A growing list of countries, mostly in Europe, have put the AstraZeneca vaccine on hold following reports of blood clots in a handful of patients following vaccination.
They acknowledge there is no evidence that the vaccine is causing the problems.
"Today's decision is a purely precautionary measure," German Health Minister Jens Spahn said Monday.
Some experts say the decision may do more harm than good.
Children's Hospital of Philadelphia vaccine expert Paul Offit told VOA it is a bad idea to stop an effective vaccine in the middle of a pandemic over theoretical concerns.
"There is nothing theoretical about this virus, which is killing thousands of people every day," he said. "I would argue that the choice to just withhold that vaccine is not a conservative choice, but rather, a radical choice."
AstraZeneca's vaccine is one of the most widely used in the world. It makes up the bulk of the WHO-backed COVAX facility, which aims to reduce inequities between high- and lower-income countries in vaccine access. More than 40 countries have authorized the vaccine.
Health officials say they aim to balance the vaccine's importance in fighting the pandemic with the risk, however slim, that it is causing adverse reactions.
"There is good evidence that the vaccine is both safe and effective," said Søren Brostrøm, head of the Danish Health Authority. "But both we and the Danish Medicines Agency have to react to reports of possible serious side effects."
Denmark was the first to announce a pause last week following a report of a 60-year-old woman who died of a blood clot after receiving the AstraZeneca vaccine.
"At present, it cannot be concluded whether there is a link between the vaccine and the blood clots," the Danish Health Authority said Thursday.
Austria put on hold one batch of the vaccine after another patient died from blood clots 10 days after receiving the shot. Several other countries have reported a handful of cases.
Several countries, including Germany, France and Italy, suspended the AstraZeneca vaccine pending a review by the European Medicines Agency (EMA), the European Union's drug regulatory authority.
Problem blood clots are not happening any more often in vaccinated people than in unvaccinated people, World Health Organization Chief Scientist Soumya Swaminathan told reporters Monday.
"People do get (blood clots) and people die every day," she said. "The question really is the linkage with the vaccine."
Rates of blood clots are lower among the 17 million people in Europe and Britain who received the vaccine than in the general population, AstraZeneca noted in a statement.
COVID-19 itself can cause blood clots and other coagulation issues, noted Stephen Evans, pharmacoepidemiology professor at the London School of Hygiene and Tropical Medicine.
While it is reasonable to study the issue, he said, "it seems a step too far in taking precautions that would stop people getting vaccines that would prevent disease."
More than 2.6 million people have died of COVID-19 worldwide. Meanwhile, there have been no deaths linked to the vaccine among the 300 million people who have received at least one dose, WHO's Swaminathan noted.
"You need to (compare) the benefit of protecting people against a disease that's killing millions, against the potential risks," she said.
WHO continues to recommend the vaccine, as does the EMA.
Officials at WHO and EMA are meeting Tuesday to consider the evidence and decide whether they need to change their recommendations.
Even if they reaffirm that the vaccine is safe, confidence in it may already have suffered damage that will be hard to repair, Offit said.
"You scare people. It's hard to unscare them," he said. "It's hard to unring the bell."