Does Home COVID-19 Testing Bias COVID-19 Vaccine Effectiveness Estimates?

COVID-19 home testing became widely available in early 2021. Care seeking for acute respiratory illness and COVID-19 vaccine effectiveness estimates may be affected by home testing.

In a test-negative case-control study, between January and May 2022, U.S. Flu VE Network outpatients were asked about their home COVID-19 testing before seeking care for acute respiratory illness and were laboratory tested for SARS-CoV-2. Associations among home testing, care seeking, and COVID-19 vaccine effectiveness were assessed using multivariable logistic regression.

Among 2,614 enrollees, home COVID-19 testing was significantly associated with current (≤6 months) COVID-19 vaccination (AOR=1.70; 95% CI=1.19, 2.42), cough (AOR=1.69; 95% CI=1.22, 2.34), and having a college degree or higher (AOR=1.67; 95% CI=1.37, 2.03) and was negatively associated with a prior positive COVID-19 test. COVID-19 illness was associated with cough (AOR=3.07; 95% CI=2.04, 4.61), contact with a COVID-19 case (AOR=2.41; 95% CI=1.93, 3.0), and home testing (AOR=1.87; 95% CI=1.53, 2.29) and was negatively associated with a prior positive COVID-19 test. In unadjusted modeling with only the association between current vaccination and COVID-19 infection, the OR was 0.78 (95% CI=0.66, 0.92). Adjusting for patient race/ethnicity, age, days from onset to enrollment, prior COVID-19 illness, and study site, the AOR was 0.65 (95% CI=0.55, 0.78). Estimated vaccine effectiveness was 35% (95% CI=22%, 45%). Adding home testing to this adjusted model, vaccine effectiveness was 37% (95% CI=13%, 46%). Among patients who home tested, vaccine effectiveness was 31% (95% CI=13%, 46%) versus 43% (95% CI=24%, 57%) among patients who did not.

Patients reporting home testing for COVID-19 before seeking outpatient care for acute respiratory illness differed from patients not using home tests, which could affect estimates of COVID-19 vaccine effectiveness in some populations.