close up hand of doctor put label covid-19 vaccine sticker on vaccination certificate card and passport
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Having vaccine- and infection-derived immunity is tied to much lower rates of COVID-19 when compared with being unvaccinated, according to a new public health study that spanned the rise of the SARS-CoV-2 delta variant.

Image of Eli S. Rosenberg, Ph.D.; Image credit: University at Albany
Eli S. Rosenberg, Ph.D.; Image credit: University at Albany

Investigators tracked case and hospitalization rates in California and New York during May through November 2021. Before the delta variant became dominant, case rates were lowest among vaccinated persons who did not have a prior COVID-19 diagnosis. 

Once the delta variant became widespread, however, case rates shot up in those who were vaccinated and had no prior infection, but rose only slightly among those with a previously diagnosed infection — whether or not they were vaccinated. The rise of delta coincided with waning vaccine efficacy in many study subjects, the authors noted.

The results suggest not only that vaccination protects against acquiring COVID-19 and developing severe illness, but that surviving a prior infection prevents reinfection, wrote Eli S. Rosenberg, Ph.D., of the New York State Department of Health, and University at Albany School of Public Health, State University of New York.

The public health takeaway is that vaccination and booster doses are still recommended for all eligible individuals, he and his colleagues at the California Department of Public Health concluded. 

“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,” they wrote. “Additional future recommendations for vaccine doses might be warranted as the virus and immunity levels change,” the authors concluded.

Full findings were published in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report.