According to a study revealed on Tuesday, members of the US military who were inoculated against COVID-19 showed higher-than-expected rates of heart inflammation, although the condition was still extremely rare,.

The study found that 23 previously healthy males with an average age of 25 complained of chest pain within four days of receiving a COVID-19 shot. The incident rate was higher than some previous estimates would have anticipated, it said.

All the patients, who at the time of the study’s publication had recovered or were recovering from myocarditis – an inflammation of the heart muscle – had received doses made by either Pfizer Inc and BioNTech SE or Moderna Inc.

US health regulators added last week a caution to the literature that accompanies those mRNA vaccines to flag the rare risk of heart inflammation seen primarily in young males. But they said the benefit of the shots in preventing COVID-19 clearly continues to outweigh the risk.

The study, which was published in the JAMA Cardiology medical journal, said 19 of the patients were current military members who had received their second vaccine dose. The others had either received one dose or were retired from the military.

General population estimates would have predicted eight or fewer cases of myocarditis from the 436,000 male military members who received two COVID-19 shots, the study said.

An outside panel of experts advising the US Centers for Disease Control and Prevention (CDC) said last week that reports of myocarditis were higher in males and in the week after the second vaccine dose than would be anticipated in the general population. A presentation at that meeting found the heart condition turned up at a rate of about 12.6 cases per million people vaccinated.