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Overall, the vaccines reduced infections among vaccinated workers by 80 percent from Dec. 14, when the U.S. vaccination campaign began, to Aug. 14, compared with unvaccinated workers. (The results were adjusted for factors including occupation, demographic characteristics, frequency of close social contact and mask use.)
But while the shots reduced infections by 91 percent before the emergence of the Delta variant, their protectiveness dropped to 66 percent as the variant became dominant in each region.
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Another C.D.C. study released on Tuesday analyzed infections and hospitalizations in Los Angeles County from May 1 to July 25 of this year. While vaccinated people did become infected, the researchers concluded that among the unvaccinated, infection rates were 4.9 times as high, and the hospitalization rate was 29 times as high.
Of 43,127 known infections in Los Angeles County among residents 16 and older, 25 percent were in fully vaccinated people, 3.3 percent in partly vaccinated people and 71.4 percent in unvaccinated people. (The proportion of fully vaccinated Los Angeles County residents increased to 51 percent on July 25, from 27 percent on May 1.)
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But boosters seem to provide enough extra protection. Dr. Fauci described experiments at the National Institutes of Health that mixed blood serum from people who had had two doses of Moderna and serum from those who had also had a third dose with viruses engineered to carry Omicron’s surface proteins.
These “pseudoviruses” evaded many antibodies from people who had received two doses, but the level of antibodies conferred by the boosters blocked viruses from invading cells.