Europe's medicine regulator said on Tuesday that it finds possible link between Johnson & Johnson's (J&J) COVID-19 vaccine and very rare cases of unusual blood clots with low blood platelets.

The European Medicines Agency (EMA) said a warning about the rare blood clots should be added to labels for J&J's COVID-19 vaccine. The agency said these rare blood disorders should be considered as "very rare side effects of the vaccine."

Last week, J&J halted its European roll-out of the vaccine after U.S. officials recommended a pause in the vaccine when they detected six 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., which consisted of 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.

On Monday, World Health Organization Director-General Tedros Adhanom Ghebreyesus said more than 5 million new coronavirus cases were confirmed worldwide last week, the highest-ever number in a single week. He noted that cases and hospitalizations among younger people were "increasing at an alarming rate."

(With input from AP.)