Roughly one in four people in the 18-to-25 age group said they would “probably” or “definitely” not get the vaccine, according to data collected in March of this year by the U.S. Census Bureau, in collaboration with other government agencies. More than 12% of the U.S. population falls into that age group.

For herd immunity to be reached, the study’s authors say about 80% of the population must be vaccinated. According to the Mayo Clinic’s vaccine tracker, 48.3% of the U.S. population is currently fully vaccinated.

The study, which is out Wednesday in the Journal of Adolescent Health, found that among 5,082 respondents, 83% had not been vaccinated at the time. Of those, 10% said they wouldn’t get the vaccine, and 14% said they probably wouldn’t.

Among the group, the study found that more than half were concerned about side effects, and more than half said they may wait and get it later. A third said they did not trust the vaccines.

A study released in early June from the U.K.’s Office of National Statistics shows similarly worrisome results. It found that 14% of teenagers 16 and 17 years old were hesitant to get the vaccine, 10 points higher than the rate of hesitancy for adults.

“Long COVID” worries

Vaccine hesitancy among young people has huge implications as the Delta variant spreads. Even in countries with already high vaccination rates, like the U.S. and U.K., that spread has been driven by the unvaccinated young, some of whom have become eligible for the vaccine only in recent weeks.

Younger people broadly tend to get less sick from COVID-19 even if they are unvaccinated, but scientists and health officials have increasingly warned that young people both spread the virus and are still vulnerable to its impacts.

“Young adults who have had COVID, regardless of symptoms, may be vulnerable to long-term complications and debilitating symptoms that may include respiratory difficulties, loss of smell, and brain fog, often referred to as ‘long COVID,’” said Sally Adams, the lead author of the paper and the data coordinator and analyst at the UCSF National Adolescent and Young Adult Health Information Center.

The U.S. paper pointed toward concerns about rare side effects from the vaccines as contributing to the hesitancy, including cases of heart inflammation in young people after taking the Pfizer and Moderna vaccines. Researchers pointed out that the rates of the inflammation are only slightly higher for the vaccinated than for the unvaccinated.