The European Medicines Agency (EMA) concluded Wednesday that "unusual blood clots with low blood platelets" should be listed as a "very rare" side effect of AstraZeneca's COVID-19 vaccine, but that the benefits of the shot still outweigh the risks.

Why it matters: The AstraZeneca vaccine is the centerpiece of the global COVAX initiative, and one of the very few vaccines that is available, affordable and easy to store for many developing countries.

• The concerns about AstraZeneca in Europe have already spread to countries in Africa, some of which have delayed their rollouts and warned of increased vaccine hesitancy.
• The vaccine has been administered to over 20 million people in Europe and has played a key role in Britain's world-leading vaccine rollout.

U.K. health officials said at their own press briefing Wednesday that due to the rare cases of blood clots, Britons under the age of 30 will be offered alternative vaccines to AstraZeneca when possible.

What they're saying: "EMA is reminding healthcare professionals and people receiving the vaccine to remain aware of the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within 2 weeks of vaccination," the agency said in a statement.

• Most of the cases reported so far have occurred in women under 60 years of age within two weeks of vaccination, according to the EMA. Specific risk factors are unconfirmed.
• The blood clots occurred in veins in the brain and the abdomen, the EMA reports, and people who develop symptoms of this combination of blood clots and low blood platelets should seek immediate medical assistance.

The bottom line: The regulator still concluded that the benefits of the AstraZeneca vaccine outweigh the risks and that it is effective at preventing COVID-19 hospitalizations and death.

This story is breaking news. Please check back for updates.