U.S. health officials closely tracking possible side effects of the first authorized COVID-19 vaccine say they have seen six cases of severe allergic reaction out of more than a quarter million shots given.

The Centers for Disease Control and Prevention said more than 272,000 shots of the Pfizer vaccine were given nationwide as of Saturday morning. The half-dozen cases of allergic reaction were reported as of Friday night, and included one person with a history of vaccination reactions.

Health officials are keeping close watch for such side effects.

U.S. vaccine recipients are supposed to hang around after their injections in case signs of an allergy appear. The CDC says all cases occurred within the recommended observation window and were promptly treated.

The numbers were discussed at a meeting of a committee that advises the CDC on vaccines. The group on Saturday endorsed Moderna’s COVID-19 vaccine, which was granted emergency authorization on Friday.

Less severe side effects have also been rare. Among the first 215,000 people to get vaccinated in the U.S., fewer than 1.5% of them had problems that left them unable to perform their normal activities or required medical care.

Many vaccines can cause temporary discomfort, such as a sore arm or certain flu-like symptoms. COVID-19 vaccines tend to cause more of those reactions than a flu shot, and some hospitals are staggering the times their employees get vaccinated to avoid staffing problems.

The United States reached a record of nearly a quarter million coronavirus cases reported in 24 hours. An additional 2,814 people died nationwide, pushing the death toll to more than 313,000. Some areas of California are “just right at that cusp of getting overrun,” says Dr. Anthony Fauci, the nation’s top infectious disease expert.