It has been a difficult week for two COVID-19 vaccines. On 13 April, US regulators urged health-care providers to temporarily stop using a vaccine made by Johnson & Johnson (J&J) of New Brunswick, New Jersey, because of 6 suspected cases of unusual blood clotting among nearly 7 million vaccine recipients.
The move came after European regulators expressed concerns about a possible link between rare blood clots and the Oxford–AstraZeneca vaccine, developed in the United Kingdom by AstraZeneca in Cambridge and the University of Oxford.
Both decisions are having a global impact. Although researchers and regulators stress that the benefits of the vaccines outweigh the risks, several countries are restricting the use of the AstraZeneca vaccine to certain age groups, and Denmark has opted out of using it altogether. J&J, meanwhile, has paused distribution of its vaccine to some countries.
How could a COVID vaccine cause blood clots? Scientists race to investigate
“The way that it’s happened has just made us all feel that the world is a bit crazy,” says Susan Goldstein, a public-health specialist and deputy director of the SAMRC Centre for Health Economics and Decision Science in Johannesburg, South Africa. “There’s been a huge amount of confusion.”
Some of that confusion stems from an urgent need to act quickly on the basis of messy, incomplete and capricious real-world data. As regulators are forced to make decisions, scientists are still racing to investigate the rare clotting disorder and its link to the vaccines.
Here are some of the key questions that they are hoping to answer.