The study did not involve cases from the recent omicron rise.
Both vaccination and a previous infection provided protection against another infection and hospitalization due to COVID-19 during the U.S. 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 effect 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 previous infection; and vaccinated individuals with a previous infection.
The analysis showed that before delta became the dominant variant in June, vaccination provided better protection against breakthrough infections than previous infection against re-infection. However, after delta became dominant, this trend changed, with previous infection providing slightly better protection. However, this also coincides with a time when many Americans were several months out of their shots and before boosters were approved.
In particular, the study was conducted before the advent of the omicron variant and before the widespread availability of booster shots, thus warning researchers that the results could not be directly applied to the country's current rise. In addition, the analysis did not include data on the severity of the initial infections, and hospitalization data were obtained only from California.
During the delta wave, two doses of a vaccine provided excellent protection against hospitalization, and in particular, researchers stressed that being vaccinated is still a safer option than being infected with COVID-19.
Vaccine immunity fades over time, the study showed, and the further out a person is from one's last vaccine shot or a previous infection, the more likely they are to experience a breakthrough infection.
When repeatedly asked during a press briefing on Wednesday whether the data showed that when delta was prominent, having an infection provided greater protection against a subsequent infection than against being vaccinated, a CDC representative insisted that vaccination was still is the safest way to protect yourself. .
Researchers also suggest that the study reinforces the evidence that "vaccination remains the safest strategy to protect against COVID-19."
The CDC also cited a recent study showing that as time increases after an infection, vaccination provides ever greater protection against COVID-19 compared to previous infection alone, emphasizing "the importance of keeping up to date with COVID-19 vaccination. "
Later in the week, the CDC said it will release additional data on COVID-19 vaccines and boosters while omicron has been circulating.