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 six suspected cases of unusual blood clotting among nearly seven 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.