Both vaccination and a previous infection offered strong protection against COVID-19 during the delta wave last year, according to a CDC study published Jan. 19.
Researchers analyzed the risk of contracting COVID-19 or being hospitalized with the virus among unvaccinated and vaccinated people with or without a prior infection. They used data on 1.1 million cases confirmed in California and New York between May and November 2021.
Three things to know:
1. Before delta became the nation's dominant strain in June, unvaccinated people without a previous infection had the highest case rates. Case rates were lowest in vaccinated people without a prior infection. This trend shifted by October once delta took hold. People who had a previous infection had lower case rates than people who were vaccinated but did not have a past infection.
"Infection-derived protection was higher after the delta variant became predominant, a time when vaccine-induced immunity for many persons declined because of immune evasion and immunologic waning," the CDC said.
2. As previous studies have confirmed, hospitalization rates were highest among people who weren't vaccinated or had not previously gotten COVID-19.
3. The results may not apply to the current situation, as the study did not include data for the omicron variant and came before boosters were widely available, according to the agency.
"Although the epidemiology of COVID-19 might change as new variants emerge, vaccination remains the safest strategy for averting future SARS-CoV-2 infections, hospitalizations, long-term sequelae and death," the CDC said.