TABLE 1—

Selected Information From Studies in the Review

Citation	Time Frame	Method and Study Design	Sample and Demographics	Geographic Location	Study Contained Key Results Related to	Key Results
Atwell et al.16	2005–2010	Quantitative, time-series cohort	All kindergarten students with nonmedical exemptions in CA and all cases of pertussis from 2005 to 2010	CA	C, E	Census tracts within a NME cluster were more likely to be in a pertussis cluster than those outside an NME cluster. Both NME and pertussis clusters were associated with high SES characteristics.
Birnbaum et al.17	2010–2011	Quantitative, cross-sectional	1018 private and public schools; kindergarteners	AZ	T, C	Rates of PBE are highly spatially aggregated in AZ and within Phoenix. Schools with highest proportion of White students had the highest PBE rates. Charter schools and those with low prevalence of free and reduced lunches had significantly higher rates of PBE.
Blank et al.11	2011–2013	Mixed, descriptive, and cross-sectional	50 states	US	S	Fourteen, 15, and 15 states were found to have easy, medium, and difficult exemption policies, respectively. States that granted philosophical exemptions and states with simpler exemption procedures had higher rates of exemption. No association was found between strictness of religious exemption policy language and percentage of children exempted for religious reasons. From 2011 to 2012, bills were introduced in 4 states restricting scope of NMEs and in 10 states broadening it. However, only bills that tightened exemption policies passed.
Britten18	2008–2009	Quantitative, cross-sectional, mathematical model	5 communities; kindergarteners	CA	T, E	Using a hypothetical population, each percentage decrease in exemption coverage was found to lead to a significant increase in severity of the outbreak and duration of the outbreak. The most significant increase in severity happened between 93% and 90% coverage, dependent on population size.
Buttenheim et al.19	2008–2010	Quantitative, cross-sectional	> 7000 private and public schools; around 500 000 kindergartners	CA	T	Crude PBE rates in CA increased 25% from 2008 to 2010, and PBE rate per 100 kindergartners increased from 1.9 to 2.3. The percentage of schools with a high PBE rate also increased from 2.2% to 2.6%. The interaction index increased from 1.6 to 2.0, and the aggregation index increased from 14.7 to 15.6, indicating increased interactions of exempted kindergarteners with other exempted kindergartners. Aggregation indexes varied widely among counties.
Centers for Disease Control and Prevention20	May–June 2005	Mixed, descriptive, cross-sectional	34 measles patients	Cincinnati, OH	E	A 17-year-old girl not vaccinated for measles contracted measles in Bucharest, Romania, during an outbreak and returned to Indiana. 34 people were later identified with measles, only 2 of whom were partially or fully vaccinated.
Centers for Disease Control and Prevention21	March 2004	Mixed, descriptive, cross-sectional	1 case of measles	Cedar Rapids, IA	E	28 students from a college with a high exemption rate went to India, and 6 measles cases occurred among the students. It was recommended they not return to the US until after the period of infectivity, but 1 student went against these precautions, alerting public health officials about the potential risk of outbreak.
Centers for Disease Control and Prevention22	March–June 2011	Mixed, descriptive, cross-sectional	2 measles outbreaks, 13 cases of measles	UT	E	Two outbreaks occurred in Utah in April and May 2011. Thirteen people were confirmed to have measles; 9 (69%) were unvaccinated and had PBEs. The infection was acquired during international travel (1 case), in households (8 cases), and schools (3 cases). Source of infection was unknown in 1 case.
Centers for Disease Control and Prevention23	April 9–July 7, 1996	Mixed, descriptive, cross-sectional	107 measles reports	Washington County, UT	E	Of the 99 measles cases who were vaccine-eligible, 64 (64%) had not been vaccinated. At the high school at which the outbreak was initially reported, 27 (3%) of 879 students were unvaccinated, and 780 (89%) had received only 1 dose of the MMR vaccine. Seventeen of the unvaccinated students had philosophic exemptions, and the other 10 had no record of measles vaccination.
Centers for Disease Control and Prevention24	January 1–April 25, 2008	Mixed, descriptive, cross-sectional	64 measles reports	NY, AZ, CA, MI, WI, HI, IL, NY, PA, VA	E	Of the 64 patients with measles 63 were unvaccinated or had unknown or undocumented vaccination status. Of these, 14 were not vaccinated because of religious or personal beliefs. 5 who had traveled abroad were unvaccinated, 2 because of personal beliefs.
Centers for Disease Control and Prevention25	January–July 2008	Mixed, descriptive, cross-sectional	131 measles reports	IL, NY, WA, AZ, CA, WI, HI, MI, AR, DC, GA, LA, MO, NM, PA, VA	E	Among the 131 measles patients, 123 were US residents, of whom 112 (91%) were unvaccinated or had unknown vaccination status. Among these 112 patients, 95 (85%) were eligible for vaccination, and 63 (66%) of those were unvaccinated because of philosophical or religious beliefs.
Centers for Disease Control and Prevention26	2009–2010	Quantitative, descriptive, cross-sectional	47 states and DC	US	T	From 2009 to 2010, total kindergarten exemption rates ranged from < 1% to 6.2% across states; 15 states had a total exemption rate ≥ 3.0%, and 15 states had exemption rates < 1%. Nonmedical exemptions ranged from 0.2% (Rhode Island) to 5.8% (Washington) among the states that allow NMEs.
Centers for Disease Control and Prevention27	2011–2012	Quantitative, descriptive, cross-sectional	49 states and DC	US	T	From 2011 to 2012, total kindergarten exemption rates ranged from < 0.1% in Mississippi to 7.0% in Alaska, with 10 reporting rates < 1%, and 9 reporting > 4% total exemption rates. The median total exemption level was 1.5%, a median increase of 0.2 percentage points compared with the 2009–2010 school year. Arkansas reported the largest increase in exemptions with an increase of 3.4 percentage points, and Nebraska reported the largest decrease, with a decrease of 2.3 percentage points. The median NME level was 1.2%, where allowed, with a range from 0.04% in Delaware and Kentucky to 5.8% in Oregon.
Centers for Disease Control and Prevention28	2012–2013	Quantitative, descriptive, cross-sectional	49 states and DC	US	T	From 2012 to 2013, the percentage of kindergarteners with an exemption was < 1% for 9 states and > 4% for 11 states, ranging from < 0.1% in Mississippi to 6.5% in Oregon, with a median of 1.8%. Georgia and West Virginia had the largest increases in total exemptions, each with an increase of 1.0 percentage point; 4 states reported decreases of > 1.0 percentage points. The median NME level was 1.5%, ranging from 0.2% in New Mexico to 6.4% in Oregon.
Ernst & Jacobs29	2009–2011	Quantitative, cross-sectional, descriptive	Vaccine coverage levels in 18 states; county-level PBE rates in 8 states	US	S, E	A significant trend of increasing vaccine coverage was found with increasing difficulty of obtaining PBE for DTP–DTaP–DT and poliovirus vaccines (P < .05) and a modest association for MMR and HepB vaccines (P < .01). Although mean vaccination coverage remains > 90% even in states in which exemptions are easy to obtain, geographic heterogeneity exists in vaccine exemptions; in Arizona, PBE rates ranged from 0.6% to 8.5% among counties, and in Washington, rates ranged from 1% to 25.3%.
Fair et al.30	1992–2000	Mixed, descriptive, cross-sectional	15 cases of tetanus in children aged < 15 y	TN, MT, MO, IN, FL, MI, CA, OH, PA, TX, WV	T, E	Fifteen cases of tetanus in children aged < 15 y were reported in 11 states. 12 (80%) children were unprotected because of lack of vaccination. Among all unvaccinated cases, objection to vaccination, either religious (n = 9) or philosophic (n = 3), was the reported reason for choosing not to vaccinate.
Feikin et al.31	1987–1998	Quantitative, time-series cohort	All children aged 3–18 y in CO	CO	S, E	The percentage of philosophical exemptions among school-aged children in Colorado increased from 1.02% to 1.87%. Exemptors were 22.2 times more likely to acquire measles and 5.9 times more likely to acquire pertussis than vaccinated children. The frequency of exemptors in a county was associated with the incidence rate of measles and pertussis in vaccinated children. Schools with pertussis outbreaks had more exemptors than schools without outbreaks. At least 11% of vaccinated children in measles outbreaks acquired infection through contact with an exemptor.
Fiebelkorn et al.32	2001–2008	Quantitative, time-series cohort	557 cases, 38 outbreaks	US	E	In the US, 557 confirmed cases of measles and 38 outbreaks were reported, 232 (42%) of which were imported from other countries. A total of 285 US-resident case-patients (65%) were considered to have preventable measles. From 2004 to 2008, a total of 68% of vaccine-eligible US-resident case-patients claimed exemptions for personal beliefs.
Gaudino & Robison33	2004–2005	Quantitative, retrospective cohort	1588 parents of OR elementary school students	OR	T, C	Exempting parents reported more markers of lower SES than nonexemptors. Exemptors were significantly more likely to have: strong vaccine concerns, > 1 childbirths at a nonhospital, distrust of local doctors, chiropractic health care, and knowledge of someone with a vaccine-hurt child, although this varied by specific communities. Exemptors were less likely to have pro-vaccine beliefs and less likely to report relying on print materials.
Gullion et al.34	Not mentioned	Qualitative, semistructured interviews	25 nonvaccinating parents	TX	C	Most (88%) of the interviewees mentioned aspects of their lifestyle that could be categorized as ‘‘alternative living.” Participants engaged in sophisticated data collection and analysis in formulating their stance on vaccinations and were skeptical of the medical community, although they placed a high value on scientific knowledge.
Gust et al.35	July and August 2003	Quantitative, cross-sectional	642 parents with at least 1 child aged < 6 y	US	C	Parents who disagreed that they had enough immunization information were more likely to report that they would not have their child immunized if it were not required by law; to believe states should grant exemptions; to mistrust the government to establish immunization policy; and to believe that they should be allowed to obtain exemptions for their child even if it raised the risk of disease for everyone else.
Gust et al.36	May 2004–February 2006	Mixed: interviews, focus groups, cross-sectional	100 cases of parents who would not immunize children if not required or who had considered filing or filed a NME and 100 controls	US	C	Parents who had filed exemptions or considered it did not differ demographically from those who did not file exemptions but were significantly more likely to have negative attitudes about immunizations, including safety, number of immunizations, and trust. A brochure intervention was found not to have improved parents’ immunizations attitudes compared with controls.
Imdad et al.37	2000–2011	Quantitative, time-series cohort	All schools in NY; all pertussis cases among children aged < 19 y	NY	T, S, E	Religious exemptions in NY increased from 0.23% to 0.45% in the past 10 y, although not uniformly among counties. Counties with religious exemption prevalence rates > 1% had a higher incidence of pertussis, and a 0.1% increase in exemption rate corresponded with an increased pertussis incidence of 5 in 100 000. The mean incidence of pertussis among exempted children was 14 times that among vaccinated children. High exemption rates in a county increased pertussis risk for both vaccinated and exempted children.
Kennedy et al.38	2002	Quantitative, cross-sectional	1527 parents with at least one child aged ≤ 18 y	US	T, C, S	A parent’s belief regarding compulsory vaccination for school entry is significantly associated with beliefs in the safety and utility of vaccines and the intention to have the youngest child fully vaccinated. Supportive parents were more likely to be White, to have a higher household income, and to have a smaller household size. Residence in a state that permits philosophical exemptions was also associated with a parent’s opposition to compulsory vaccination.
Kennedy & Gust39	Sometime between 2005 and 2008	Mixed, focus group, interview, cross-sectional	6 church members for focus group; 12 study households involved in the outbreak for interview	IN	C	Outbreak households recognized the importance of vaccines, yet had concerns or doubts about their safety and necessity, believing that childhood vaccinations may cause serious side effects or learning disabilities. All believed in the right to refuse vaccines but were open to alternatives such as quarantine during an outbreak, and all reported that they had access to enough information on vaccination. Most said that the outbreak experience did not make their opinion of vaccines more positive.
Luthy et al.40	Not mentioned	Qualitative, cross-sectional questionnaire	287 parents	UT	C	Five overarching themes were identified regarding PBEs: parental perceptions of vaccine harm (such as the belief that vaccines caused autism), health care systems issues (insofar as filing for a PBE was more convenient), chronic disease concerns, immune system concerns, and adverse reaction concerns.
Luthy et al.41	Not mentioned	Quantitative, cross-sectional questionnaire	801 parents who have an exempted child	UT	C	The most commonly reported reason for seeking a personal exemption was vaccination conflicting with philosophical beliefs. Most parents communicated their vaccine concerns with their health care provider before seeking exemption. The majority of exempting parents did not use the Internet when researching vaccines even though they had Internet access.
Mergler et al.42	2002–2003, 2005	Quantitative, cross-sectional	1367 parents and 551 providers	CO, MA, MO, WA	C	Parents who agreed that a child’s immune system could be weakened by too many immunizations or, conversely, who the community benefits from having children fully vaccinated; that the child benefited from vaccination; and that vaccines were very safe had greater odds of having a provider who shared those beliefs.
Omer et al.43	1993–2004	Quantitative, time-series cohort	4495 schools, 1111 cases	MI	S, E	A total of 23 significant clusters of high exemption rates and 6 clusters of pertussis cases were identified. There was a statistically significant geographic overlap between exemptions and pertussis case clusters. Census tracts in exemptions clusters were 3 times more likely to also be in a pertussis cluster than census tracts outside any exemptions cluster.
Omer et al.44	1991–2004; 1986–2004	Quantitative, time-series cohort	48 states, kindergarten or first-grade data	US, except MS and WV	S, E	States that easily granted exemptions had higher NME rates than states with medium and difficult exemption processes, as well as increased pertussis incidence. Although the mean exemption rate increased an average of 5% for easy-exemption-process states, there was no significant change in states with only religious exemptions or with medium or difficult exemption processes.
Omer et al.45	2005–2011	Quantitative, time-series cohort	50 states	US	S	Unadjusted rates of NME in states with easy exemption policies were 2.31 times as high as those of states with difficult exemption policies. By 2011, exemption rates in states with easy, medium, and hard exemption policies increased to 3.3%, 2.0%, and 1.3%, with annual rates of 13%, 18%, and 8%, respectively.
Peterson et al.46	February–May 2010	Quantitative, cross-sectional	2052 students in elementary, middle, and high schools in rural school district	WA	T	A total of 5.4% of children in kindergarten and 4.74% of children in kindergarten through 12th grade were exempted from immunizations, with higher exemptions in rural districts. Correcting school immunization records resulted in an increase in the number of students classified as fully immunized. After conducting school-based immunization clinics, the number of fully immunized students also increased.
Richards et al.47	1994–2009	Quantitative, time-series cohort	6392 schools	CA	T, C	The average school PBE rate increased from 0.6% in 1994 to 2.3% in 2009, an average of 9.2% per year. The average PBE rate among private schools was 1.77 times that among public schools, and its annual rate of increase was higher. Schools located in rural census tracts had 1.66 times higher PBE rates than those in urban census tracts. Exemption rates were also found to be associated with race, population density, education, and income.
Rota et al.48	January 1998	Mixed, cross-sectional survey	48 states, distributed to state health department immunization program managers	US, except MS and WV	S	Sixteen states delegated sole authority for processing exemptions to school officials, and 9 states had written policies informing parents who seek an exemption of the risks of not immunizing. The complexity of the exemption process was inversely associated with the proportion of exemptions filed.
Safi et al.8	2001–2010	Quantitative, time-series cohort	All students with exemptions in AR	AR	T, S	Exemptions increased steadily from 2003 after philosophical exemptions became allowed. Kindergarten had the steepest increase in exemptions. Medical exemptions declined by 55%, and religious exemptions declined and then increased. In the 2009–2010 school year, 70.8% of exemptions were requested for all vaccines, 9.2% were requested for ≥ 2 vaccines, and 20% were requested for a single vaccine. Of the single-vaccine exemptions, 93% were for the MMR and 4.6% were for HepB and varicella.
Salmon et al.49	1985–1992	Quantitative, time-series cohort	Mapping of exemptors by county in CA; individuals aged 5–19 y	US	E	On average, exemptors were 35 times more likely to contract measles; relative risk varied by age and year. When mapping exemptors by county in CA, exempt populations tended to be clustered in certain geographic regions. If the number of exemptors doubled, incidence of measles in nonexempt populations would increase by 5.5%, 18.6%, and 30.8%, respectively, for intergroup mixing ratios of 20%, 40%, and 60%.
Salmon et al.50	1998–2004	Quantitative, case-control	1367 parents, 391 of exempt children and 976 of fully vaccinated children	CO, MA, MO, WA	T, C, S	Most children (75%) with NMEs received at least some vaccines. Parents of exempt children were significantly more likely to report low perceived vaccine safety and efficacy, a low level of trust in the government, low perceived susceptibility to and severity of vaccine-prevented diseases, lower confidence in government sources for information, and higher confidence in alternative medicine professionals.
Salmon et al.51	May 2001– June 2002	Quantitative, cross-sectional	695 schools, surveys mailed to elementary school personnel who had completed state immunization report	CO, MA, MO, WA	C	Greater perceived disease susceptibility and severity of vaccines were associated with a decreased likelihood of a child in the school having an exemption. Children in schools at which the respondents were nurses or who had confidence in health departments were significantly less likely to be given an exemption. Use of professional organizations, government resources, and vaccine companies and pharmacists for vaccine information were associated with decreased likelihood of a child having an exemption.
Salmon et al.52	2001–2002	Quantitative, cross-sectional	1000 school immunization personnel in CO, MA, MO, and WA	CO, MA, MO, WA	S	School policies associated with an increased likelihood of children having exemptions included lack of provision of written instructions for completing the immunization requirement before enrollment, administrative procedures making it easier to claim an exemption, and granting of philosophical exemptions. A correlation was found between the number of procedures that make administration of exemptions difficult and a lower odds of actual exemptions.
Salmon et al.53	Not mentioned	Quantitative, case-control	780 parents	WI	T, C	Varicella vaccine and HepB vaccine were the top vaccines often not received by exempt children. The top reasons for seeking exemptions included the belief that vaccines might cause harm, that it was better to get natural disease, that the child was not at risk for disease, and that the child might develop autism. Exempt parents were also less likely to believe in disease susceptibility, severity, and vaccine efficacy and safety.
Smith et al.54	1995–2001	Quantitative, time-series cohort	151 720 children aged 19–35 mo, 795 of whom were unvaccinated	US	T, C, S	Undervaccinated children tended to be Black; have a younger mother who was not married and did not have a college degree; live in a poorer household; and live in a central city. Unvaccinated children tended to be White; have a mother who was married and had a college degree; live in a wealthier household; and have parents who expressed concerns regarding safety of vaccines. States that allowed philosophical exemptions also had significantly higher estimated rates of unvaccinated children.
Sugerman et al.55	2008	Mixed, discussion groups, survey, cross-sectional	839 patients	CA	T, C, E	PBE rates increased, and higher PBE rates in public schools were associated with higher median income. There was no significant effect of income in public charter and private schools. On the parent level, nearly all parents who reported declining or delaying vaccination were White and college educated. Most reported substantial skepticism of the government, pharmaceutical industry, and medical community; believed vaccination was unnecessary; and felt vaccines can produce a number of adverse health effects.
Thompson et al.56	2001–2002, 2002–2003, 2003–2004 school years	Quantitative, time-series cohort	Immunization exemptions granted for all AR school attendees, K–12	AR	T, S	Philosophical exemptions were found to be clustered geographically. After AR started allowing philosophical exemptions, the total number of exemptions granted increased by 23% from year 1 to year 2, by 17% from year 2 to year 3, and by 50% from year 3 to year 4. NMEs accounted for 79% of exemptions granted in years 1 and 2, 92% in year 3, and 95% in year 4.
Wenger et al.57	007	Mixed, cross-sectional	359 Amish parents	OH	T, C	A total of 68% stated that all of their children had received ≥ 1 immunization, and 17% reported that some of their children had received ≥ 1 immunization. Only 14% of the parents reported that none of their children had received immunizations. Reasons Amish parents resisted immunizations include concerns about adverse effects such as side effects, dangerous chemicals, and injection of a disease.

Note. C = predictors or correlates; DT = diphtheria and tetanus; DTaP = diphtheria, tetanus, and pertussis; DTP =  diphtheria, pertussis, and tetanus; E = epidemiological implications; HepB = hepatitis B; MMR = measles–mumps–rubella; NME = nonmedical exemption; PBE = personal beliefs exemption; S = characteristics and impact of state-level exemption policies; SES = socioeconomic status; T = prevalence and trends.