Fresh concerns have emerged about the single-shot Covid-19 vaccine from Johnson & Johnson, which is expected to play a major part in the Irish vaccine roll-out.The European Medicines Agency yesterday announced a review assessing reports of rare blood clotting in people who received the vaccines.Four cases of unusual thromboembolic events with low blood platelets have now been reported; one during a clinical trial and three during the American roll-out. One person has died.
The EMA said in a statement: These reports point to a safety signal, but it is currently not clear whether there is a causal association between vaccination with Covid-19 Vaccine Janssen and these conditions. 
The EMA will shortly confirm whether further action is needed, but indicated this usually consists of an update to the product information.
This vaccine is currently only used in America but was authorised for the EU on March 11. 
Ireland was expected to receive its first doses of the J&J vaccine before the end of April, with some 605,000 doses due to arrive here by the end of June. 
Any change to this would have a significant impact on the Irish roll-out as the other vaccines all require two doses each, some spread out by 12 weeks.
The Johnson & Johnson vaccine is based on an adenovirus vector, as is the AstraZeneca vaccine.
The news of the review comes just days after the EMA confirmed a link between the AstraZeneca vaccine and very rare blood clots but stressed the benefts of the vaccine far outweigh the risk.
Separately, France has advised people under-55 who received a first shot of the AstraZeneca vaccine to be switched to either BioNTech/Pfizer or Moderna for the second. These use mRNA technology.
French health authorities previously allocated AstraZeneca to healthcare workers and mRNA vaccines to older people. Three weeks ago they limited AstraZeneca to over-55s following reports of rare blood clotting.
Yesterday, they followed Germany in announcing a mixed-dose strategy.
The World Health Organisation cautioned there is no clinical trial evidence to support mixing vaccines.
Earlier this week the EMA head of data analytics and methods task force Dr Peter Arlett said there are no European guidelines on this as no data was submitted for approval. 
He said: There is a theoretical reason to think that mixing vaccines could work and could be a safe and effective approach.”