The study, conducted between December 2021 and February 2022, looked at 11,123 hospital admissions and emergency room visits that did not result in hospitalization. Both the Delta and Omicron variants were circulating at that time.
The study, funded by Pfizer, shows that for the Omicron variant, efficacy against hospitalizations was 85% for those who received a third dose within three months, but fell to 55% for people who have received it before.
A similar drop in efficacy is also seen against the Delta variant, although a third dose appears increasingly effective.
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