In the first six months after the release of the Pfizer-BioNTech and Moderna mRNA COVID-19 vaccines, 298 million doses were given in the United States. An assessment of their side effects reveals reactions were overwhelmingly mild and brief. The findings may not be surprising, and are unlikely to change anti-vaxxers' minds, but are important in the context of the mRNA vaccines under development for conditions such as cancer and HIV.

The finding coincides with research showing even mild cases of COVID-19 itself can damage the brain far more than any vaccine.

Two systems were used to track reactions to the COVID-19 mRNA vaccines; Vaccine Adverse Event Reporting System (VAERS) and v-safe. A study published in Lancet Infectious Diseases examines data from both for vaccinations given between December 2020 and June 2021.

During that time, 340,522 negative reactions were reported to VAERS from the two vaccines. This number is a little higher than the deaths from COVID-19 during that period, although of course far less than the number who became seriously ill from the disease in that period alone.

More importantly, 92 percent of these were not deemed serious – usually involving headaches, fever, and pain at the site of injection. Even the 27,023 cases (0.009 percent of doses) not classified as mild more often than not ended in a full recovery.

The VAERS data is backed up by COVID-specific v-safe, involving questions sent to vaccine recipients' phones daily for the first week after vaccination and less frequently thereafter.

Most v-safe participants reported effects like fatigue and headache on the first day, particularly after the second injection. However, this usually passed within 48 hours, although many people were unable to work or perform normal duties for a day. Under 1 percent of v-safe participants reported even seeking medical care post-vaccination, and major treatment such as hospitalization was rarer still.

VAERS was established in 1990 and encourages vaccine recipients, their healthcare providers, or manufacturers to report any post-vaccine reactions possibly caused by the vaccine. Many reactions turn out to have causes unrelated to vaccination, but are reported out of an abundance of caution. Reporting is mandatory in cases of death, even where the cause is obviously not vaccine-related, such as for murders or car accidents.

Critics of VAERS claim reactions are under-reported, but the data revealed in this study discredit that idea. If more than 300,000 mild reactions were reported to VAERS, it is hard to explain how cases that bring more contact with the medical system would escape notice.

Similarly, unless they had actually died – in which case autopsies should bring the cause to light – few among the 8 million people enrolled in v-safe would ignore repeated questions by text message if seriously ill.

VAERS did highlight 4,500 cases where people died soon enough after the vaccine to be reported, but the evidence does not support vaccination playing a part. The median age of death was 76 and half of deaths were over 10 days after vaccination, making any connection particularly improbable.

“The rapid pace at which COVID-19 vaccines were administered under emergency use, especially among older populations, was unprecedented,” author Dr David Shay of the CDC said in a statement. "Due to their age, this group already has a higher baseline mortality rate than the general population and our results follow similar patterns of deaths rates for people in this age group following other adult vaccinations,”

The findings confirm smaller studies, including clinical trials which led to the vaccine's approval, that also didn't find serious reactions outside very rare cases.

In a commentary piece for the Lancet, Professor Elizabeth Phillips of Vanderbilt University noted; “It is reassuring that there were no unexpected signals other than myopericarditis and anaphylaxis, already known to be associated with mRNA vaccines.” Myopericarditis is a far more common symptom of the disease itself than the vaccine.