Vaccine regulators have delivered a clear verdict: In most settings, the benefits of the COVID-19 vaccines made by AstraZeneca and Johnson & Johnson (J&J) far outweigh the small risk they will cause an unusual and sometimes deadly clotting disorder. But many questions remain about who is most at risk, how the risk-benefit calculus changes when cases fall, and what the side effects mean for the future of these vaccines, which use adenoviruses to ferry the gene for SARS-CoV-2’s spike protein into human cells.

A major concern is how the rest of the world will respond to some European countries’ moves to limit the use of the AstraZeneca and J&J vaccines, and the brief suspension of the J&J shot in the United States. The AstraZeneca vaccine—named Vaxzevria, or Covishield when it’s produced by the company’s Indian partner, the Serum Institute of India—is the cornerstone of the COVID-19 Vaccines Global Access (COVAX) Facility, a scheme aimed at vaccinating billions in the developing world. J&J is expected to provide hundreds of millions of doses of its one-shot vaccine to COVAX this year.

“Once there are clear policies from the West regarding in what age groups to use these vaccines, it’s going be very hard to recommend anything different here,” says John Amuasi, an epidemiologist at the Kwame Nkrumah University of Science and Technology in Ghana, one of the first countries in the world to receive the AstraZeneca vaccine through COVAX. But if no other vaccines are available, limiting use of these could ultimately cause many more COVID-19 cases—and deaths.

Signs the AstraZeneca vaccine could lead to an unusual reaction that causes clots throughout the body, accompanied by low levels of platelets, first surfaced 2 months ago.

Many of the first cases of what scientists now call vaccine-induced immune thrombotic thrombocytopenia or thrombosis with thrombocytopenia syndrome (TTS), were in women under the age of 60. But that may just be because many European countries used the shots in health care workers and educators, most of whom are women and under age 65.

Indeed, the gender imbalance has started to even out as more cases came to light. Among 209 people affected in the United Kingdom, 87 were men and 120 women; 139 cases were in people younger than 60. Overall, one in roughly 120,000 AstraZeneca shots has triggered the side effect in the country. ...