COVID-19 vaccine trial volunteers note occasional harsh side effects

Most report mild side effects, but those experiencing worse still recommend the shots

Jocelyn Edwards wasn’t sure she got Moderna Inc.’s experimental Covid-19 vaccine or a placebo when she received her first of two doses in August. Hours after the second shot, she said she was sure it was the genuine article.

“I woke up around midnight freezing,” said the 68-year-old retired nurse. “For the next 24 hours I had intense chills, serious neck pain, headache, all my joints were aching.” She had a fever that peaked at 102.4 and poured out so much sweat that she lost 3 pounds, she said. The following day she woke up and felt fine.

Ms. Edwards, like the other 30,000 volunteers who took part in Phase 3 clinical trials for Moderna’s Covid-19 vaccine, wasn’t told whether she got the vaccine or a placebo. However, she said a trial researcher attributed her symptoms to her body mounting a strong immune response to what was most likely the vaccine. “It’s better having 36 hours of feeling really rough than getting Covid,” she said.

Moderna declined to comment for this article.


As the first vaccine from Pfizer Inc. and BioNTech SE rolls out this week and the next one from Moderna looks poised to start reaching people soon, some Americans have expressed reservations about getting vaccinated. One concern has been possible side effects. While the data show that some Moderna and Pfizer trial volunteers experienced side effects, even those who had harsh reactions recommend the shots.

A Food and Drug Administration advisory panel is set to review the safety and efficacy of Moderna’s experimental vaccine on Thursday. The company released data Tuesday that showed that its shot is 94.1% effective at preventing Covid-19 with certain symptoms. The data also show that after the second of two doses, about twice as many trial volunteers aged 18 to 64 who received the vaccine experienced side effects compared with those injected with a placebo. About 17% got a fever compared with less than 1% of the placebo group, and 48% got chills versus 6% of placebo recipients. Fatigue and headaches were also more common among vaccine recipients.

Pfizer’s vaccine, which uses technology similar to Moderna’s, showed similar side effects, according to data released last week. Among its volunteers aged 18 to 55 receiving their second dose, 15.8% got a fever, compared with 0.5% of the placebo group; 35% got chills versus 4% of placebo recipients; and they also got more headaches and were more fatigued than those who got the placebo. Volunteers in both trials who received the vaccine also reported pain at the injection site more frequently than placebo recipients.

Two of the first people vaccinated last week with the Pfizer shot in the U.K. had an allergic reaction following the injection. Both recovered after receiving treatment. They each had a history of allergies and carried adrenaline autoinjectors to treat themselves. The U.K.’s medical regulator issued guidance warning those with a history of significant allergic reactions against having the inoculation.


Pfizer has said its vaccine was generally well tolerated with no serious safety concerns reported by the independent data monitoring committee.

In both the Pfizer and Moderna trials, most side effects were reported as mild or moderate, and they occurred at a lower rate in older volunteers.

“It’s a really good sign that there is a signal from your body that there is something different inside you,” said Paul Duprex, director of the Center for Vaccine Research at the University of Pittsburgh. “It’s being recognized by your immune system to make all important SARS-CoV-2 antibodies.”

The FDA granted Pfizer’s vaccine an emergency-use authorization on Friday and health-care workers started receiving the vaccine this week.

Brad Hoylman, a 55-year old state senator from New York spent the evening after his second Pfizer shot shivering under a pile of blankets as he ran a fever around 102, had severe body aches and a splitting headache. By the next morning the symptoms were gone, though he felt fatigued for a few days. “It’s definitely worth getting the shot,” said Mr. Hoylman. “It beats dying from Covid.”


To reach herd immunity for Covid-19, public health authorities estimate that 60% to 70%, but possibly as low as 50%, of a given population would need antibodies to protect against infection. If Americans decline to be vaccinated in large numbers for any reason, including fear of side effects, it may cost the nation a chance to stamp out the disease.

Amy Warren, a 48-year-old nurse practitioner in Kansas City, got chills, a fever and severe joint and muscle pain after getting her second dose of Moderna’s vaccine over the summer as part of its Phase 2 trial. She didn’t know what to expect so hadn’t planned to take off from work the following day.

“I felt like death, and I’m no wimp,” she later posted to a Facebook group she started. She said she created the group in part to warn trial volunteers to take a day off after the second shot, in case they need time to recover.

Ms. Warren subsequently took tests that showed antibodies for Covid-19, she said, suggesting she got the experimental vaccine.

“We’re pretty sure when someone has that kind of reaction, they didn’t get the placebo,” said her doctor, Jed Ervin, medical director at the Center for Pharmaceutical Research in Kansas City, who is testing eight Covid-19 vaccine candidates, including Moderna’s.


Despite the possibility of harsh side effects, Ms. Warren still recommends everyone get vaccinated. “Get the shot because it can save your life and your family’s life,” she texted from a beach in Mexico last Wednesday where she was vacationing. She felt safer traveling because she was vaccinated, she wrote. She said she still practices social distancing, wears a mask and frequently washes her hands since it isn’t clear yet whether vaccinated people can still catch and shed the virus even if they are protected from developing symptoms themselves.

Jackie Stone wanted to volunteer for a Covid-19 vaccine trial as soon as one was available. The 35-year-old resume writer from Lafayette, Colo., had already quarantined for a period before the Covid-19 pandemic, to protect her immune-compromised son, who had been born prematurely.

She had a reaction to her second shot during Pfizer’s vaccine trial, with symptoms she said were similar to a mild cold coming on. She spent a day in bed and was glad to have family quarantining with her to help with child care. She believes she got a vaccine and not a placebo also because of an antibody test she said was positive.

Ms. Stone now handles shopping for the family, and is looking forward to returning to her rock climbing gym in the new year, something she wouldn’t have had the confidence to do without the vaccine, she said.

“One day of feeling crappy in bed is totally worth getting your life back,” she said.

—Jared S. Hopkins contributed to this article.