Top 5 Takeaways

1. Highest Risk Group: Unvaccinated individuals without a previous COVID-19 diagnosis had the highest rates of COVID-19 cases and hospitalizations.
2. Impact of Delta Variant: After the Delta variant became predominant, infection-derived protection was higher, and vaccine-induced immunity waned.
3. Vaccination vs. Previous Infection: Initially, vaccinated individuals without previous infection had lower case rates, but after Delta’s emergence, those with previous infections had lower rates than vaccinated individuals alone.
4. Hospitalization Rates: Hospitalization rates were consistently highest among unvaccinated individuals without a previous COVID-19 diagnosis, even after Delta variant predominance.
5. Public Health Recommendation: Vaccination remains the safest strategy to prevent SARS-CoV-2 infections, hospitalizations, and deaths, with recommendations for primary, additional, and booster doses for all eligible individuals.

Original Article Author and Citation

Corresponding Author

Tomás M. León, tomas.leon@cdph.ca.gov

Suggested Citation

León TM, Dorabawila V, Nelson L, et al. COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status and Previous COVID-19 Diagnosis — California and New York, May–November 2021. MMWR Morb Mortal Wkly Rep 2022;71:125–131. DOI: http://dx.doi.org/10.15585/mmwr.mm7104e1

Summary

This report examines the impact of primary COVID-19 vaccination and previous SARS-CoV-2 infection on COVID-19 incidence and hospitalization rates in California and New York from May to November 2021. The study analyzed four cohorts based on vaccination status and previous COVID-19 diagnosis. The findings indicate that unvaccinated individuals without a previous COVID-19 diagnosis had the highest rates of COVID-19 cases and hospitalizations. After the Delta variant became predominant, infection-derived protection increased, while vaccine-induced immunity waned. Vaccination remains the safest strategy to prevent SARS-CoV-2 infections and associated complications.

Methods

The study used statewide testing, surveillance, and COVID-19 immunization data from California and New York. Four cohorts of adults aged ≥18 years were analyzed: unvaccinated without previous COVID-19 diagnosis, vaccinated without previous COVID-19 diagnosis, unvaccinated with previous COVID-19 diagnosis, and vaccinated with previous COVID-19 diagnosis. Age-adjusted hazard rates of incident COVID-19 cases and hospitalizations were compared among these cohorts using a lifetable method. Data were stratified by timing of previous diagnoses and primary series vaccine product.

Discussion

The analysis revealed that vaccination and previous infection both provide protection against COVID-19 and related hospitalizations. However, the protection dynamics changed with the emergence of the Delta variant, which saw increased infection-derived protection and waning vaccine-induced immunity. The study underscores the importance of vaccination, including booster doses, to maintain robust protection against COVID-19, especially as new variants emerge.

Conclusion

Vaccination is the primary strategy to prevent SARS-CoV-2 infections, hospitalizations, and deaths. The study highlights the need for ongoing public health measures, including vaccination and booster doses, to adapt to changing virus and immunity levels. Future recommendations for vaccine doses may be necessary as new variants continue to emerge.

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