race, non-Hispanic White; by ratio of family income to FPL, ≥200%; by US Census region, Northeast; and by urban–rural classification, large central metropolitan.
“Other” category consists of non-Hispanic Asian, non-Hispanic American Indian or Alaska Native (along and with any other group), other single race, and multiple races.
Family income as a percentage of FPL is defined as the ratio of family income to federal poverty threshold (ie, minimum amount of income needed to cover the cost of basic needs). Due to high nonresponse for the survey item on family income, we used 10 publicly available imputed income files to estimate missing values for the ratio of income to FPL. 24 The US Census Bureau’s FPL is based on family income and family size.
Based on the 2013 National Center for Health Statistics Urban–Rural Classification Scheme for Counties. 23 Large central metropolitan areas are counties of ≥1 million population containing the principal city; large fringe metropolitan areas are counties of ≥1 million population not containing the principal city; medium metropolitan areas are counties of 250 000-999 999 population; small metropolitan areas are counties of <250 000 population; micropolitan areas have an urban cluster population of 10 000-49 999; and noncore areas include nonmetropolitan areas that did not qualify as micropolitan, including those without an urban cluster population of ≥10 000.
Results of the adjusted analysis were similar to results of the unadjusted pairwise comparisons (Table 3). In the analysis of asthma–allergy comorbidity, APRs were higher among children and adolescents who were older (vs younger), those who were non-Hispanic Black (vs non-Hispanic White), those whose family income was <100% FPL (vs ≥200% FPL), and those living in large fringe metropolitan areas (vs large central metropolitan areas).
Table 3.
Adjusted prevalence ratios a (95% CIs) of asthma–allergy symptom comorbidity b among children, adolescents, and adults, United States, 2021 c
| Characteristic | Children and adolescents aged 2-17 years |
Adults aged ≥18 years |
||||||
|---|---|---|---|---|---|---|---|---|
| Current asthma only | Any allergy symptoms only | Both asthma and allergy symptoms | Neither asthma nor allergy symptoms | Current asthma only | Any allergy symptoms only | Both asthma and allergy symptoms | Neither asthma nor allergy symptoms | |
| Sex | ||||||||
| Male | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] |
| Female | 0.60 (0.38-0.96) d | 0.99 (0.92-1.06) | 0.80 (0.64-1.00) | 1.05 (1.00-1.10) | 1.09 (0.86-1.38) | 1.13 (1.09-1.16) d | 1.67 (1.50-1.86) d | 0.81 (0.79-0.84) |
| Age group, y | ||||||||
| 2-4 | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | NA | NA | NA | NA |
| 5-11 | 4.75 (1.77-12.75) d | 1.11 (1.01-1.23) | 2.56 (1.64-4.00) d | 0.86 (0.81-0.92) d | NA | NA | NA | NA |
| 12-17 | 3.91 (1.45-10.55) d | 1.18 (1.07-1.31) d | 3.21 (2.08-4.96) d | 0.81 (0.76-0.86) d | NA | NA | NA | NA |
| 18-34 | NA | NA | NA | NA | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] |
| 35-54 | NA | NA | NA | NA | 0.85 (0.61-1.16) | 1.00 (0.96-1.03) | 0.97 (0.84-1.12) | 1.01 (0.97-1.06) |
| 55-64 | NA | NA | NA | NA | 0.98 (0.69-1.41) | 0.97 (0.93-1.02) | 1.05 (0.89-1.23) | 1.02 (0.97-1.07) |
| ≥65 | NA | NA | NA | NA | 0.94 (0.67-1.30) | 0.92 (0.89-0.96) d | 0.78 (0.68-0.91) | 1.12 (1.08-1.18) |
| Race and ethnicity | ||||||||
| Hispanic | 0.96 (0.54-1.68) | 0.86 (0.77-0.94) d | 0.92 (0.68-1.23) | 1.11 (1.05-1.19) d | 0.97 (0.68-1.39) | 0.78 (0.74-0.82) d | 0.60 (0.49-0.73) d | 1.36 (1.30-1.41) |
| Non-Hispanic Black | 2.64 (1.43-4.89) d | 0.93 (0.82-1.05) | 1.90 (1.35-2.67) d | 0.93 (0.84-1.03) | 1.26 (0.89-1.77) | 0.83 (0.78-0.87) d | 1.29 (1.10-1.51) d | 1.16 (1.11-1.22) |
| Non-Hispanic Other e | 0.39 (0.15-1.01) | 1.00 (0.89-1.12) | 1.17 (0.84-1.64) | 1.00 (0.92-1.09) | 0.86 (0.53-1.39) | 0.86 (0.82-0.92) d | 0.89 (0.73-1.08) | 1.20 (1.13-1.27) |
| Non-Hispanic White | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] |
| Ratio of family income to FPL f | ||||||||
| <100% | 1.37 (0.72-2.59) | 0.78 (0.69-0.88) d | 1.62 (1.19-2.22) d | 1.10 (1.02-1.18) d | 2.11 (1.55-2.87) d | 0.86 (0.82-0.91) d | 1.32 (1.11-1.56) d | 1.08 (1.03-1.14) |
| 100% to <200% | 1.37 (0.79-2.39) | 0.88 (0.80-0.96) d | 0.97 (0.74-1.28) | 1.09 (1.02-1.15) d | 1.74 (1.33-2.27) d | 0.89 (0.85-0.92) d | 1.17 (1.03-1.33) d | 1.09 (1.04-1.13) |
| ≥200% | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] |
| US Census region | ||||||||
| Northeast | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] |
| Midwest | 1.56 (0.70-3.46) | 1.10 (0.97-1.26) | 1.41 (0.95-2.09) | 0.90 (0.83-0.98) d | 0.82 (0.58-1.16) | 1.02 (0.97-1.07) | 1.08 (0.91-1.29) | 0.98 (0.92-1.03) |
| South | 1.28 (0.56-2.94) | 1.23 (1.09-1.39) d | 1.13 (0.78-1.64) | 0.86 (0.79-0.93) d | 0.60 (0.43-0.84) d | 1.08 (1.03-1.13) d | 0.92 (0.78-1.09) | 0.95 (0.90-1.00) |
| West | 1.29 (0.54-3.06) | 1.06 (0.93-1.21) | 1.40 (0.96-2.04) | 0.93 (0.86-1.01) | 0.78 (0.54-1.12) | 1.04 (0.99-1.10) | 1.19 (1.00-1.41) | 0.94 (0.89-0.99) d |
| Urban–rural classification g | ||||||||
| Large central metropolitan | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] | 1 [Ref] |
| Large fringe metropolitan | 1.04 (0.56-1.93) | 1.00 (0.91-1.10) | 1.47 (1.10-1.96) d | 0.96 (0.90-1.03) | 1.04 (0.75-1.44) | 1.02 (0.98-1.07) | 1.03 (0.88-1.21) | 0.97 (0.93-1.01) |
| Medium and small metropolitan | 0.88 (0.49-1.60) | 1.08 (0.98-1.18) | 1.29 (0.98-1.70) | 0.93 (0.88-0.99) d | 1.36 (0.99-1.85) | 1.06 (1.02-1.11) d | 1.07 (0.94-1.23) | 0.92 (0.88-0.96) d |
| Micropolitan and noncore | 0.91 (0.45-1.86) | 1.19 (1.06-1.34) d | 1.16 (0.79-1.71) | 0.87 (0.79-0.96) d | 1.34 (0.94-1.93) | 1.03 (0.98-1.09) | 1.34 (1.13-1.59) d | 0.91 (0.86-0.97) d |
Abbreviations: FPL, federal poverty level; NA, not applicable; ref, reference.
Adjusted for age, sex, race and ethnicity, ratio of family income to FPL, US Census region, and urban–rural classification using multinomial logistic regression models.
The 4 outcomes are mutually exclusive and were represented as one 4-level dependent variable in multinomial logistic regression.
Data source: National Center for Health Statistics, National Health Interview Survey, 2021. 20
The 95% CI excludes 1.
The “other” category consists of non-Hispanic Asian, non-Hispanic American Indian or Alaska Native (along and with any other group), other single race, and multiple races.
Family income as a percentage of FPL is defined as the ratio of family income to federal poverty threshold (ie, minimum amount of income needed to cover the cost of basic needs). Due to high nonresponse for the survey item on family income, we used 10 publicly available imputed income files to estimate missing values for the ratio of income to FPL. 24 The US Census Bureau’s FPL is based on family income and family size.
Based on the 2013 National Center for Health Statistics Urban–Rural Classification Scheme for Counties. 23 Large central metropolitan areas are counties of ≥1 million population containing the principal city; large fringe metropolitan areas are counties of ≥1 million population not containing the principal city; medium metropolitan areas are counties of 250 000-999 999 population; small metropolitan areas are counties of <250 000 population; micropolitan areas have an urban cluster population of 10 000-49 999; and noncore areas include nonmetropolitan areas that did not qualify as micropolitan, including those without an urban cluster population of ≥10 000.
Adults aged ≥18 years
The prevalence of any allergy symptom only and asthma–allergy comorbidity was higher among women than among men (Table 2). Adults aged ≥65 years were more likely than adults aged 18 to 34 years to have neither asthma nor allergy symptoms. Compared with non-Hispanic White adults, Hispanic adults had a lower prevalence of allergy symptoms only and asthma–allergy comorbidity, and non-Hispanic Black adults had a higher prevalence of asthma–allergy comorbidity. Adults with family income <100% FPL and 100% to <200% FPL were less likely than adults with family income ≥200% FPL to have allergy symptoms only but more likely to have asthma only and asthma–allergy comorbidity. Adults living in micropolitan and noncore areas were more likely than adults living in large central metropolitan areas to have allergy symptoms only and asthma–allergy comorbidity.
Results of the adjusted analysis were similar to results of the unadjusted pairwise comparisons (Table 3). For asthma–allergy comorbidity, women (vs men), non-Hispanic Black adults (vs non-Hispanic White adults), those with income <100% FPL and 100% to <200% FPL (vs ≥200% FPL), and those living in micropolitan and noncore areas (vs large central metropolitan areas) had higher APRs.
Prevalence of Asthma–Allergy Comorbidity and Asthma Attacks Among People With Asthma
Overall, among the 7.8% of people with asthma, 82.1% (95% CI, 80.4%-83.6%) had comorbid allergy symptoms. This 82.1% is a sum of those with allergy symptoms and a lifetime allergy diagnosis (67.3%) and those with allergy symptoms with no lifetime allergy diagnosis (14.8%) (data not shown).
Children and adolescents aged 2 to 17 years
Among children and adolescents with asthma, 79.0% (95% CI, 74.4%-82.9%) had comorbid allergy symptoms. Children and adolescents of non-Hispanic other race and ethnicity were less likely than those who were non-Hispanic White to have asthma only, and those whose family income was 100% to <200% FPL had a lower prevalence of allergy symptoms without an allergy diagnosis than those whose family income was ≥200% FPL (Figure).
Figure.
Asthma–allergy comorbidity among people with current asthma, by sex, age, race and ethnicity, and family income, United States, 2021. Comorbidity categories are mutually exclusive. Inverted triangles indicate significant Wald χ2 P value (<.05) across categories and comorbidity outcomes. Closed circles indicate significant pairwise difference from referent group, using the following reference groups: by sex, male; by age, 2 to 4 years among children and adolescents, 18 to 34 years among adults; by race and ethnicity, non-Hispanic White; by income, ≥200% FPL. Data source: National Center for Health Statistics, National Health Interview Survey, 2021. 20 Abbreviation: FPL, federal poverty level.
Among children and adolescents with asthma, the occurrence of asthma attacks was not significantly associated with having allergy symptoms (ie, any allergy in model 1 and respiratory allergy, skin allergy, or food allergy in model 2) (Table 4) or lifetime allergy diagnosis (Supplemental Table).
Table 4.
Association between having ≥1 asthma attack in the past 12 months and allergy symptoms among children, adolescents, and adults with current asthma, United States, 2021 a
| Model | Children and adolescents aged 2-17 years (n = 5220) |
Adults aged ≥18 years (n = 24 570) |
||||
|---|---|---|---|---|---|---|
| ≥1 Asthma attack, % (95% CI) | PR (95% CI) of asthma attack | APR (95% CI) of asthma attack | ≥1 Asthma attack, % (95% CI) | PR (95% CI) of asthma attack | APR (95% CI) of asthma attack | |
| Model 1 b | ||||||
| Any allergy symptoms | ||||||
| Yes | 39.3 (33.9-45.0) | 1.09 (0.78-1.53) | 1.10 (0.80-1.52) | 40.9 (38.4-43.5) | 1.27 (1.07-1.51) c | 1.23 (1.04-1.46) c |
| No | 35.6 (26.1-46.5) | Reference | Reference | 32.3 (27.2-37.7) | Reference | Reference |
| Model 2 d | ||||||
| Respiratory allergy symptoms | ||||||
| Yes | 39.2 (33.5-45.2) | 1.03 (0.77-1.39) | 1.06 (0.79-1.42) | 41.7 (39.1-44.4) | 1.32 (1.13-1.55) c | 1.22 (1.04-1.42) c |
| No | 37.0 (28.3-46.5) | Reference | Reference | 31.6 (27.1-36.4) | Reference | Reference |
| Skin allergy symptoms | ||||||
| Yes | 45.8 (36.9-55.1) | 1.32 (0.99-1.78) | 1.30 (0.99-1.71) | 46.5 (41.4-51.6) | 1.24 (1.09-1.42) c | 1.17 (1.03-1.34) c |
| No | 35.2 (29.4-41.5) | Reference | Reference | 37.4 (34.8-40.0) | Reference | Reference |
| Food allergy symptoms | ||||||
| Yes | 32.9 (23.2-44.2) | 0.84 (0.58-1.21) | 0.82 (0.58-1.18) | 48.4 (43.6-53.4) | 1.32 (1.16-1.49) c | 1.23 (1.08-1.39) c |
| No | 39.9 (34.5-45.7) | Reference | Reference | 36.8 (34.2-39.5) | Reference | Reference |
Abbreviations: APR, adjusted prevalence ratio; PR, prevalence ratio.
Data source: National Center for Health Statistics, National Health Interview Survey, 2021. 20
The outcome for model 1 was asthma attack; independent variables were having any allergy symptoms and the demographic variables shown in Table 3.
The 95% CI excludes 1.
The outcome for model 2 was asthma attack; independent variables were symptoms of respiratory allergy, skin allergy, and food allergy, and the demographic variables shown in Table 3.
Adults aged ≥18 years
Among adults with asthma, 82.8% (95% CI, 81.0%-84.5%) had comorbid allergy symptoms. Women were less likely than men to have asthma alone and more likely to have allergy symptoms and a lifetime allergy diagnosis. Adults with family income <100% FPL and 100% to <200% FPL were less likely than adults with family income ≥200% FPL to have allergy symptoms with a lifetime allergy diagnosis and more likely to have asthma only (Figure).
Among adults with asthma, having asthma attacks was significantly associated with having any allergy symptoms (APR = 1.23; 95% CI, 1.04-1.46). In the model with each of the 3 types of allergy symptoms, the APRs of having an asthma attack were significant for each type. A sensitivity analysis showed similar associations: having an asthma attack was significantly associated with a lifetime allergy diagnosis and for specific diagnoses of respiratory and food allergy (Supplemental Table).
Discussion
In 2021, 6.4% of the US population had comorbid asthma and allergies. Among the population with asthma, 82.1% had comorbid allergy symptoms and two-thirds had a lifetime allergy diagnosis. The increased morbidity associated with asthma–allergy comorbidity is well known,5-13 but our study, to our knowledge, is the first nationally representative study to estimate the prevalence of allergies among people with asthma. Clinical recommendations highlight the importance of assessing patients with asthma for respiratory allergies and vice versa.1,15,27
Asthma–allergy comorbidity patterns differed between children and adolescents aged 2 to 17 years and adults in our study. Among children and adolescents, the prevalence of asthma–allergy comorbidity was higher among boys than among girls. Among adults, women had a higher prevalence of any allergy and asthma–allergy comorbidity than men. In general, asthma prevalence varies by sex during the lifespan, with a higher prevalence among boys than among girls in childhood and a higher prevalence among women than among men in adulthood. 25 Our study reflects this pattern and additionally shows higher asthma–allergy comorbidity among women than among men. Other disparities also exist: among children and adolescents, those who were non-Hispanic Black were more likely than those who were non-Hispanic White to have asthma only; among people aged ≥2 years, the prevalence of asthma–allergy comorbidity was higher among those who were non-Hispanic Black (vs non-Hispanic White) and those with family income <100% FPL (vs ≥200% FPL).
Although numerous studies have documented the increased risk of adverse asthma outcomes among children with asthma–allergy comorbidity,6,7,9,10,28 we found no increased risk of asthma attacks among children and adolescents with comorbid asthma and allergy. However, respiratory infections are a major trigger for asthma attacks in children, 29 and we could not account for the effect of respiratory infections in our study. Yet, asthma–allergy comorbidity is still important among children and adolescents. One review presented evidence that allergic sensitization in preschool-aged children was associated with lower-than-normal growth of lung function. 30 A study of urban children aged 5 to 12 years found that allergic sensitization was not associated with lung function but was associated with persistent wheezing. 31 Other studies have also found that childhood eczema and respiratory allergies are associated with the persistence of asthma into adulthood and adult-onset asthma.32,33 These studies highlight differences in how allergy comorbidity may affect children, adolescents, and adults with asthma.
Adult-onset asthma without allergic comorbidity, especially among women, has been highlighted as a clinical phenotype with a high risk for severe asthma.34,35 Studies identifying asthma phenotypes have found that comorbid asthma–allergy often has early onset, 36 typically continues into adulthood, and has varying severity levels. 16 In our study, which used cross-sectional population-based data, the assessment of asthma phenotypes was not possible. Regardless, the results from the NHIS data show a greater prevalence of asthma–allergy comorbidity among women than among men.
Our study assessed both self-reported allergy symptoms and lifetime diagnosis of allergies. Requiring a diagnosis from a health care professional could strengthen the definition of allergic conditions. However, a lifetime diagnosis does not always correspond to having current symptoms.10,11,37 Allergic conditions may abate, be controlled, or remain undiagnosed. In our study population, allergic rhinitis/respiratory allergies were the most prevalent allergies. A previous study found that most people use over-the-counter medication to treat their respiratory allergy symptoms, and only 44.3% had a physician diagnosis. 37 In our study, slightly more than half of people with respiratory allergy symptoms reported receiving a lifetime allergy diagnosis.
Limitations
This study had several limitations. The first was recall bias. Second, cross-sectional survey data cannot be used to assess causation. Third, as with most epidemiologic studies, uncontrolled confounding likely existed. Because the NHIS includes allergy questions only every third year, and for the first time in 2021 for adults, we could not group years of NHIS data to increase sample size. Due to small sample size, we did not have sufficient statistical power to detect differences among subgroups of children and adolescents. For example, a previous study found higher asthma prevalence among children with food allergy than among those without food allergy. 14 In our study, in which the prevalence was lowest for the asthma–food allergy comorbidity, a single year of data for children and adolescents may have been insufficient to detect an association between asthma attacks and food allergy. Fourth, diagnoses of asthma and allergies can be challenging, especially in young children. 38 Fifth, the categories of allergies included in the NHIS are nonspecific, and respondents may not have reported allergic sensitization types previously demonstrated to affect asthma control (eg, allergies to cockroaches and dust mites). 39 Finally, the 2021 NHIS was conducted during the COVID-19 pandemic. Most studies reported a lack of effect or a protective effect of asthma on risk for COVID-19. 40 However, no studies, to our knowledge, have assessed the effect of the pandemic on the prevalence of asthma and allergies, although decreases in air pollution and exposure to infectious diseases during lockdown periods could have affected asthma incidence. It is unknown how other exposures known to be risk factors for developing asthma (eg, environmental tobacco smoke) changed during the pandemic, although exposures likely varied by housing and other characteristics. 41 Using the NHIS interactive summary health statistics tool 42 to compare prepandemic and postpandemic prevalence estimates for asthma, we found that the prevalence among those aged ≤17 years in 2021 (6.5%) was comparable to 2019, 2022, and 2023 (7.0%, 6.2%, and 6.7%, respectively); the 2020 estimate (5.8%) was the lowest estimate. Among adults, the prevalence of asthma in 2021 (8.0%) was the same as in 2019 (8.0%) and slightly lower than in 2020, 2022, and 2023 (8.4%, 8.7%, and 8.9%, respectively). That is, prevalence estimates for asthma were generally comparable from 2019 through 2023. The NHIS questions on allergies in 2018 among children and adolescents differed from NHIS questions in 2021, so these data were not comparable. Allergy prevalence estimates for adults were not available before 2021.
Conclusion
The prevalence of asthma and allergies is high in the US population. Most people with asthma also have comorbid allergies. We observed disparities in comorbid asthma–allergy previously observed for asthma43,44: women (vs men), people with low family income (vs ≥200% FPL), and non-Hispanic Black people (vs non-Hispanic White people) were more likely to have both conditions. Among adults with asthma, comorbid allergy was associated with a higher risk of having ≥1 asthma attack in the previous 12 months. These findings support and can inform national asthma management guidelines that highlight the importance of identifying and treating comorbid allergies among people with asthma.
Supplemental Material
Supplemental material, sj-docx-1-phr-10.1177_00333549251358658 for Asthma and Allergy Comorbidity Among the US Population Aged 2 Years or Older, National Health Interview Survey, 2021 by Cynthia A. Pate, Lara J. Akinbami, Carol Johnson, Joy Hsu and Hatice S. Zahran in Public Health Reports®
Footnotes
Disclaimer: The findings and conclusions in this article are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.
Authors’ Note: Cynthia A. Pate and Lara J. Akinbami are co–first authors.
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding: The authors received no financial support for the research, authorship, and/or publication of this article.
ORCID iDs: Lara J. Akinbami, MPH 
https://orcid.org/0000-0001-6876-6262
Joy Hsu, MD
https://orcid.org/0000-0001-8410-9334
Supplemental Material: Supplemental material for this article is available online. The authors have provided these supplemental materials to give readers additional information about their work. These materials have not been edited or formatted by Public Health Reports’s scientific editors and, thus, may not conform to the guidelines of the AMA Manual of Style, 11th Edition.
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Supplementary Materials
Supplemental material, sj-docx-1-phr-10.1177_00333549251358658 for Asthma and Allergy Comorbidity Among the US Population Aged 2 Years or Older, National Health Interview Survey, 2021 by Cynthia A. Pate, Lara J. Akinbami, Carol Johnson, Joy Hsu and Hatice S. Zahran in Public Health Reports®