Risk of COVID-19 Reinfection and Vaccine Breakthrough Infection, Madera County, California, 2021

The probability of either testing COVID-19 positive or dying for three cohorts in Madera County, California in 2021 was compared. These cohorts included 1. those unvaccinated, 2. those vaccinated and 3. persons with a previous COVID-19 infection. The three groups were made generally comparable by matching on age, gender, postal zip code of residence, and the date of either COVID-19 infection or of vaccination. The hazard ratio (HR) for death (from all causes) after COVID-19 infection vs. vaccination was 11.7 (95% CI 5.91-23.1, p14 days after initial COVID-19 infection or after completing primary COVID-19 vaccination was 1.98 (95% CI 1.53-2.58 p<0.001). As the majority of positive COVID-19 tests in the post COVID-19 cohort occurred within 90 days of the initial infection, and as these early positives may not represent a new infection, we also compared rates of testing COVID-19 positive ≥ 90 days after initial infection or vaccination. After removing these early positive COVID-19 tests that occurred between days 14-90, the HR ratio for testing COVID-19 positive is now lower for the post COVID-19 cohort compared with the vaccinated cohort. The risk for having a positive COVID-19 test occurring 90 days after an initial COVID-19 infection or after vaccination was 0.54 (95% CI 0.33-0.87, p<0.05) for the post COVID-19 group vs Vaccinated group. Thus the risk for testing COVID-19 positive was higher in the first 90 days after COVID-19 infection compared to those vaccinated. However, from 90 to 300 days after COVID-19 infection, the post COVID-19 infection cohort had a lower risk of testing COVID-19 positive than those fully vaccinated.