Immune thrombocytopenia is easily diagnosed and treated, James Bussel, a pediatrics professor at Weill Cornell Medicine who studies the condition, told me in an email. But the unusual combination of blood clots and low platelets is trickier. For example, one standard treatment for clots is a blood thinner called heparin, but the drug can, in very rare cases, cause the exact combination of low platelets and blood clots that doctors are concerned about. Experts now fear that heparin might make the potential vaccine reaction even worse. This combined condition also seems to be more dangerous than immune thrombocytopenia, but the prognosis may improve as more doctors learn how to treat it.
U.S. officials expect the J&J pause to last no more than a few days, as experts review the safety data and potentially revise recommendations. After a similar pause and review of the AstraZeneca data in Europe, several countries restricted that vaccine to older residents. (Most of the 86 blood-clot cases observed with the AstraZeneca vaccine in Europe were in women under 60, as were all six cases observed with the J&J vaccine in the U.S.) The U.K. now recommends that people younger than 30 be offered a different vaccine if possible.
The recommendations take into account individual risk: For older people at high risk of severe COVID-19 complications, the benefits of the vaccine clearly outweigh the risks of a blood clot. But for young people at lower risk from the coronavirus, the benefits are not so clear. For regulators, that balance also depends on whether a country has any other vaccines available and the severity of its local COVID-19 outbreak. The European Union and the U.K. do not have as many mRNA vaccines as the U.S., and less wealthy nations have even less supply. Ultimately, every country will have to do its own benefit-risk calculation.
The U.S.’s recommendations may end up diverging from other countries’, but they may also influence them. Sean O’Leary, a pediatrician at the University of Colorado and a liaison to the CDC’s Advisory Committee on Immunization Practices, notes two historical examples. Although the United States has discontinued use of the oral polio vaccine—which is more effective and easier to administer than the shot, but also carries a one in 2.5 million risk of paralysis after infection with the live virus in the oral vaccine—the World Health Organization continues to recommend it in countries where polio is endemic. But when the U.S. in 1999 stopped using a vaccine against rotavirus because of rare reports of intestinal blockage, the rest of the world fell in line, despite the fact that the virus was killing about half a million kids worldwide each year. “The decision was made, essentially, if it’s not good enough for you, it’s not good enough for us,” O’Leary says. Eventually, two newer rotavirus vaccines with a lower risk of complications were developed. They are now used in the U.S. and around the world.