(HealthDay News) — Vaccination and previous COVID-19 infection were protective against COVID-19 infection and hospitalization during the alpha and delta severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) waves, according to research published in the Jan. 19 early-release issue of the U.S. Centers for Disease Control and Prevention Morbidity and Mortality Weekly Report.
Tomás M. León, Ph.D., from the California Department of Public Health, and colleagues examined the impact of primary COVID-19 vaccination and previous SARS-CoV-2 infection on COVID-19 incidence and hospitalization rates by analyzing data from California and New York. Four cohorts of adults were considered: unvaccinated with no previous COVID-19 diagnosis; vaccinated with no previous COVID-19 diagnosis; unvaccinated with a previous COVID-19 diagnosis; and vaccinated with a previous COVID-19 diagnosis.
The researchers found that in both states, COVID-19 incidence was highest among unvaccinated individuals without a previous COVID-19 diagnosis compared with the other three groups. During the week beginning May 30, 2021, in California and New York, COVID-19 case rates were 19.9-fold and 18.4-fold lower, respectively, among vaccinated persons without a previous diagnosis; 7.2-fold and 9.9-fold lower, respectively, among unvaccinated persons with a previous COVID-19 diagnosis; and 9.6-fold and 8.5-fold lower, respectively, among vaccinated persons with a previous COVID-19 diagnosis compared with unvaccinated persons without a previous COVID-19 diagnosis. Hospitalization rates in California followed a similar pattern. In late June and July, these relationships changed after the SARS-CoV-2 delta variant became predominant. By the week beginning Oct. 3, 2021, the corresponding case rates were 6.2-fold and 4.5-fold lower among vaccinated persons without previous COVID-19 diagnosis, 29.0-fold and 14.7-fold lower among unvaccinated persons with previous diagnosis, and 32.5-fold and 19.8-fold lower among vaccinated persons with a previous diagnosis compared with unvaccinated persons without a previous COVID-19 diagnosis.
“Vaccination protected against COVID-19 and related hospitalization, and surviving a previous infection protected against a reinfection and related hospitalization during periods of predominantly alpha and delta variant transmission, before the emergence of omicron,” the authors write.