Vaccinated people with prior COVID infection had strongest protection during delta, CDC study finds

Both vaccination and a prior infection provided protection against another infection and hospitalization due to COVID-19 during the United States' delta wave, according to a study released Wednesday by the Centers for Disease Control and Prevention.

Between May and November 2021, researchers analyzed data from New York and California to determine the impact of vaccination and previous COVID-19 infection on cases and hospitalization rates.

The study focused on four core groups of people -- those who were unvaccinated, without a previous infection; those who were unvaccinated, with a previous infection; vaccinated people, without a previous infection; and vaccinated people, with a previous infection.

The analysis found that before delta became the predominant variant in June, vaccination offered better protection against breakthrough infections than prior infection offered against reinfection. But after delta became dominant, this trend shifted, with prior infection offering slightly better protection. However, this also coincides with a time when many Americans were several months out from their shots, and before boosters were authorized.

Notably, the study was conducted prior to the emergence of the omicron variant, and before the widespread availability of booster shots, thus, researchers warned that results cannot be directly applied to the nation's current surge. In addition, the analysis did not include data pertaining to the severity of initial infections, and hospitalization data was only pulled from California.

During the delta wave, two doses of a vaccine offered excellent protection against hospitalization, and notably, researchers stressed that getting vaccinated remains a safer option than contracting COVID-19.

Vaccine immunity does fade over time, the study found, and the further out an individual is from one's last vaccine shot, or a prior infection, the more likely it is that they will experience a breakthrough infection.

When asked repeatedly on Wednesday during a press briefing whether the data were showing that when delta was prominent, having had an infection provided greater protection against a subsequent infection than from being vaccinated, a CDC representative insisted that vaccination is still the safest way to protect oneself.

Scientists also suggest the study reinforces the evidence that "vaccination remains the safest strategy for protecting against COVID-19."

The CDC also cited a recent study, which demonstrates that as time increases after an infection, vaccination still provides greater protection against COVID-19 compared to prior infection alone, thus underscoring "the importance of being up to date on COVID-19 vaccination."

Later this week, the CDC said it will publish additional data on COVID-19 vaccines and boosters while omicron has been circulating.