The clots reported after J&J immunization—called cerebral venous sinus thromboses—are in an unusual class. A very similar clotting problem has been reported after injections of AstraZeneca’s COVID-19 vaccine, which strongly resembles the J&J shot in formulation; several European countries have restricted the AstraZeneca shot to use in older adults only. One benefit of publicizing the potential problem with J&J right now is that federal health officials can alert doctors to the right treatments. They have advised against deploying heparin, one of the most common blood thinners prescribed for clots, in these patients, because it has the potential to exacerbate this specific condition. Luckily, an entire arsenal of alternative treatments exists. “It’s really important to know what to be looking for right now,” Céline Gounder, an infectious-disease physician at Bellevue Hospital in New York, where some patients have been receiving the J&J vaccine upon discharge, told me.
For people still waiting to get vaccinated, the pause could introduce some temporary, regional speed bumps into the rollout. But Johnson & Johnson shots already made up a minority of the injections being doled out to Americans, and supply has sputtered because of a recent factory snafu that compromised up to 15 million doses. Many people who were signed up for these shots in the near future should be able to switch to an mRNA-based alternative. (I was signed up to get a J&J shot this week; I’ll be getting Pfizer instead.) The CDC and FDA are also expected to issue updates within the next few days, leaving open the possibility that vaccines will continue largely on schedule thereafter.
The forecast further out is murky. A panel of experts must now determine whether the vaccine and these clots are conclusively linked. This will involve comparing the rate at which blood clots occur shortly after a J&J vaccination to their incidence in the general, uninoculated population. These cases could still be chalked up to coincidence, or possibly to an unidentified underlying condition that afflicted all six women. Clinical trials found the J&J vaccine to be safe as well as effective; that verdict could certainly hold after this new round of federal vetting. Pauses, Omer told me, aren’t revocations or excoriations of a product. Instead, they make space for experts to suss out a situation and revise the playbook as needed.
“Hopefully one outcome would be that we develop a better understanding of this issue, and who’s specifically at risk,” Dean told me.
If researchers establish that there is a risk relationship, they’ll next need to determine its extent, and weigh it against the relative risk of remaining unvaccinated. Since the start of the pandemic, the coronavirus has killed nearly 3 million people around the globe, and saddled many thousands of others with lasting, debilitating symptoms. Contracting the coronavirus, too, has been linked to blood clots, which appear, by some estimates, in about 20 percent of COVID-19 patients, and 31 percent of patients who end up in intensive care. Several experts told me that they’d need to see many, many more clotting events to even consider pulling an otherwise highly successful and effective vaccine from the global market, especially while many countries struggle to acquire, store, and administer vaccines.