US health authorities are calling for a pause in the use of the Johnson & Johnson Covid-19 vaccine, after reports of extremely rare blood clotting cases.
The Food and Drug Administration (FDA) said it was acting "out of an abundance of caution".
It said six cases of severe blood clotting had been detected in more than 6.8 million doses of the vaccine.
The recommendation follows similar rare cases in the AstraZeneca vaccine, which has prompted some curbs in its use.
In a series of tweets, the FDA said it and the Centers for Disease Control and Prevention (CDC) were reviewing "six reported US cases of a rare & severe type of blood clot in individuals after receiving the vaccine. Right now, these adverse events appear to be extremely rare".
"We are recommending a pause in the use of this vaccine out of an abundance of caution," it said.
This was to "ensure that the health care provider community is aware of the potential for these adverse events".
A joint statement from the FDA and CDC clarified that the blood clotting was cerebral venous sinus thrombosis (CVST).
It said that this type of blood clot needed a different treatment than usual.
The common treatment - an anticoagulant drug called heparin - "may be dangerous", it said and an alternative was required.
All six cases were in women aged between 18 and 48, with symptoms six to 13 days after vaccination.
The New York Times quoted officials as saying one woman had died and a second, in Nebraska, was in a critical condition.
Johnson & Johnson statement
The joint statement said that "people who have received the J&J vaccine who develop severe headache, abdominal pain, leg pain, or shortness of breath within three weeks after vaccination should contact their health care provider".
Johnson & Johnson, a US health care company, issued a statement saying that safety was its "number one priority" and that it shared "all adverse event reports" with the health authorities.
It added: "We are aware that thromboembolic events including those with thrombocytopenia have been reported with Covid-19 vaccines. At present, no clear causal relationship has been established between these rare events and the Janssen (J&J) Covid-19 vaccine."
It said it would continue to work closely with the regulators.
AstraZeneca vaccinations
The Oxford-AstraZeneca vaccine, which has been given safely to tens of millions of people, has also seen some extremely rare blood clotting cases.
The reports led some nations to suspend its use but most have now resumed, although in a number of cases with a recommended minimum age, for example 60 and over in Germany.
In the UK, authorities advised that those under 30 should be offered an alternative.
https://www.bbc.com/news/world-us-canada-56733715
