About one in 4000 people who got an mRNA-based COVID-19 vaccine in a new study had an anaphylactic reaction - far higher rate than previous research has found.
But only one of the 65,000 patients looked at in the study needed treatment in intensive care, and all recovered just fine.
mRNA vaccines are new on the scene, the COVID-19 shots from Pfizer/BioNTech and Moderna being the first-ever approved for use outside of trials. They work by teaching the body's cells how to make the SARS-CoV-2 virus' spike protein, teaching our immune system what it looks like, without using any of the actual virus itself.
Trials and real-world data so far have found them to be incredibly effective, particularly for a disease that's been around for little more than a year, and relatively safe - with no deaths yet linked to them, despite the speed of which they've been rolled out to combat a pandemic that's left at least 2.6 million dead already.
Researchers looked at healthcare workers in Boston who'd been injected with at least one dose of either the Pfizer or Moderna vaccines, and what side-effects they experienced over the following few days. Forty percent got the Pfizer jab and 60 percent Moderna.
Of the 65,000, 2 percent reported an allergic reaction - the vast majority were minor. But 16 were determined to be anaphylaxis - a potentially life-threatening overreaction by the body's immune system to a foriegn substance.
On average it took 17 minutes for an anaphylactic reaction to kick in. Of the 16, nine needed epinephrine and one intensive care, and all recovered. None went into shock, and none needed a breathing tube - not even temporarily. Five of the 16 had a history of anaphylaxis, 10 of them allergies, and 15 of the 16 were female. The average age was 41.
"Given that approximately 5 percent of adults have severe food allergy histories and 1 percent of adults have severe drug allergy histories, this... cohort likely included almost 4000 individuals with severe food or medication allergy histories who were safely vaccinated," the study said.
The rate of anaphylaxis - 2.47 per 10,000 vaccinations, or 0.0247 percent, is about 22 times what the US Centers for Disease Control and Prevention (CDC) reported in January for the mRNA vaccines, based on reports of allergic reactions to the 17.5 million doses delivered by that point.
Kimberly Blumenthal of Massachusetts General Hospital - who led the research - said the CDC figures relied on voluntary reporting from patients, while her study proactively asked people and had experts on hand to make diagnoses.
"Sometimes there is key information missing in [ Vaccine Adverse Event Reporting System] reports or missing records," she tweeted in response to questions about the discrepancy, adding that there could also be "differences potentially based on geography or employee demographics... 2.47 per 10,000 is NOT scary."
The difference was also noted in the study itself, published this week in journal JAMA.
"The incidence rate of confirmed anaphylaxis in this study is larger than that reported by the Centers for Disease Control and Prevention based on passive spontaneous reporting methods. However, the overall risk of anaphylaxis to an mRNA COVID-19 vaccine remains extremely low and largely comparable to other common health care exposures."
Dr Blumenthal compared the risk of anaphylaxis from an mRNA vaccine, based on her study's results, to getting antibiotics - and said it was important people understood the risk is "still exceedingly low".
"The COVID-19 mRNA vaccines are the first vaccines of their kind, and they have remarkable efficacy and safety across all populations. It is critical to have accurate information on allergic reactions to these vaccines, not only for our current situation, but also because this new vaccine platform is so important for future pandemic responses."
In comparison, the present case fatality rate for COVID-19 in New Zealand is 1.1 percent - 44 times higher than the chance of anaphylaxis from the Pfizer vaccine, which we're currently rolling out.
The exact cause of the reactions isn't yet known.