New studies on vaccine efficacy against Delta: vaccines' efficacy against infection weakens

The COVID-19 vaccines available in the United States are still highly effective at preventing hospitalization but their effectiveness against new infections has decreased as the Delta variant spread, according to new studies published on Wednesday in Morbidity and Mortality Weekly Report. None of the studies could tell whether the breakthrough infections were due to waning immunity, reduced protection against the Delta variant, or a combination of factors.
Still, the U.S. Centers for Disease Control and Prevention (CDC) announced on Wednesday that COVID-19 booster shots would be made available to all Americans beginning in September. The new studies found:
- Vaccine effectiveness against any SARS-CoV-2 infection - mild or severe - dropped to 53.1% in late June to early August, from 74.7% before Delta became predominant, according to a study of U.S. long-term care facilities;
- Vaccine efficacy for preventing new infections dropped from 91.7% to 79.8% between early May and late July, though efficacy at preventing hospitalization remained above 90%, according to a study by New York State health officials.
- Protection from hospitalization lasts at least six months, according to a study in 18 U.S. states. It found that 24 weeks after full vaccination with an mRNA vaccine, efficacy was 84%, and it was 90% among adults without immunocompromising conditions.
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