US health authorities recommended a pause in the use of Johnson & Johnson’s Covid-19 vaccine in order to investigate rare but critical cases of blood clots, a setback to vaccination efforts racing against virus variants.The US Food and Drug Administration and the US Centers for Disease Control and Prevention revealed the move on Tuesday, after discovering that six women between the ages of 18 and 48 years old who received the vaccine had developed blood clots and one died. The severe side effects were rare-more than 6.8 million doses of J&J’s shot have been administered in the US-but health authorities stated that they moved quickly out of excessive caution. “The fact that a pause was done, I think, is just a testament to how seriously we take safety,” said Anthony Fauci, the government’s elite infectious-disease official. The action prompted the US government to suspend all its vaccination sites administration of J&J’s vaccine, while some states and other authorities moved to administer other authorized shots. Some countries, meanwhile, also moved quickly to limit the shot’s use.