Prior COVID-19 Vaccination and Infection Most Effective Against Delta Variant

Mutations have changed the epidemiology of COVID-19, and today the US Centers for Disease Control and Prevention (CDC) reported that during the time that Delta was the predominant variant, persons who were fully vaccinated and had recovered from prior infection had the lowest COVID-19 incidence rates.

In their Morbidity and Mortality Weekly Report(MMWR), the CDC announced that COVID-19 infections and hospitalizations are highest among persons who had not been vaccinated nor previously infected.

The study included laboratory testing, hospitalization surveillance, and immunization registry data in California and New York from May-November 2021. Notably, the study was conducted before Omicron became the dominant variant, and before COVID-19 booster doses were widely recommended to prevent waning immunity.

Initially, infection rates were lowest among vaccinated persons without a prior COVID-19 diagnosis. After the emergence of the Delta variant, immunity decreased and cases increased significantly in this group; cases increased only slightly among vaccinated and unvaccinated individuals who had contracted and recovered from COVID-19.

Across the entire study period, people who were previously vaccinated and infected had much lower case rates than unvaccinated persons. According to the report, “These results suggest that vaccination protects against COVID-19 and related hospitalization and that surviving a previous infection protects against a reinfection. Importantly, infection-derived protection was greater after the highly transmissible Delta variant became predominant, coinciding with early declining of vaccine-induced immunity in many persons.”

The CDC emphasized that vaccination is still the safest strategy for reducing the risk of COVID-19 infection, hospitalization, and death. They noted, “Additional future recommendations for vaccine doses might be warranted as the virus and immunity levels change.”