The AstraZeneca COVID-19 vaccine can cause blood clots, two teams of scientists have independently claimed - but the good news is they're extremely rare and easily treatable.
A number of countries recently halted rollouts of the vaccine, developed in partnership with the University of Oxford, over concerns it could cause potentially fatal blood clots.
AstraZeneca said the problem didn't show up in its trials, and the World Health Organization urged nations to keep using it because the risk was extremely low and outweighed by the risks of contracting COVID-19.
Most nations resumed using it after a review by the European Medicines Agency found the rate of clots found in trials was similar to the general population.
But it now appears the vaccine can, in rare cases, prompt blood clots. A team of German scientists looked at cases found there in people who'd received the AstraZeneca vaccine, and independently, researchers in Norway were doing the same thing.
They both found the vaccine appears to trigger an overly strong antibody immune response in some people, leading to the clots.
"There is no other thing than the vaccine that can explain this immune response," Oslo University Hospital professor Pål Andre Holme told local newspaper VG.
"We have no other history in these patients that can give such a strong immune response. I'm pretty sure it's these antibodies that's the cause, and I see no other reason than that it's the vaccine that triggers it."
One death in Denmark has been linked to the clots.
"We take the vaccine to get an immune response to what we are to be protected against," said Dr Holme. "Then you get, among other things, the development of antibodies.
"Some antibodies can then react so that they can activate the platelets, as in these cases, and cause a blood clot. And because we have these antibodies on the surface, they are removed from the circulation, thus they get too low platelets."
The German team said if the theory is correct, the clotting can be identified and treated, the Wall Street Journal reported.
"Very, very few people will develop this complication," said University of Greifswald professor Andreas Greinacher. "But if it happens, we now know how to treat the patients."
Not everyone is convinced. In response to the findings, the European Medicines Agency said there remained "no increase in overall risk of blood clots" from the vaccine.
"A causal link with the vaccine is not proven but is possible and deserves further analysis," the regulator said in a statement.
AstraZeneca declined to comment when approached by VG. On Tuesday it revealed details from its new trial, claiming the vaccine had 79 percent efficacy against symptomatic COVID-19 and was 100 percent effective at "severe disease and hospitalisation".
"It is clear this vaccine has very good efficacy (remember that 60 percent was, prior to any trials being started, regarded as a good target), and that this efficacy does not show a notable decline at older ages," Stephen Evans, professor of pharmacoepidemiology at the London School of Hygiene & Tropical Medicine, told Reuters.
The efficacy is lower than that reported by vaccines made by Pfizer/BioNTech (which New Zealand is getting) and Moderna, but the AstraZeneca vaccine has the advantage of being much cheaper to produce and easier to transport.
The company didn't reveal how many people in the placebo group suffered serious illness however, with some experts saying the 100 percent figure is "probably very misleading" as a result.
"Encouraging people to think they have perfect protection from serious harm if they are vaccinated is a terribly risky message," expert Hilda Bastian told Science.
No cases of blood clotting were found in the trial's 21,583 volunteers. The new trial was undertaken after mistakes marred its initial testing, including giving wrong doses to participants.
Recently testing in South Africa found the AstraZeneca vaccine was almost completely ineffective against the local variant of the disease. South Africa on Monday sold its remaining AstraZeneca stock to other African nations, opting instead for the Johnson & Johnson-developed vaccine.