Blood clots tied to vaccines can also occur spontaneously, scientists find

In February, 2021, as health authorities around the globe raced to blunt the COVID-19 pandemic, reports began to emerge of a rare but serious adverse effect associated with one of the first vaccines to reach the public.That vaccine, manufactured by Britain-based pharmaceutical company AstraZeneca, was soon connected to dangerous blood clots in about one in 66,000 recipients – a rate too low for the effect to have turned up in clinical trials of the vaccine.