The US recommendation came on the heels of a finding last week from the EU’s European Medicines Agency (EMA) of a possible link between a vaccine made by British-Swedish firm AstraZeneca – of which more than 30 million doses have been administered in Europe – and rare cases of blood clots.

01:46

One-shot coronavirus vaccine by Johnson & Johnson is safe and effective, US FDA finds

One-shot coronavirus vaccine by Johnson & Johnson is safe and effective, US FDA finds

The products made by CanSino, AstraZeneca, Johnson and Johnson and Russia’s Gamaleya Institute are all adenovirus-vectored vaccines. They work by taking a virus known to cause common colds and engineering it to trigger an immune response to the pathogen that causes Covid-19.

While the technology is the same for all four products, their developers used different formulations of adenoviruses to construct their vaccines.

CanSino’s product is based on a common cold virus known as Ad5, while Johnson and Johnson used a rarer human adenovirus. The Gamaleya Institute used a combination of the two for its Sputnik V vaccine, and AstraZeneca relied on a virus found in chimpanzees.

CanSino noted this difference its announcement to investors on Wednesday, but said the company “would closely monitor the situation and make further announcement(s) as and when appropriate”.

06:18

SCMP Explains: What’s in a Covid-19 vaccine?

SCMP Explains: What’s in a Covid-19 vaccine?

Gamaleya, which developed the Sputnik V vaccine, said in a statement that a “comprehensive analysis of adverse events during clinical trials and over the course of mass vaccinations with the Sputnik V vaccine showed there were no cases of cerebral venous sinus thrombosis”, referring to the type of brain blood clot seen in the cases under investigation in Europe and the US.

The maker noted several technical differences between its vaccine and those of AstraZeneca and Johnson and Johnson, adding “all vaccines based on adenoviral vector platform are different and not directly comparable”.

The US FDA said that while it was investigating whether there was a link between the Johnson and Johnson vaccine and the six blood-clotting cases – all of them women aged between 18 and 48 – the adverse events appeared to be “extremely rare”.

The EMA said it had received reports of 169 cases of a brain blood clot known as cerebral venous sinus thrombosis and 53 cases of blood clots in the abdomen following AstraZeneca vaccinations as of April 4, and after 34 million doses had been administered in the European Economic Area – the EU plus Norway, Liechtenstein and Iceland.

Most cases were in women under 60 years of age, and there were 18 deaths among cases as of March 22.

While experts have repeatedly said that the incidence of blood clots has yet to be linked to the vaccines, some have said it could point to an issue with the adenovirus platform.

“Although it’s too soon to draw any firm conclusions, this development does raise the possibility that at least some adenovirus vectors either of themselves or in combination with the SARS CoV2 S protein gene can cause this idiosyncratic reaction in a very small proportion of individuals,” Adam Finn, a professor of paediatrics at the University of Bristol in England, said in a report published by the Science Media Centre.

An advisory committee is expected to meet on Wednesday in the US to review the cases following the Johnson and Johnson jabs. The EMA’s safety committee said last week it was also reviewing the reports of blood clotting in that vaccine. No causal link has been established.

Several European countries including Britain and Italy have made recommendations to limit the use of the AstraZeneca vaccine in younger people, following the EMA decision to list unusual clotting as a rare side effect of the vaccine last week.