Understanding the Patterns of Serological Testing for COVID-19 Pre- and Post-Vaccination Rollout in Michigan

Abstract

Testing for SARS-CoV-2 antibodies is commonly used to determine prior COVID-19 infections and to gauge levels of infection- or vaccine-induced immunity. Michigan Medicine, a primary regional health center, provided an ideal setting to understand serologic testing patterns over time. Between 27 April 2020 and 3 May 2021, characteristics for 10,416 individuals presenting for SARS-CoV-2 antibody tests (10,932 tests in total) were collected. Relative to the COVID-19 vaccine roll-out date, 14 December 2020, the data were split into a pre- (8026 individuals) and post-vaccine launch (2587 individuals) period and contrasted with untested individuals to identify factors associated with tested individuals and seropositivity. Exploratory analysis of vaccine-mediated seropositivity was performed in 347 fully vaccinated individuals. Predictors of tested individuals included age, sex, smoking, neighborhood variables, and pre-existing conditions. Seropositivity in the pre-vaccine launch period was 9.2% and increased to 46.7% in the post-vaccine launch period. In the pre-vaccine launch period, seropositivity was significantly associated with age (10 year; OR = 0.80 (0.73, 0.89)), ever-smoker status (0.49 (0.35, 0.67)), respiratory disease (4.38 (3.13, 6.12)), circulatory disease (2.09 (1.48, 2.96)), liver disease (2.06 (1.11, 3.84)), non-Hispanic Black race/ethnicity (2.18 (1.33, 3.58)), and population density (1.10 (1.03, 1.18)). Except for the latter two, these associations remained statistically significant in the post-vaccine launch period. The positivity rate of fully vaccinated individual was 296/347(85.3% (81.0%, 88.8%)).

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“…Within this framework, the observation that individuals who received positive test results were more prone to indicate greater severity of symptoms compared to those who yielded negative outcomes [20] further accentuates the significance of comprehensive testing approaches. Additionally, the relationship between test positivity and the vaccination status of subjects should also be taken into account due to potential immunological interferences that may occur [21,22].…”

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“…Our study has several limitations. Our rate of COVID positivity (31.9%) is higher than rates reported in the literature, such as the seropositivity rate of 8.8% (1818/20 614) among health care workers in the BLAST COVID study [16] and the 9.2% seropositive rate among prevaccinated patients in Michigan Medicine, a primary regional health system, in a study by Zhao et al [29] , suggesting that our population may be self-selective. In other words, those who were diagnosed with COVID, had COVID-like symptoms, or were seropositive were more likely to participate.…”