From the Journals

Side effects of COVID mRNA vaccines are mild and short, large study confirms


Data from the first 6 months after the rollout of mRNA COVID-19 vaccines in the United States released today show that adverse effects from shots are typically mild and short-lived.

Findings of the large study, compiled after nearly 300 million doses were administered, were published online March 7 in The Lancet Infectious Diseases.

Researchers, led by Hannah G. Rosenblum, MD, with the Centers for Disease Control and Prevention COVID Response Team, used passive U.S. surveillance data collected through the Vaccine Adverse Event Reporting System (VAERS), and the active system, v-safe, starting in December 2020 through the first 6 months of the U.S. COVID-19 vaccination program. V-safe is a voluntary, smartphone-based system set up in 2020 specifically for monitoring reactions to COVID-19 and health effects after vaccination. The health effects information from v-safe is presented in this study for the first time.

Of the 298.7 million doses of mRNA vaccines administered in the U.S. during the study period, VAERS processed 340,522 reports. Of those, 313,499 (92.1%) were nonserious; 22,527 (6.6%) were serious (nondeath); and 4,496 (1.3%) were deaths.

From v-safe reporting, researchers learned that about 71% of the 7.9 million participants reported local or systemic reactions, more frequently after dose 2 than after dose 1. Of those reporting reactions after dose 1, about two-thirds (68.6%) reported a local reaction and 52.7% reported a systemic reaction.

Among other findings:

  • Injection-site pain occurred after dose 1 in 66.2% of participants and 68.6% after dose 2.
  • One-third of participants (33.9%) reported fatigue after dose 1 and 55.7% after dose 2.
  • Headache was reported among 27% of participants after dose 1 and 46.2% after dose 2.
  • When injection site pain, fatigue, or headaches were reported, the reports were usually in the first week after vaccination.
  • Reports of being unable to work or do normal daily activities, or instances of seeking medical care, occurred more commonly after dose 2 (32.1%) than after dose 1 (11.9%). Fewer than 1% of participants reported seeking medical care after dose 1 or 2 of the vaccine.
  • Reactions and health effects were reported more often in female than in male recipients, and in people younger than 65 years, compared with older people.
  • Serious adverse events, including myocarditis, have been identified following mRNA vaccinations, but the events are rare.

The authors wrote that these results are consistent with preauthorization clinical trials and early postauthorization reports.

“On the basis of our findings, mild to moderate transient reactogenicity should be anticipated,” they said, “particularly among younger and female vaccine recipients.”

‘Robust and reassuring data’

“The safety monitoring of the mRNA COVID-19 vaccines stands out as the most comprehensive of any vaccine in U.S. history. The use of these complementary monitoring systems has provided robust and reassuring data,” Matthew S. Krantz, MD, with the division of allergy, pulmonary, and critical care medicine at Vanderbilt University, Nashville, Tenn., and Elizabeth J. Phillips, MD, with the department of pathology, microbiology, and immunology at Vanderbilt, wrote in a related commentary in The Lancet Infectious Diseases.

They point out that the v-safe reports of reactions are consistent with those reported from clinical trials and a large population study in the United Kingdom.

Dr. Phillips said in a press release, “[A]lthough approximately one in 1,000 individuals vaccinated may have an adverse effect, most of these are nonserious. No unusual patterns emerged in the cause of death or serious adverse effects among VAERS reports. For adverse events of special interest, it is reassuring that there were no unexpected signals other than myopericarditis and anaphylaxis, already known to be associated with mRNA vaccines.”

The study authors and editorialists have disclosed no relevant financial relationships.

A version of this article first appeared on

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