Americans who received their primary series of vaccines and previously contracted Covid-19 had the highest protection against reinfection and hospitalization during the Delta variant-fueled outbreak, according to a new study from the Centers for Disease Control and Prevention.
The study, published Wednesday, looked at four categories of people in New York and California — individuals who were unvaccinated with and without a prior infection and vaccinated people with and without a prior infection.
The study said protection against reinfection and hospitalization grew significantly among unvaccinated and vaccinated individuals who had previously contracted Covid-19 from the time period before the Delta variant emerged to after it became the dominant strain in the U.S. Prior infection provided better protection than vaccination alone during Delta. The case rates among the unvaccinated and vaccinated individuals with a previous diagnosis and without a previous diagnosis were relatively similar in the California population during the same time period.
The study did not explain why protection against reinfection and hospitalization grew among those individuals with a prior infection during Delta. During a call with reporters Wednesday, Benjamin Silk, a CDC scientist and one of the authors, did not explain the CDC’s thinking on to what degree a previous Covid-19 infection can protect against reinfection and hospitalization versus vaccination, but stressed that the agency continues to recommend all Americans receive their primary series and booster shots as soon as possible.
“A Covid vaccination helps protect by creating an antibody response without the person having to experience severe illness and death,” Silk said. “Vaccination provides safer protection.”
Eli Rosenberg, deputy director for science at the New York State Department of Health who helped with the study, said "the totality of the evidence suggests ... that both vaccination and having survived Covid each provide protection against subsequent infection and hospitalization." "Either of those provides protection, and only one of those is the safe choice that we would recommend. And that's vaccination," he said.
In a press statement, the agency said the results of the Delta study could not be compared to the Omicron strain but that it would release other studies focused on the new variant in the coming days.
“[The study] offers a framework for analyzing surveillance data that we know will help evaluate infection in vaccinated persons and reinfection as new variants like Omicron continue to emerge,” Silk said.
