LONDON (AP) — Experts at the agency that regulates drugs for the European Union said Tuesday that they found a “possible link” between the Johnson & Johnson COVID-19 vaccine and very rare blood clots after a small number of cases were reported in the United States.

The European Medicines Agency said a warning about very unusual blood clots should be added to labels for Johnson & Johnson’s COVID-19 vaccine. The agency said these rare blood disorders should be considered “very rare side effects of the vaccine.”

Last week, Johnson & Johnson halted its European roll-out of the vaccine after U.S. officials recommended a pause in the vaccine, when they detected six very rare blood clot cases among nearly 7 million people who had been vaccinated.

European officials said they considered all currently available evidence from the U.S., including eight reports of serious cases of rare blood clots associated with low blood platelets, including one death.

J&J advised European governments to store their doses until the EU drug regulator issued guidance on their use; widespread use of the shot in Europe has not yet started.

The delay was a further blow to vaccination efforts in the European Union, which have been plagued by supply shortages, logistical problems and concerns over unusual blood clots also in a small number of people who received the AstraZeneca COVID-19 vaccine. Experts worry the temporary halt on J&J’s shot could further shake vaccine confidence and complicate worldwide COVID-19 immunization efforts.

Last week, South Africa suspended its use of the vaccine in the wake of the U.S. pause, and countries including Italy, Romania, the Netherlands, Denmark and Croatia put their J&J doses into storage.

The blood clots linked to the J&J vaccine are occurring in unusual parts of the body, such as veins that drain blood from the brain. Those patients also have abnormally low levels of blood platelets, a condition normally linked to bleeding, not clotting.

With the AstraZeneca vaccine, scientists in Norway and Germany have suggested that some people are experiencing an abnormal immune system response, forming antibodies that attack their own platelets.

It’s not yet clear if there might be a similar mechanism with the J&J vaccine. But both the J&J and AstraZeneca vaccines, as well as a Russian COVID-19 vaccine and one from China, are made with the same technology. They train the immune system to recognize the spike protein that coats the coronavirus. To do that, they use a cold virus, called an adenovirus, to carry the spike gene into the body.

Earlier this month, the EU drug regulator said there was a “possible link” between the AstraZeneca shot and rare blood clots but said the benefits of vaccination far outweigh the risks of COVID-19. It noted the risk is less than the blood clot risk that healthy women face from birth control pills.