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A quarter of young adults in the United States ages 18 to 25, a group most likely to transmit the coronavirus, say they “probably” or “definitely” won’t get vaccinated against COVID-19, according to a study released Wednesday.

The UC San Francisco study says this reluctance among young adults to get vaccinated threatens the health of older unvaccinated adults and may contribute to the rise of viral variants. The hesitancy also will make it more difficult for the U.S. to achieve herd immunity, which requires about 85% of the population to be vaccinated, the study authors say.

The study analyzed March data from the Household Pulse Survey, an online nationally representative population sample conducted by the the U.S. Census Bureau in collaboration with the National Center for Health Statistics and other agencies.

Eighty-three percent of 5,082 young adults said they had not been vaccinated. Ten percent said they definitely would not get a vaccine and 14% said they probably would not.

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