Roxanne Khamsi writes:

For weeks, Americans looked on as other countries grappled with case reports of rare, sometimes fatal blood abnormalities among those who had received the AstraZeneca vaccine against COVID-19. That vaccine has not yet been authorized by the FDA, so restrictions on its use throughout Europe did not get that much attention in the United States. But Americans experienced a rude awakening this week when public-health officials called for a pause on the use of the Johnson & Johnson vaccine, after a few cases of the same, unusual blood-clotting syndrome turned up among the millions of people in the country who have received it.

The world is now engaged in a vaccination program unlike anything we have seen in our lifetimes, and with it, unprecedented scrutiny of ultra-rare but dangerous side effects. An estimated 852 million COVID-19 vaccine doses have been administered across 154 countries, according to data collected by Bloomberg. Last week, the European Medicines Agency, which regulates medicines in the European Union, concluded that the unusual clotting events were indeed a side effect of the AstraZeneca vaccine; by that point, more than 220 cases of dangerous blood abnormalities had been identified. Only half a dozen cases have been documented so far among Americans vaccinated with the Johnson & Johnson vaccine, and a causal link has not yet been established. But the latest news suggests that the scope of this problem might be changing.

Whether the blood issues are ultimately linked to only one vaccine, or two vaccines, or more, it’s absolutely crucial to remember the unrelenting death toll from the coronavirus itself—and the fact that COVID-19 can set off its own chaos in the circulatory system, with blood clots showing up in “almost every organ.” That effect of the disease is just one of many reasons the European Medicines Agency has emphasized that the “overall benefits of the [AstraZeneca] vaccine in preventing COVID-19 outweigh the risks of side effects.” The same is true of Johnson & Johnson’s. These vaccines are saving countless lives across multiple continents.

But it’s also crucial to determine the biological cause of any vaccine-related blood conditions. This global immunization project presents a lot of firsts: the first authorized use of mRNA vaccines like the ones from Pfizer and Moderna; the first worldwide use of adenovirus vectors for vaccines like AstraZeneca’s, Johnson & Johnson’s, and Sputnik V; and the first attempt to immunize against a coronavirus. Which, if any, of these new frontiers might be linked to serious side effects? Which, if any, of the other vaccines could be drawn into this story, too? How can a tiny but disturbing risk be mitigated as we fight our way out of this pandemic? And what might be the implications for vaccine design in the years to come?

To answer these questions, scientists will have to figure out the biology behind this rare blood condition: what exactly causes it; when and why it happens. [Continue reading…]