The European Union (EU) drug watchdog is looking at when people should be given the second dose of AstraZeneca and Oxford University’s Covid-19 vaccine, rather than at the size of the first dose, as part of its review of the shot, a source at the agency said on Wednesday.
Britain on Wednesday became the first country in the world to approve the coronavirus vaccine developed by the British duo, with the authorities recommending that a second shot is administered between four and 12 weeks after the first.
Two full doses three months apart were found to be 80% efficacious, the UK drug regulator said, higher than the average that the developers themselves had found in their late-stage trials.
For the European Medicines Authority (EMA), the crucial point is the level of protection people have once they have received the first shot and then have to wait between four and 12 weeks for the second one, the source said.
“This is exactly what we are looking at. You must have sufficient coverage with the first dose given that this is a vaccination campaign in the middle of a pandemic,” the source, who declined to be named due to the sensitivity of the matter, said.
Earlier on Wednesday, AstraZeneca said it had submitted full data to pursue conditional marketing authorisation from the EMA, but the regulator said it still needs more information for approval.
In a statement, the EMA said it expected more information from ongoing trials in January and it has requested additional scientific data from the company. It did not respond to Reuters’ request for comment on its analysis of timing and dosages.
The Oxford/AstraZeneca shot has been plagued with questions about its most effective dosage since data showed some surprising results.
Two full doses were given to the majority of participants and were shown to be 62% efficacious – but a smaller group of volunteers received by accident half a dose followed by a full dose, and registered 90% efficacy.
The UK Medicines and Healthcare products Regulatory Agency (MHRA) said on Wednesday that the 90% rate had not stood up to analysis.
The issue of the first shot at half dosage is also not on the table at EMA, with the regulator only looking at the two full dosage regimen.
It makes more sense for the timing of the second shot to affect efficacy rather than the dosage size in this case, the source at EMA said.
“The 12-week thing is reassuring from a scientific point of view because there is a logical explanation for these types of vaccines, but that the half dose was more effective than the full dose is something that actually clashes with logic,” the source said.
However, the source said the variations in the trial have caused a complexity that makes it more difficult for regulators to assess it.
AstraZeneca’s late-stage trial was originally designed for two injections four weeks apart. But in late-stage trial data published in The Lancet medical journal on December 12., the company said most participants had delays in receiving their second shot.
The median time between two standard doses in UK volunteers was about 10 weeks, and six weeks in Brazil, it said.
“Some people received the dose four weeks later, others 12 weeks later. It was all very variable. And that’s why it’s very difficult to interpret these studies,” the EMA source said.