In the United States alone, 300,000 to 600,000 people a year develop blood clots, according to C.D.C. data. But the particular blood clotting disorder that the vaccine recipients developed, known as cerebral venous thrombosis, is extremely rare. All of the women developed the condition between six and 16 days of vaccination, and government experts are concerned that an immune system response triggered by the vaccine was the cause.
The decision is a fresh blow both to Johnson & Johnson and to the administration’s plans. Late last month, the company discovered that workers at a Baltimore plant run by its subcontractor had accidentally contaminated a batch of vaccine, forcing the firm to throw out the equivalent of 13 million to 15 million doses. That plant was supposed to take over supply of the vaccine to the United States from Johnson & Johnson’s Dutch plants, which were certified by federal regulators earlier this year.
The Baltimore plant’s certification by the F.D.A. has now been delayed while inspectors investigate quality control issues, sharply reducing the supply of Johnson & Johnson vaccine. The sudden drop in available doses led to widespread complaints from governors and state health officials who had been expecting much bigger shipments of Johnson & Johnson’s vaccine this week than they got.
States have been using the vaccine in a broad range of settings, including at mass vaccination sites and on campuses. Officials have also directed it to transient, rural and isolated communities where following up with a second dose is more complicated.
It is common for regulators to investigate so-called “safety signals” in new vaccines and other medical products. Very often, the signals prove not to be of concern. But the concerns about Johnson & Johnson’s vaccine mirror concerns about AstraZeneca’s, which European regulators began investigating last month after some recipients developed blood clots.
Out of 34 million people who received the vaccine in Britain, the European Union and three other countries, 222 experienced blood clots that were linked with a low level of platelets. The majority of these cases occurred within the first 14 days following vaccination, mostly in women under 60 years of age.
On April 7, the European Medicines Agency, the main regulatory agency, concluded that the disorder was a very rare side effect of the vaccine. Researchers in Germany and Norway published studies on April 9 suggesting that in very rare cases, the AstraZeneca vaccine caused people to make antibodies that activated their own platelets.