(Reuters) -The second shot of the COVID-19 vaccine developed by AstraZeneca and the University of Oxford must not be given to anyone who has had blood clots with low blood platelets after receiving the first, Europe’s medicines regulator said on Friday. The advice for healthcare professionals was provided by the European Medicines Agency (EMA) as part of an ongoing review into rare, but severe blood clots possibly linked to inoculation after the shot and also to Johnson & Johnson’s coronavirus vaccine. The EMA has been looking into such clots in the abdomen and brain since March and has recommended that both vaccine labels carry a warning on the clotting issues while maintaining that the overall benefits outweighed any risks. Both the AstraZeneca and J&J vaccine use different versions of a cold virus to deliver instructions for making coronavirus proteins into cells to produce an immune response. "While blood clots with low blood platelets following vaccination are very rare, EMA continues to advise people to be aware of symptoms ... so they can receive prompt specialised medical treatment if needed," the EMA said here on Friday. The watchdog said people should be on guard for any signs of any blood clots or low platelets within three weeks of receiving the first shot of AstraZeneca’s vaccine, Vaxzevria, and that the latest recommendations will be added to the vaccine’s product information. AstraZeneca and the University of Oxford had no immediate comment. The new guidance adds to the problems that have dogged the AstraZeneca vaccine, including supply cuts, production snags and legal action from the European Union for under-delivering on promised doses. At the same time, evidence is mounting that it is effective. On Thursday, a British study suggested that two doses of the AstraZeneca-Oxford vaccine may be around 85% to 90% effective against symptomatic COVID-19. A media report this week also said a study found that a third booster dose of the vaccine could increase antibodies in people. Reporting by Pushkala Aripaka in Bengaluru; Editing by Ramakrishnan M. And Barbara Lewis Our Standards: The Thomson Reuters Trust Principles.