A man who boosted his Pfizer COVID-19 vaccine with a shot from Moderna said his side effects were a 'little more severe' the third time round

  • Scientists in the US are starting a booster shot trial, giving fully vaccinated people another jab.

  • Participants are given a booster shot of the Moderna vaccine - whether they previously got 2 shots of an mRNA vaccine (from Pfizer or Moderna) or 1 adenovirus (J&J) shot.

  • One participant told Insider his booster dose of Moderna gave him similar side effects to his second Pfizer shot, but the third was "a little bit more severe."

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Virus expert Joseph Hyser has been fully vaccinated since the end of January, and he's not really worried that his COVID-19 protection from the Pfizer vaccine has waned much since then.

Yet, when he got an email last month asking if he'd be interested in getting a third dose booster shot - this time from Moderna - he did not hesitate to say yes.

The virologist at Baylor College of Medicine signed right up, and took the elevator down six floors to Baylor's Vaccine Research Center, where he offered up his arm for a third jab.

"Oooh I got boosted!" he remembered bragging to family and friends afterwards, who were nonplussed about his unique booster shot experience.

Perhaps they didn't realize that Hyser is one of the first Americans to participate in a groundbreaking COVID-19 mix-and-match study, aimed at assessing whether boosters work, and whether different COVID-19 shots are safe - and possibly more effective - when they're mixed together.

Though Hyser says he experienced "similar types of side effects" from both Moderna's and Pfizer's mRNA shots, he said the side effects were "a little bit more obvious with the Moderna."

Is it safe to mix shots from Moderna, Pfizer, and Johnson & Johnson?

dr. robert atmar stands with his arms crossed and lab coat on
Dr. Robert Atmar is leading the booster dose study at Baylor College of Medicine in Texas. Baylor College of Medicine

This is not Hyser's first vaccine trial, he's been in studies of smallpox shots, anthrax vaccines, and annual flu jabs.

"I'm a dyed in the wool nerd, so I mean, of course I think that this is interesting," he said of the trial. "I like being involved in being a part of producing science and information for the world."

The study that he's now part of, funded by the National Institutes of Health, will evaluate mixing different coronavirus booster shots to assess whether they will be safe and effective for adults of all ages across the US.

For now, the study is boosting only with a single dose of Moderna's vaccine, whether participants' first shots were from Moderna, or from Pfizer or Johnson & Johnson - the other two US-authorized vaccines. Other combinations of the vaccines, and which order they're taken in, will be added in to the study later on.

Study participants will get routine blood draws (Hyser already had his first), and initial results could be ready within months.

A big question this study will explore is whether boosting people who've had J&J's one-shot adenovirus vaccine with Moderna's mRNA shot might make the performance similar. (J&J's vaccine did not protect as well against mild and moderate disease compared to Pfizer and Moderna's two dose mRNA regimens during clinical trials.)

"Is it because of the [vaccine] construct, or is it because of the one dose? We don't really have the answer to that," lead study researcher Dr. Robert Atmar, from the Vaccine Research Center at Baylor, told Insider of J&J's lower efficacy.

A day of chills, body aches, and ibuprofen after the 3rd dose

woman in scrubs prepares covid-19 vaccine, with patient and doctor chatting in background
Chanei Henry, senior research coordinator of molecular virology and microbiology at Baylor College of Medicine, prepares a COVID-19 vaccine. Baylor College of Medicine

While Hyser's Moderna side effects were a little more intense, he said they faded with some ibuprofen and a day of rest.

"I did, for Pfizer, have a little bit of chills, a little bit of muscle ache, but nothing that prevented me from conducting my normal day to day activities," he said, remembering how the day after his second shot in January he did some work, went grocery shopping, and made a pot of chili.

"With the booster, it was a little bit more severe," he said, describing waking up the day afterwards feeling "like I had done a very rigorous gym workout," with chills and arm soreness like he'd gotten a "hard punch in the shoulder." He popped some ibuprofen, which helped ease the aches, and by the following day, he felt like himself again.

"Given the dramatic impact that [the coronavirus] has had on the world, as a scientist, it's actually really heartening that these clinical trials are still going on and the secondary and tertiary questions are still being asked," Hyser said, estimating that he'll make about $600 total (compensation for his time) over the course of this study.

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