People Are Reporting Unexpected Side Effects After COVID-19 Vaccination—But That’s Actually Normal

Abigail, a 29-year-old from New York City who asked to use a pseudonym to preserve her privacy, knew to expect some side effects after she got her second Pfizer-BioNTech COVID-19 vaccine in February. The fever, body aches and fatigue she felt were nothing surprising. But when she began experiencing heavy, “stupidly painful, debilitating” menstrual periods, she felt blindsided. “I had not heard a single person talking about it,” she says.

While scientists have not confirmed that COVID-19 vaccines can cause menstrual changes like Abigail’s, she feels confident the shot was her trigger, since she experienced something similar after a probable case of COVID-19 last spring. As of mid-April, more than 19,000 people had reported similar experiences in a University of Illinois survey. Even without scientific confirmation, their stories are drawing attention to the fact that vaccination side effects can be far more varied than what’s often discussed in the mainstream.

Side effects—while temporarily uncomfortable—are a standard part of vaccination, says Dr. Stanley Perlman, a professor at the University of Iowa’s Carver College of Medicine and a member of the U.S. Food and Drug Administration’s vaccine advisory committee. They show that the body is responding to the shot and mounting an immune response. (Perlman says he considers side effects different from serious adverse events, like the rare blood clots reported in connection with Janssen/Johnson & Johnson’s shot. Most side effects are unpleasant but normal, whereas serious adverse events are potentially fatal issues that demand medical care.)

The U.S. Centers for Disease Control and Prevention (CDC) lists common COVID-19 vaccine side effects as pain, swelling or redness at the injection site; fatigue; headaches; muscle pain; chills; fever; and nausea. But that list isn’t exhaustive. In fact sheets describing each vaccine, manufacturers listed additional possible side effects—including diarrhea, joint pain, swollen lymph nodes, hives, rashes and facial swelling—and noted that there could be even more side effects beyond those specified.

Menstrual changes may fall into that category, if conclusive evidence links them to the vaccines. (In statements to TIME, a spokesperson for Pfizer said clinical trial participants did not report any menstrual changes. A spokesperson for Janssen said menstrual changes were not tracked as part of its study. A Moderna rep did not return requests for comment by press time.) There have also been reports of people developing rashes and other skin conditions, like so-called “COVID arm,” after vaccination. Others have experienced brain fog that feels akin to a marijuana high. In rare cases, people have also developed shingles after COVID-19 vaccination, as small numbers of people do after taking other types of drugs, but those findings are preliminary.

There are bound to be more side effects observed and reported by the general population than are uncovered during a clinical trial, Perlman says. Each COVID-19 vaccine trial had to have at least 30,000 participants. That’s a lot, but it’s a small fraction of the people who have been vaccinated in the U.S., now that 215 million doses have been administered. Even side effects that were uncommon during clinical trials may affect a relatively large number of people once vaccines are widely distributed, Perlman says.

“If you have a trial where you’ve looked at 30,000 people and something happens to 0.1% of them, that’s 30 people,” Perlman says. “But then when you change that 30,000 to 30 million, now there’s many more people having the same side effect.”

Not everything that happens post-vaccination is caused by the vaccine, of course. When millions of people get vaccinated every single day, some are bound to experience health problems following but unrelated to the vaccine, Perlman says.

Even still, it’s important for people to monitor their health in the days following vaccination and alert their doctors to anything out of the ordinary, so health authorities know which side effects to emphasize or look into further, says Paul Duprex, director of the University of Pittsburgh’s Center for Vaccine Research. Members of the public can report their side effects using the CDC’s Vaccine Adverse Event Reporting System or V-safe tracker.

“Those voices matter,” Duprex says. “Patterns can only be recognized [if people who get vaccinated] voice what happened and people who hear those voices collect and collate” the data.

There’s a tendency among researchers to discount observations if their cause can’t be proven, Duprex admits. “There’s a fundamental difference between correlations and a cause,” he says. But “none of those things should be dismissed.” Only when researchers have all the data can they adequately assess what’s really happening and communicate that to the public.

“There are things that we can only learn,” Duprex says, “if those vaccinated voices are heard.”

That’s what people like Abigail are trying to make sure of, by speaking out about potential side effects like menstrual changes. Abigail says she remains glad she got vaccinated, and recommends that everyone do the same. She just wants others to go into the experience feeling more informed than she did.

Correction, April 22

The original version of this story misstated how many people in the U.S. have been vaccinated against COVID-19. More than 215 million doses have been administered, but fewer than 215 million people have received at least one dose.