Here’s what you need to know about the side effects from the coronavirus vaccine

Physician Alister Martin received one of the first doses of the Pfizer-BioNTech coronavirus vaccine from RN Jennifer Lisciotti at Massachusetts General Hospital on Wednesday.Craig F. Walker/Globe Staff

A coronavirus vaccine from Cambridge-based Moderna was expected to be approved by the US Food and Drug Administration on Friday, a week after the Pfizer-BioNTech vaccine was approved.

The initial rollout of vaccines has been a spark of hope for a nation wracked by the pandemic. But lately some stories have emerged of a handful of possible adverse reactions.

While initial supplies are limited and the shots are being rolled out in a phased fashion, here’s what you need to know about side effects when it’s your turn to roll up your sleeve and feel the pinch.

What are the side effects of the vaccines?

The most common side effects of both vaccines, according to data gathered during clinical trials, were pain at the site of the injection, fatigue, headache, muscle pain, joint pain, and chills. Less common was a fever.

“It’s a sore arm for a day or so for most people. Somewhere between 10 and 30 percent may get more generalized side effects like not feeling terrific for a few hours, having a headache, and, less commonly, having a low-grade fever,” said Dr. Daniel Kuritzkes, chief of the division of infectious diseases at Brigham and Women’s Hospital. “These symptoms typically occur within one or two days and typically resolve within one or two days. It shouldn’t really be a concern to the public.”

He said that people may get some of the more unpleasant symptoms, such as fever and aches and pains, after they get the second of the two shots required for the vaccine. He also noted people under 55 tended to feel worse.

Dr. Eric Rubin, editor-in-chief of the New England Journal of Medicine and adjunct professor of immunology at Harvard’s T.H. Chan School of Public Health, said, “We don’t know what side effects might arise as more and more people get vaccinated, but the side effects that showed up in the original trials weren’t all that concerning. It was the usual vaccine stuff. Nothing extraordinary.”

How do the side effects compare with other vaccinations?

Kuritzkes said the side effects may resemble something like the side effects of the shingles shot, which “more commonly will cause diffuse muscle aches or low-grade fever.”

Rubin said he thought the symptoms would be fairly similar to flu shot side effects. “We have vaccines out there that cause more reactions than this one and vaccines that cause fewer reactions. This sits somewhere in the middle,” he said.

What about severe allergic reactions and Bell’s palsy?

There have been reports of health care workers — two in Britain, two in Alaska — having a severe allergic reaction after receiving the Pfizer-BioNTech vaccine. All four have apparently recovered. The FDA says it is closely monitoring the situation.

Dr. Megan Ranney, an emergency physician and researcher in Rhode Island, said, “There are going to be very few people who have those types of reactions to any medications.” She also said anaphylaxis, the sudden allergic response, is “totally treatable.”

“Almost any vaccine may very rarely cause an allergic reaction,” said Kuritzkes, noting FDA guidance, which already requires people to be monitored for 15 minutes after the injection to see if they have a reaction, or 30 minutes if they have a history of severe allergic responses.

“The only concern is if you have had a severe reaction to another vaccination, then you should really check with your health care provider before getting the COVID vaccination,” he said.

A handful of people in the Pfizer and Moderna clinical trials also developed Bell’s palsy, which causes what is typically only a temporary facial paralysis. FDA officials have said there’s no clear basis to conclude the vaccine caused the Bell’s palsy cases, but they’re looking into it.

“It’s something to watch for, but we don’t know that it’s associated with the vaccine,” said Rubin.

Why do we get side effects anyway?

Ranney said side effects simply show that the vaccine is “mounting an immune response that will help to protect you from COVID.”

“It’s stimulating your immune system so you will be able to fight off the infection if you’re exposed to it,” she said.

It’s important that the public understands, Rubin said, that the side effects don’t mean that a person has somehow contracted the coronavirus.

“The side effect you’re seeing has nothing to do with COVID-19,” he said. “It has to do with the ingredients in the vaccine. It’s the same thing you’ll get from any vaccine.”

What do we not know?

More needs to be learned about the vaccine, anaphylaxis, and Bell’s palsy, said Kuritzkes.

It’s also not known if there will be some very rare serious side effects that crop up once the vaccine is rolled out to millions, or hundreds of millions, of people.

“We don’t know whether there are late-appearing side effects. It’s not expected that there would be such effects ... but anything is possible,” Susan Ellenberg, a biostatistician at the University of Pennsylvania Perelman School of Medicine, said in an e-mail.

The good news is that side effects from vaccines typically crop up within days, said Kuritzkes.

“That’s why the FDA was satisfied with having an average follow up of two months,” he said. “They felt that that would give them a more than sufficient view of the safety of the vaccine.”

Rubin, who is a member of an FDA advisory panel that recommended approval of the Pfizer vaccine last week and the Moderna vaccine this week, said he would have wanted more data from the two vaccines’ clinical trials, but it was an easy call when there’s “a disease that’s killing thousands of people every day.”

“While we’d like to have more information, we’d also like not to have people dying,” he said.

Material from Globe wire services was used in this report.