On March 15, Germany, France, and Italy joined several other European and Asian countries to suspend the distribution of the AstraZeneca/University of Oxford Covid-19 vaccine. Public health leaders in these countries stated that the pause was “precautionary” after a very small number of people developed blood clots at some point after receiving the vaccine.
Scientists don’t yet have data directly connecting these clots to the vaccine itself, but the vaccines’ reputation has already taken a hit. Millions of Europeans aren’t willing to take the shots, even in areas where they’re still widely, available because of rumors about side effects of the two-dose immunization.
Health officials’ initial concern about blood clots, which can be lethal if left untreated, is based on anecdotes of dozens of people who received the AstraZeneca vaccine and later developed blood clots.
But more than 17 million people have received the AstraZeneca vaccine, according to a statement from the company. As of March 8, only 37 people subsequently developed blood clots. The European Medical Agency pointed out that this is actually fewer than medical experts would expect to see occur in the general population. The number of people who develop them per year is around one per 1,000 people.
“The nature of the pandemic has led to increased attention in individual cases,” Ann Taylor, the chief medical officer of AstraZeneca, said in a statement. “We are going beyond the standard practices for safety monitoring of licensed medicines in reporting vaccine events, to ensure public safety.” The company also stated it would be keeping all evidence under close review, although it had no reason to believe vaccines were the cause of any clots.
Nor is there earlier data suggesting blood clots could be a side effect of the jab. Regulatory agencies in the UK, the EU, Canada, Australia, and several countries in Asia and Africa cleared AstraZeneca’s Covid-19 vaccine for emergency use at the end of 2020 and beginning of 2021. They did so based on data from a phase 3 clinical trial, which followed roughly 11,600 participants, some of which received vaccines and some of which received a placebo. In the trial, researchers deemed the vaccine safe (mild side effects like temporary aches and pains occurred in some participants, but these were no worse than any other Covid-19 vaccine like the ones from Pfizer or Moderna), and it protected against roughly 70% of symptomatic Covid-19 cases.
The World Health Organization is still conducting its own investigation of the vaccine, which will likely be concluded this week. The organization currently endorses the use of AstraZeneca’s vaccine. The European Medical Agency is also convening an emergency meeting on Thursday (March 18) to review the safety of the vaccine as well.
AstraZeneca’s vaccine for Covid-19 uses a tried and true vaccine platform to encourage the body to mount an immune response. It’s a viral vector vaccine, just like AstraZeneca’s previously authorized vaccines against tuberculosis, MERS, and Ebola. It uses a genetically-altered virus that causes a mild respiratory infection in chimpanzees to transport mRNA for the SARS-CoV-2 spike protein. (This virus wouldn’t ordinarily make humans sick, but it’s been engineered not to reproduce, either.)
This mRNA works just like it does in the Moderna and Pfizer-BioNTech vaccines: it teaches our cells to make the SARS-CoV-2 spike proteins which trigger an immune response capable of protecting against the actual virus. The advantage of using a virus as a vessel for the mRNA is that it has the same appearance to the body as other potential pathogens.
It’s the frustrating chorus of the coronavirus pandemic: We don’t know.
The AstraZeneca vaccine was a leader early in the vaccine race because it was one of the few candidates that didn’t require deep freezing. This made it potentially more accessible to billions of people or poorer countries or rural areas, where those freezers would be a limiting factor. (Now there are other vaccines, including Johnson & Johnson’s, which do not need to be kept at such low temperatures.)
Even if scientists affirm that AstraZeneca’s vaccine is after more reinvestigation, health officials will likely have to double down on their messaging to the public in an order to rebuild trust in the jab. Fortunately, there’s still an ongoing clinical trial using AstraZeneca’s vaccine: One in the United States with some 30,000 participants. As STAT reports, scientists are likely to release early figures from this trial as soon as this week, which could provide even more data to evaluate the vaccine’s safety profile.