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The US COVID-19 surveillance environment: An ecological analysis of the relationship of testing adequacy in the context of vaccination

Abstract Background COVID-19 testing is a critical component of public health surveillance and pandemic control, especially among the unvaccinated, as the nation resumes in-person activities. This study examined the relationships between COVID-19 testing rates, testing positivity rates and vaccination coverage across US counties. Methods Data from the Health and Human Services’ Community Profile Report and 2016-2020 American Community Survey 5-Year Estimates were used. 3,114 US counties were analyzed from January through September 2021. Associations among the testing metrics and vaccination coverage were estimated using multiple linear regression models with fixed effects for states and adjusted for county demographics. COVID-19 testing rates (PCR testing per 1,000), testing positivity (percentage of all PCR tests that were positive), and vaccination coverage (percentage county population that was fully vaccinated). Results Nationally, median daily COVID-19 testing rates were highest in January and September (35.5 and 34.6 tests per capita, respectively) and lowest in July (13.2 tests per capita). Monthly testing positivity was between 0.03 and 0.12 percentage points (pp) lower for each pp of vaccination coverage, and monthly testing rates were between 0.08 and 0.22 tests per capita higher for each pp of vaccination coverage. Conclusions The quantity of COVID-19 testing was associated with vaccination coverage, implying counties having populations with relatively lower protection against the virus are conducting less testing than counties with relatively more protection. Monitoring testing practices in relation to vaccination coverage may be used to monitor the sufficiency of COVID-19 testing based on population susceptibility to the virus.


Introduction
Timely and efficient testing and contact tracing are cornerstones of infectious disease 2 surveillance and outbreak response [1]. In the first year of the COVID-19 pandemic, large-scale 3 testing was the primary strategy to initiate quarantine and isolation protocols to disrupt SARS- We obtained daily COVID-19 testing and vaccination data for states and counties from the 20 Health and Human Services Community Profile Report (CPR) from January 1 st through 21 September 30 th 2021 [9]. The CPR compiles these data from the COVID-19 Electronic Lab

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Reporting system. Daily testing and vaccination data were used to derive two metrics of COVID- 19 testing and one metric of vaccine coverage at the nation-, state-, and county-levels. Of all 1 3,220 US counties included in the data, 107 counties were excluded from analyses due to 2 missing information, yielding a final sample of 3,113 counties. Testing rates reflect the quantity 3 of testing activities conducted by public health authorities, and maintaining low test positivity 4 has been recommended as a measure of testing adequacy. The testing rate, or tests per 1,000 5 persons, was calculated as the number of tests per month divided by population size multiplied  Statistical analysis 17 We derived the monthly national distribution of testing rates and positivity, and vaccination. We 18 estimated the state-and county-level correlations between testing rates, test positivity, and 19 vaccination coverage by month. To examine unadjusted and adjusted associations between three 20 measurements, we transformed daily CPR data into weekly data in which mean testing rates, test testing rates, test positivity and vaccination coverage, and testing rates and vaccination coverage. 1 We fitted unadjusted and adjusted linear mixed effects models. In unadjusted model, we only 2 include fixed effects for state and we additionally controlled for other covariates including 3 county-level total population, median household income, percentage of population aged 65 and 4 over and percentage of black residents for adjusted models. All analyses were conducted using R 5 statistical software, v.3.6.2.

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At the national level, median daily COVID-19 testing rates were highest in January and 9 September (35.5 and 34.6 tests per 1,000, respectively) and lowest in July and June (13.2 and 10 14.3 tests per 1,000, respectively) ( Table 1). Month-to-month trends in COVID-19 test positivity 11 by month mirrored testing rates, such that test positivity peaked in January and August (11.5% 12 and 10.2%, respectively). National-level cumulative vaccination coverage was 1.5% in January 13 and reached 54.2% in September. 14 15 Figure 1 shows correlation between state-and county-level testing rates and test positivity. We  (Table 2). In both results, the relationship between testing rates and 21 test positivity was stronger in January, August, and September in which test positivity and test 22 per capita were highest.
County-level vaccination coverage was inversely associated with testing positivity (Table 2; 2 Figure 2) and positively associated with testing rates (Table 2; Figure 3). Adjusted regression 3 model shows that monthly test positivity was between 0.02 (July) and 0.12 (September) pp lower 4 for each pp of the county population that was fully vaccinated. In contrast, the associations 5 between county vaccination and testing rate were positive in all months and was highest in 6 March (0.20 tests per 1,000; 95%CI: 0.14, 0.25) in which vaccination coverage in the county was 7 lower than 40%.   [13,14], that prevent hospitalization if offered early in the disease course. 1 Therefore, from a public health perspective, areas with lower vaccine uptake should more 2 aggressively engage in testing for COVID-19 to prevent outbreaks and thus ensure safety in 3 congregate settings such as schools and workplaces. In reality, the positive relationship between 4 vaccination coverage and testing rates indicates that counties with less vaccinated populations 5 were conducting less testing than counties with relatively more protection. Furthermore, counties 6 with lower vaccination coverage had relatively higher test positivity. Together, these findings 7 indicate potential insufficient testing among counties with larger shares of unvaccinated 8 populations, which are more susceptible to the virus and severe outcomes.  The consistency of associations across differing variants suggests that associations between 19 vaccination and testing were not dependent on the transmissibility of the variant as much as 20 testing policies of states and testing behaviors of individuals. 21 We also found that the state-level association was stronger than county-level associations in all 22 months. The stronger state-level associations may be due to more random variation in the   This study has some limitations. Testing data were not available by age, and we therefore opted 9 to estimate all indicators among the entire county population, irrespective of age. While age-10 eligibility for vaccination changed over the study period, the denominator for vaccination 11 coverage was not restricted by age. Nevertheless, analyses by study month allow for This study provides important information as states and counties continue to consider adequacy 2 of current policies to mitigate the harms of COVID-19, especially as in-person activities resume.

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Maintaining adequate testing infrastructure and public awareness of the importance of testing is 4 critical for appropriate public health response. Monitoring test positivity and testing rates in 5 relation to vaccination coverage is an important strategy to guide adequate testing activities, 6 coverage, and interpretation among communities that remain highly susceptible to the virus and 7 as new and highly transmissible variants of the virus emerge (e.g., Omicron [15]).   Table 1. National-level median test positivity rates, total number of tests per 1,000 persons and 2 percentage of population fully vaccinated by month       Table 2.