COVID-19 Cases and Hospitalizations by COVID-19 Vaccination Status andPrevious COVID-19 Diagnosis — California and New York, May–November 2021

During the study period, COVID-19 incidence in
both states was highest among unvaccinated persons without a
previous COVID-19 diagnosis compared with that among the
other three groups. During the week beginning May 30, 2021,
compared with COVID-19 case rates among unvaccinated

persons without a previous COVID-19 diagnosis, COVID-19
case rates were 19.9-fold (California) and 18.4-fold (New York)
lower among vaccinated persons without a previous diagnosis;
7.2-fold (California) and 9.9-fold lower (New York) among
unvaccinated persons with a previous COVID-19 diagnosis;
and 9.6-fold (California) and 8.5-fold lower (New York) among
vaccinated persons with a previous COVID-19 diagnosis.
During the same period, compared with hospitalization rates
among unvaccinated persons without a previous COVID-19
diagnosis, hospitalization rates in California followed a similar
pattern. These relationships changed after the SARS-CoV-2
Delta variant became predominant (i.e., accounted for >50% of
sequenced isolates) in late June and July. By the week beginning
October 3, compared with COVID-19 cases rates among unvaccinated persons without a previous COVID-19 diagnosis, case
rates among vaccinated persons without a previous COVID-19
diagnosis were 6.2-fold (California) and 4.5-fold (New York)
lower; rates were substantially lower among both groups with
previous COVID-19 diagnoses, including 29.0-fold (California)
and 14.7-fold lower (New York) among unvaccinated persons
with a previous diagnosis, and 32.5-fold (California) and
19.8-fold lower (New York) among vaccinated persons with
a previous diagnosis of COVID-19. During the same period,
compared with hospitalization rates among unvaccinated persons
without a previous COVID-19 diagnosis, hospitalization rates
in California followed a similar pattern. These results demonstrate that vaccination protects against COVID-19 and related
hospitalization, and that surviving a previous infection protects
against a reinfection and related hospitalization.

Importantly,
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.

https://www.cdc.gov/mmwr/volumes/71/wr/mm7104e1.htm