“We have been reviewing these cases with European health authorities,” the company said. “We have made the decision to proactively delay the rollout of our vaccine in Europe.”

Biden’s top medical adviser Anthony Fauci stressed that,given six clotting cases have been identified and around seven million Johnson&Johnson doses have been administered,the chance of any serious side effects appeared to be less than one in a million.

Nevertheless,he said it was important for health experts to investigate the clotting issue and make recommendations to medical staff and the American public.

“We want to get this worked out as quickly as we possibly can,” he said.

The women were between the ages of 18 and 48 and experienced symptoms six to 13 days after vaccination. One woman died,and another remains in critical condition,federal health officials said.

The brain clots,known as cerebral venous sinus thrombosis,occurred in combination with low levels of blood platelets.

Fauci said there had been no “red flag signals” for the Pfizer and Moderna vaccines,which are the most widely used vaccines in the US.

The Johnson&Johnson vaccine is based on the adenovirus technology,as is the AstraZeneca shot which has also beenlinked to extremely rare cases of blood clots. The Moderna and Pfizer vaccines use a different technology known as mRNA.

Health experts from the FDA and CDC earlier said they expected the pause on the Johnson&Johnson vaccine to last for a “a matter of days”.

One possibility being examined is whether certain demographics – for example,women aged under 50 – should be directed to take the Pfizer and Moderna vaccines rather than the Johnson&Johnson.

The Morrison government this weekrevealed that it would not seek to purchase the Johnson&Johnson vaccine for use in Australia because it was too similar to the AstraZeneca jab.