Vaccination programmes have been upset after a small number of reports that recipients of the AstraZeneca inoculation have suffered extremely rare blood clots, leading some countries worldwide to suspend its use out of caution.
A senior official for the European Medicines Agency (EMA) said in an interview published on Tuesday there was a link between the vaccine and rare blood clots in the brain but the possible causes were still unknown.
The EMA later said in a statement that its review of the vaccine was ongoing. It will give an update on its investigation on Wednesday afternoon.
AstraZeneca has said previously its studies have found no higher risk of clots because of the vaccine, millions of doses of which have been administered worldwide.
While many countries have resumed using the shot, some have imposed age restrictions.
In many instances, this has left officials scratching their heads over what to do for people who received a first dose of AstraZeneca but are no longer eligible under the new rules.
While the numbers are small compared with the tens of millions being inoculated across the region, the decision is significant because it has not been tested in late-stage human trials.
Any divergence to the EMA’s marketing authorisation would also be considered as “off label use”, meaning it would not be approved by the regulator and leave individual countries responsible for any possible side-effects.
The EMA had no immediate comment when asked about mixing and matching vaccines and referred to the briefing later on Wednesday.
Some experts say that, because all of the vaccines target the same outer “spike” protein of the virus, they could work together to train the body to fight off the virus. There is no evidence it will be as effective.