Three studies that drew data from different U.S. regions evaluated the protective power of the vaccines. One looked at more than 600,000 virus cases in 13 states, representing about one quarter of the U.S. population, between April and July, and concluded that individuals who were not fully vaccinated were far more susceptible to infection and death from the virus.

They were 4.5 times more likely than vaccinated individuals to become infected, 10 times more likely to be hospitalized, and 11 times more likely to die from the coronavirus, the study found.

Vaccine protection against hospitalization and death remained strong even when the Delta variant was the dominant form of infection. But the vaccines’ effectiveness in preventing infection dropped from 91 percent to 78 percent, the study found.