On Monday, Germany, France and Italy joined a group of smaller European countries that have temporarily stopped administering vaccine, saying the move was precautionary amid a small number of cases of serious blood clotting reported on the continent.

AstraZeneca and U.K. medicine regulators say there is no evidence showing a connection between clotting and the vaccine. The shot has been given to around 11 million people in the U.K., and officials there have said there is no data suggesting a link with blood clotting. AstraZeneca has said based on data on people who have been vaccinated, cases of blood clotting among those recipients is lower than the general population. The European Medicines Agency, which regulates drugs in the European Union, has also said it has seen no link.

Still, the EMA is conducting a deeper probe into the cases and is expected to provide a recommendation as early as Tuesday. The vaccine isnt approved for use in the U.S.

Q: How does the vaccine work?

Unlike shots by

Pfizer Inc.

and

Moderna Inc.,

which both use a novel genetic technique called mRNA, the AstraZeneca shot employs a method called viral-vector delivery. The shot uses a modified chimpanzee cold virus to ferry genetic material into human cells to trigger immunity.

Q: Why isnt it approved in the U.S.?

AstraZeneca is just wrapping up large-scale human trials in the U.S. Once those are analyzed and releasedand assuming they show the vaccine to be safe and effective against Covid-19the company can apply for emergency use authorization from the Food and Drug Administration.

Q: Where is it being used?

The U.K. has administered about 11 million doses, a key reason why that countrys rollout is going quicker than others. The European Union has administered about six million doses. It has been approved for use in India and dozens of other countries.

AstraZeneca promised to make three billion doses of the vaccine at no profit. Because of that pledgeand its promise to also distribute the shot around the worldit has become a key plank in vaccinating the developing world.

Q: How effective is the shot?

In results from late-stage human trials late last year, AstraZeneca showed a range of results for effectiveness against symptomatic Covid-19, between 62% and 90%. The highest effectiveness, however, only came in a small subgroup of relatively young trial participants, who all received a smaller initial dose. The results of U.S. trials should provide a more definitive efficacy rate.

In the U.K., where it has been most widely used, researchers have found that among older Britons who have received it so far, the shot was highly effectivearound 80%in preventing serious disease.

Q: Whats the safety concern in Europe?

Last week, reports surfaced of a potential clotting issue, with one death and a case of severe illness, in Austria. That country suspended one batch of the vaccine but said it didnt have evidence of a connection between the health incidents and the shot and kept using it otherwise.

On Thursday, Denmark, Norway and Iceland halted use of the vaccine altogether, citing additional cases and concerns. Danish authorities said they would wait at least two weeks before administering it again. The EMA, which acts much like the Food and Drug Administration in regulating medicine across the European Union, has already said serious blood clots werent any more common among vaccinated people than among the general population. It has said it is investigating the reported cases of multiple thrombosis, or the formation of blood clots within blood vessels, and similar conditions.

Alerts and web-browser tools can help you book a Covid-19 vaccine appointment. WSJs Joanna Stern met up with Kris Slevens, an IT guy who has booked over 300 appointments for New Jersey seniors, to learn the best tricks to compete in the vaccine-booking Hunger Games. Photo illustration: Emil Lendof for The Wall Street Journal

Write to Jenny Strasburg at [email protected]

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