Researchers revealed this week that most side effects of the mRNA vaccines from Pfizer and Moderna were mild and started fading away just a day after their administration.

The Study

A team of scientists examined the two mRNA-based COVID-19 vaccines used in the USA and presented their findings in a study published in The Lancet Infectious Diseases on Monday. The group used surveillance data collected through two system types — a passive one called the Vaccine Adverse Event Reporting System (VAERS) and an active system called v-safe. The data only included those who received their jobs during the first six months of the vaccination program in the country.

For the reports gathered through VAERS, they were categorized by the researchers as non-serious, serious, or dead. On the other hand, v-safe survey reports were examined for reactogenicity, severity, and health impacts of the vaccine dose for the entire week after vaccination. The observational study only used data from the two systems collected from Dec. 14, 2020, to June 14, 2021.

During the study period, more than 298 million doses of the mRNA vaccines were administered in the country. About 167 million accounted for vaccine doses from Pfizer, while 132 million were from Moderna. The researchers found that more than 4 million out of the almost 7 million participants who reported negative side effects complained about injection site pain after their first dose. The figure rose to 5.6 million after the second dose.

Around 33.9% or 2.3 million participants experienced fatigue after the first dose, while 55.7% or 3.1 million had the same issue after the second dose. More than 1.8 million participants suffered headaches after their initial dose, and the figure rose to 2.6 million after the follow-up dose. Overall, the team said the reactions following the vaccinations were mostly mild and began to diminish after 24 hours.

Implications of the Study

The findings of the study funded by the U.S. Centers for Disease Control and Prevention (CDC) help establish the safety of the mRNA vaccines to people who are still hesitant to get jabbed amid the ongoing pandemic. According to the researchers, the data they collected proved that most of the reported adverse effects of the Pfizer and Moderna vaccines were mild and short in duration.

“These data are reassuring that reactions to both mRNA vaccines are generally mild and subside after one or two days – confirming reports from clinical trials and post-authorization monitoring,” Tom Shimabukuro, one of the study authors, told USA Today.

The CDC’s Hannah Rosenblum, who also helped author the large-scale study, noted that their findings should “bolster confidence that the mRNA COVID-19 vaccines are safe” as the medical community continues to monitor the safety of the vaccines after the world entered the second year of the pandemic.