A team of researchers combining the health records of 6.2 million patients found no serious health effects that could be linked to the Pfizer-BioNTech and Moderna mRNA Covid-19 vaccines.
The study, published in the journal JAMA, indicates mRNA Covid-19 vaccines were not associated with significantly higher rates of 23 serious adverse events.
"These results from our safety surveillance are reassuring," said lead author Nicola Klein, from the Kaiser Permanente.
"The world is relying on safe and effective vaccines to bring an end to the Covid-19 pandemic. The Vaccine Safety Datalink is ideally suited to carry out this important surveillance and we will continue to monitor the safety of all vaccines that protect against Covid-19," Klein added.
The Vaccine Safety Datalink (VSD) is an ongoing collaboration between US health plans and the Centers for Disease Control and Prevention (CDC).
The study reported findings from mid-December 2020 through June 26, 2021.
The analyses compared specific health events among all Covid-19 mRNA-vaccinated people during the first 3 weeks after inoculation with health events among similar types of patients during the 3 to 6 weeks after mRNA vaccination.
The total number of people evaluated was 6.2 million for the first dose of either mRNA vaccine and 5.7 million for the second dose.
The authors added a comparison group of unvaccinated patients in a supplemental analysis.
The researchers examined 23 potential health effects, chosen because they had been included in previous vaccine studies, were of particular concern as an effect of Covid-19, were noted during the Covid-19 clinical trials, or were added after public health officials reported increased cases among vaccinated people.
Outcomes tracked included neurological disorders such as encephalitis and myelitis, seizures, and Guillain-Barre syndrome; cardiovascular problems such as acute myocardial infarction, stroke, and pulmonary embolism; and others such as Bell's palsy, appendicitis, anaphylaxis, and multisystem inflammatory syndrome.
Patient medical records were searched electronically and analysts carried out chart reviews of specific health outcomes to verify the medical problem and to assess whether it started before or after vaccination.
The study authors highlighted their findings about cases of confirmed myocarditis and pericarditis among young individuals, as that has become an outcome of a concern.
The VSD study identified 34 such cases in patients 12 to 39 years old; 85 per cent of them were male and 82 per cent hospitalised (for a median of 1 day), and nearly all recovered by the time the chart review took place.