Two new but extremely rare Covid-19 vaccine side effects have been detected by researchers in the largest vaccine safety study to date.
The study involved more than 99 million people from several countries including New Zealand.
Researchers confirmed how rare known vaccine complications are, with researchers stating the benefits of Covid-19 vaccines still “vastly outweigh risks”.
Anonymous electronic healthcare data was used to compare the rates of 13 brain, blood and heart conditions in people who received either the Pfizer, Moderna or AstraZeneca jabs with the rate that would be expected in the population pre-pandemic.
It proved known links between mRNA (Pfizer and Moderna) vaccines and the rare side-effects of myocarditis (inflammation of the heart muscle) and pericarditis (swelling of the thin sac covering the heart).
It also confirmed that Guillain-Barre syndrome (where the immune system attacks nerves) and cerebral venous sinus thrombosis (a type of brain blood clot) were rare side effects linked to the AstraZeneca vaccine.
But two rare new side effects were also identified in the data analysis linked to AstraZeneca vaccine.
Acute disseminated encephalomyelitis (inflammation and swelling in the brain and spinal cord) and transvere myelitis (spinal cord inflammation) were both detected by the study.
The findings were published in the international journal Vaccine on Friday.
The Australian study found the data translated to an extremely small risk of acute disseminated encephalomyelitis of 0.78 cases for every million doses, and 1.82 cases per million doses for transverse myelitis.
“For rare side effects, we don’t learn about them until the vaccine has been used in millions of people”, said Professor Jim Buttery, co-director of the Global Vaccine Data Network.
“No clinical trial can ever have the size to answer those questions and so we only find out those questions after a vaccine has been introduced.”
He added that the risk of myocarditis is even high with natural Covid infection than it is following Covid vaccination.
Vaccine expert Professor Julie Leask said that it is important to keep these findings in perspective, as Covid infection increases the risk of some of these rare conditions “more than a vaccine” does.
She said the studies also confirmed that “our vaccine experts are paying attention to when vaccines lead to serious side-effects, and they’re acting on it”.
“Being confident in a system that will detect problems and address them, is a very important part of a robust vaccination program.”
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