So are theories to explain it.
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.