US-Mexico binational COVID-19 cases in southern California border counties, California, February–June 2020

Abstract

Background

COVID-19 has had a significant public health impact on both the United States and Mexico. Cross-border mobility between southern California and Mexico raises questions of transmission trends between these jurisdictions. The objective of this project was to describe binational cases amongst California US-Mexico border county COVID-19 cases and compare incidence trends to cross-border Mexico jurisdictions.

Methods

Interview data from persons with confirmed SARS-CoV-2 infections in San Diego County, CA and Imperial County, CA from February to June 2020 were reviewed for binational cases; demographics and connection to COVID-19 outbreaks were assessed. Graphs of COVID-19 incidence in San Diego County and Imperial County were compared to incidence graphs in cross-border Mexico jurisdictions of Tijuana and Mexicali.

Results

Persons with COVID-19 and a binational case were older, more likely to be Hispanic, and reside in a border ZIP code than those without. Binational cases were a small proportion and tracked with overall cases during the study period.

Conclusions

Binational cases had different trends than non-binational cases of SARS-CoV-2 in San Diego and Imperial counties from February to June 2020. Findings could inform SARS-CoV-2 mitigation strategies specific to the US-Mexico land border, particularly recommendations regarding cross-border land travel.

1. Introduction

COVID-19 has had a significant public health impact on both the United States and Mexico. Despite non-essential travel limitations on the US-Mexico border, on average 13.8 million US citizens, lawful permanent residents, and essential workers crossed into the US from Mexico monthly in February – June 2020 (Fact sheet 2020; Border crossing/Entry data, 2020). Mexican migrants in the southern United States face vulnerabilities related to COVID-19 secondary to a high prevalence of preexisting conditions and limited access to testing and preventive services (López-Jaramillo et al., 2020). Additionally, temporary accommodations that house cross-border irregular migrants, such as camps and shelters, may have qualities that increase the risk of acquiring SARS-CoV-2 (Bojorquez-Chapela et al., 2022). It is unknown whether cross-border mobility might be a factor in SARS-CoV-2 spread in the California-Mexico border region.

Southern California counties on the US-Mexico border, San Diego County and Imperial County, have been COVID-19 hotspots (Oster et al., 2020). Mexican jurisdictions on the US-Mexico border have also been among those with highest COVID-19 incidence in Mexico (COVID-19 Mexico 2020). The Council of State and Territorial Epidemiologists (CSTE) developed the concept of a “binational case” to identify cases of infectious disease with potential cross-border exposure or dissemination (Incorporating a binational variable into the National, 2020; Example, 2020).

This study examines the impact of cross-border mobility on COVID-19 burden in San Diego County and Imperial County. We describe binational cases in persons with confirmed SARS-CoV-2 infection in San Diego County and Imperial County and compare trends with COVID-19 cases in Tijuana and Mexicali, adjoining Mexican municipalities (Fig. 1). Assessment of binational COVID-19 cases in California border counties is important to understand transmission dynamics and the extent to which binational cases may be driving increases in COVID-19 incidence in San Diego and Imperial Counties. Such information is also useful to inform public health recommendations related to contact tracing, case investigation, travel, and travel restrictions along the US-Mexico border.

Fig. 1.

Map of the California-Mexico border region, with California counties and Mexico municipalities used for analysis* highlighted in red. *Tijuana and Mexicali were chosen as cross-border municipalities for analysis based on population flow at the land border ports of entry in these jurisdictions (Border crossing/Entry data, 2020).

2. Material and methods

We reviewed data from persons with confirmed SARS-CoV-2 infection reported to San Diego County or Imperial County public health departments during February–June 2020, including demographics and presence of a binational case. Inclusion criteria included primary residence reported in San Diego County or Imperial County. A confirmed COVID-19 case tested positive for SARS-CoV-2 infection by real-time reverse transcription polymerase chain reaction by any mode of collection (Coronavirus Disease, 2019).

We used the CSTE definition to identify persons with a binational case. A binational case was defined as meeting ≥1 variable component related to binationality during the disease exposure or infectious period (Incorporating a binational variable into the National, 2020). Binational variable components collected for COVID-19 cases varied by county and over time given variation in case interview tools. Persons in San Diego County with a binational case were those travelling to Mexico during their exposure or infectious period, had a residence in Mexico, or a case contact from Mexico. San Diego County collected binational case data through free-text investigator notes, and an additional interview question regarding travel during the exposure period was collected during case interviews conducted March to April and June 2020. In Imperial County, persons with a binational case were those who travelled to Mexico during the exposure period, as collected through a direct question collected for all interviews conducted in February–June 2020.

Exposure period was defined as the 14 days prior to symptom onset, and the infectious period was two days prior to symptom onset through the release from isolation (Duration, 2020). If a person was asymptomatic the collection date of first confirmatory SARS-CoV-2 test was used (Duration, 2020). Demographic information included race and ethnicity, per self-report, age, and gender. Association with an outbreak was defined as an individual being present at an identified outbreak setting during their exposure or infectious period. COVID-19 outbreaks were identified using setting definitions from by California Department of Public Health (Non-Healthcare Congregate Facilities COVID-19, 2022). Border ZIP codes were ZIP codes that are on the US-Mexico border. Descriptive statistics were used to describe demographic information, including age, gender, and ethnicity. Chi-square tests were used to assess differences between persons with and without a binational case in regards to demographics, death, involvement in an outbreak, and resident of a border ZIP code.

Association with a COVID-19 outbreak was assessed for persons with SARS-CoV-2 infection in San Diego County, and outbreak setting type defined as congregate living facility, facility that provides food, or other. Outbreak information was not available for Imperial County. Daily case counts for Mexico were obtained from publicly available data (COVID-19 Mexico 2020). In Mexico during this time period, primarily persons hospitalized with acute severe respiratory infectious disease were being tested for COVID-19. We plotted 7-day moving averages of daily cases with and without a binational case for each county and for adjacent Mexico municipalities of Tijuana and Mexicali. Statistical analysis was conducted using R version 4.0.2, with significance defined as less than 0.05. This activity was reviewed by CDC and was conducted consistent with applicable federal law and CDC policy.§

3. Results

During February–June 2020, San Diego County and Imperial County had 25,326 COVID-19 cases, 7% (1782) of which were identified as binational. In San Diego, 7% (1164/17,647) of COVID-19 cases were binational and 8% (618/7679) of Imperial County cases were binational. Binational cases in both counties were more likely to self-identify as Hispanic (San Diego: 90% vs. 50%, p < 0.001; Imperial: 96% vs. 48%, p <0.001), be older than 50 years (San Diego: 43% vs. 33%, p = 0.001; Imperial: 39% vs. 34%; p < 0.001), and reside in border ZIP codes (San Diego: 26% vs.11%, p < 0.001; Imperial: 52% vs. 42%, p < 0.001). In San Diego County, binational cases were less likely to be involved in an identified outbreak than cases that were not binational(7% vs. 16%, p < 0.001) (Table 1).

Table 1.

Demographics of persons with COVID-19* in San Diego and Imperial Counties, California, February–June 2020.

	San Diego County				Imperial County
	All (N = 17,647) n (%)	Binational† (N = 1164)	Non-Binational (N = 16,483)	Chi-Square Test p-value (binational/non-binational)	All (N = 7679)	Binational‡ (N = 618)	Non-Binational (N = 7061)	Chi-Square Test p-value (binational/non-binational)
Age 0–4 years 5–17 years 18–49 years 50–64 years 65+ years Unknown Median age Mean age	379 (2%) 553 (3%) 10,736 (61%) 3580 (20%) 2396 (14%) 3 (<1%) 39 41.9	19 (2%) 21 (2%) 630 (54%) 310 (27%) 184 (16%) 0 (0%) 45 45.2	360 (2%) 532 (3%) 10,106 (61%) 3270 (20%) 2212 (13%) 3 (<1%) 38 41.67	<0.001	350 (5%) 519 (7%) 4151 (54%) 1669 (22%) 990 (13%) 0 (0%) 39 40.9	15 (2%) 22 (4%) 338 (55%) 161 (26%) 82 (13%) 0 (0%) 43 43.8	335 (5%) 497 (7%) 3813 (54%) 1508 (21%) 908 (13%) 0 (0%) 39 40.6	<0.001
Gender Female Male Unknown	9019 (51%) 8604 (49%) 24 (<1%)	555 (48%) 609 (52%) 0 (0%)	8464 (51%) 7995 (49%) 24 (<1%)	0.02	3962 (52%) 3714 (48%) 3 (<1%)	298 (48%) 320 (52%) 0 (0%)	3664 (52%) 3394 (48%) 3 (<1%)	0.20
Race and Ethnicity American Indian Asian Black, Non- Hispanic Multiple Race Pacific Islander White, Non-Hispanic Unknown	30 (<1%) 876 (5%) 654 (4%) 9285 (53%) 195 (1%) 110 (1%) 3546 (20%) 2951 (17%)	0 (0%) 8 (1%) 4 (<1%) 1049 (90%) 4 (<1%) 1 (<1%) 52 (4%) 46 (4%)	30 (<1%) 868 (5%) 650 (4%) 8236 (50%) 191 (1%) 109 (1%) 3494 (21%) 2905 (18%)	<0.001	70 (1%) 4 (<1%) 21 (<1%) 3954 (51%) n/a 2 (<1%) 148 (2%) 3480 (45%)	0 (0%) 0 (0%) 0 (0%) 593 (96%) n/a 0 (0%) 9 (1%) 16 (3%)	70 (1%) 4 (<1%) 21 (<1%) 3361 (48%) n/a 2 (<1%) 139 (2%) 3464 (49%)	<0.001
Number died	495 (3%)	51 (4%)	444 (3%)	0.001	223 (3%)	26 (4%)	197 (3%)	0.06
Number involved in an outbreak	2790 (16%)	83 (7%)	2707 (16%)	<0.001	N/A	N/A	N/A	N/A
Number residing in border ZIP codes	2040 (12%)	302 (26%)	1738 (11%)	<0.001	3260 (42%)	322 (52%)	2938 (42%)	<0.001

*Persons with confirmed COVID-19 tested positive for SARS-CoV-2 with PCR testing, were reported to San Diego County and Imperial County public health departments and reported primary residency in those counties.

^†Persons in San Diego County with COVID-19 cases and a binational case were those travelling to Mexico during their exposure or infectious period, had a residence in Mexico, or a case contact from Mexico.

^‡In Imperial County, persons with COVID-19 cases and a binational case were those who travelled to Mexico during the exposure period.

Of San Diego County COVID-19 cases in Hispanic persons, 11% (1049/9285) had a binational case; 28% (289/1049) of these resided in a border ZIP code. Of Imperial County's COVID-19 cases in Hispanic persons, 15% (593/3954) had a binational case; 52% (309/593) of these resided in a border ZIP code. During February–June 2020, of San Diego's 233 COVID-19 outbreaks, 22% involved an individual with a binational case. There was no statistically significant difference between the outbreaks occurring in a location with a border ZIP code (10%) or in a non-border ZIP code (4%). Outbreaks were more likely to include than not include a person with a binational case in a facility that provided food (e.g. restaurant, grocery store) (33% vs. 15%) than outbreaks in congregate living facilities (25% vs. 44%) or other (e.g. clinic, office) settings (42% vs. 41%) (Table 2).

Table 2.

Location type and ZIP code of San Diego County COVID-19 outbreaks* involving at least one person with a binational case**, February–June 2020.

	Binational Case		Total	p-value
	Binational	Non-Binational
Outbreak Location Type
Congregate living facility	13 (25%)	80 (44%)	93 (40%)	0.005
Facility that provides food	17 (33%)	27 (15%)	44 (19%)
Other	22 (42%)	74 (41%)	96 (41%)
Total	52	181	233
Outbreak Location ZIP Code
Border ZIP code	5 (10%)	7 (4%)	12 (5%)	0.19
Not in a border ZIP code	47 (90%)	174 (96%)	221 (95%)
Total	52	181	233

* Outbreaks as defined by the California Department of Public Health at the time of outbreak notification (Duration, 2020), including at least one linked case whose symptom onset, or if asymptomatic test date, occurred February–June 2020.

** Persons in San Diego County with COVID-19 cases and a binational case were those travelling to Mexico during their exposure or infectious period, had a residence in Mexico, or a case contact from Mexico.

In San Diego County, while remaining at lower incidence, persons with a binational case increased approximately 2 weeks after COVID-19 cases increased in Tijuana, from 0.10 to 0.38 cases per 100,000 (a 280% increase) over three weeks, with cases in Tijuana increasing from 0.09 to 1.47 cases per 100,000 (a 1533% increase) over three weeks (Fig. 2). Binational cases in San Diego increased approximately three weeks after persons without a binational case began to rise, from 0.22 to 1.94 cases per 100,000 (a 781% increase). In June, persons with a binational case and Tijuana cases leveled off, while cases in persons without a binational case continued to rise.

Fig. 2.

Incidence of COVID-19 cases in two US and Mexican jurisdictions, February/March–June 2020. Note: In Mexico during this time period, primarily persons hospitalized with acute severe respiratory infectious disease were being tested for COVID-19.

In Imperial County, persons with cases both with and without a binational case began to rise simultaneously; the peak in the number of persons with a binational case was closer in time to Mexicali case peak than cases in persons without a binational case (Fig. 2). Overall, the incidence curve of Imperial County more closely resembles that of Mexicali than the incidence curve of San Diego County resembles that of Tijuana.

4. Discussion

Less than 10% of persons with COVID-19 and a primary residence in San Diego County or Imperial County had a binational case, however approximately one-fifth of persons with COVID-19 who are Hispanic and residing in a border ZIP code had a binational case. The proportion of cases that were binational was relatively low compared to the total number of cases, and relatively constant during the study period; the binational cases had different trends than the non-binational cases. The similarities between overall case incidence of Imperial County and Mexicali suggests a closer relationship in disease transmission dynamics than between San Diego County and Tijuana.

The binational variable is complex and multi-faceted, and there is limited national guidance on how to best capture the data within the context of an emerging global pandemic. Data on persons with a binational case are particularly important for disease surveillance and to inform public health recommendations related to travel and travel restrictions along the border. Standardization in operationalizing the binational case variable would produce more comparable data across jurisdictions and over time. Persons with COVID-19 and a binational case highlight the importance of local, state, federal, and international collaboration in addressing SARS-CoV-2 spread, as encouraged in the World Health Organization's International Health Regulations, which include efforts to minimize health risks at ground crossings between jurisdictions (Handbook, 2020).

California, in addition to supporting a significant binational economy, fosters unique binational attributes as the only state in which US-based companies can offer Mexico-based insurance to employees, potentially leading to the mobility of ill persons and SARS-CoV-2 transmission on both sides of the border, thereby requiring international collaboration for mitigation strategies (Bustamante et al., 2012). The Technical Guidelines for United States—Mexico Coordination on Public Health Events of Mutual Interest, developed in 2012, provides the framework for ongoing exchange of binationally relevant epidemiologic information, and jointly coordinated responses to public health events that affect both countries (Centers for Disease Control and Prevention, 2022). Enhanced training for more timely and efficient exchange of information between United States and Mexican Health Departments would result in improvement management of binational cases (López-Jaramillo et al., 2020).

Our analysis has several limitations. Free-text notes in San Diego County case investigations are not standardized, and detail on binational cases might not have been systematically requested in either county. Given limited resources and varying capacity, local jurisdictions assess binational cases differently over time and from one another. Ability or prioritization of collecting detail on binational cases may vary over time, as demonstrated by San Diego County public health investigators only collecting exposure data during periods when resources allowed. Only persons identifying primary residence in each county were included, however defining residency in a mobile population is challenging and might have resulted in an underrepresentation of persons with a binational case. Persons with a binational case were likely undercounted given data limitations and persons’ concerns about how residency status and data on cross-border travel might be used. Additional undercounting may have been due to Mexico primarily testing persons hospitalized with acute severe respiratory infectious disease at the point of the pandemic discussed here.

5. Conclusions

Persons with a binational case and COVID-19 represent an important and unique demographic in the US-Mexico border region, requiring collaboration between US and Mexico border jurisdictions, including exchange of information and coordinated public health follow-up. Differences in the operationalization of the binational case variable despite current guidance results in difficulty assessing and characterizing binational cases and cross-border transmission of disease. Standardized and complete collection of the binational variable is needed throughout the border region to monitor transmission patterns and the impact of changes to cross-border ground travel on COVID-19 disease trajectory.

Disclaimers

The findings and conclusions in this report are those of the authors and do not necessarily represent the official position of the Centers for Disease Control and Prevention.

Declaration of Competing Interest

None

Biography

Dr. Chuey is an Epidemic Intelligence Service Officer with the CDC, stationed in San Diego, CA. Her primary research interests include the intersection of infectious diseases and reproductive health, and reproductive decision-making within the context of major life stressors.

§ See e.g., 45 C.F.R. part 46, 21 C.F.R. part 56; 42 U.S.C. §241(d); 5 U.S.C. §552a; 44 U.S.C. §3501 et seq.

Color is not needed for figures in print.