CDC Statement on MMWR: COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis — California and New York, May–November 2021

Today’s MMWR study finds that during the Delta wave, both COVID-19 vaccination and surviving a prior infection provided protection against infection and hospitalization from COVID-19. Scientists reviewed data from New York and California to determine the level of protection offered by COVID-19 vaccines, previous infection, and both. Between May and November 2021, people who were unvaccinated and did not have a prior COVID-19 infection remained at the highest risk of infection and hospitalization, while those who were previously infected, both with or without prior vaccination, had the greatest protection.

Viruses are constantly changing, including the virus that causes COVID-19. These changes occur over time and can lead to the emergence of new variants that have new characteristics, including ones that impact the level of immunity vaccination and/or prior infection can provide. The level of protection offered by vaccination and surviving a previous infection changed during the study period. Vaccination remains the safest strategy for protecting against COVID-19.

There are important caveats to the data presented in this study:

• The analysis was conducted before the emergence of the Omicron variant and the findings cannot be applied to the current Omicron wave.
• The study ended prior to the widespread implementation of booster doses and does not reflect the immunologic benefit of additional vaccine doses.
• The analysis did not include information on the severity of initial infection and does not reflect the risk of morbidity and mortality from COVID-19 infection.

Additionally, a recent study shows with increasing time since prior infection, vaccination provides greater protection against COVID-19 compared to prior infection alone, emphasizing the importance of being up to date on COVID-19 vaccination. Later this week, CDC will publish additional data on COVID-19 vaccines and boosters while Omicron has been circulating.

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U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

CDC works 24/7 protecting America’s health, safety and security. Whether disease start at home or abroad, are curable or preventable, chronic or acute, or from human activity or deliberate attack, CDC responds to America’s most pressing health threats. CDC is headquartered in Atlanta and has experts located throughout the United States and the world.