halted vaccinations from one batch of AstraZeneca’s vaccine. A company spokesman said at the time that no serious vaccine side effects had been confirmed.

several more countries had announced plans to suspend their vaccination campaigns, AstraZeneca issued a news release with more specifics on the numbers of side effects reported and people vaccinated in clinical trials and in immunization campaigns in Europe.

On Monday, an AstraZeneca spokesperson said the company was “working with national health authorities and European officials and look forward to their assessment later this week.” (The company refused to name the spokesperson.)

Past vaccine safety concerns have not usually been borne out by the evidence, though there are exceptions. In 1999, the first vaccine against the rotavirus infection that causes diarrhea in infants was pulled off the market because it was found to increase risk of obstruction of the intestine.

But even unsubstantiated safety concerns have deflated public confidence.

After the Food and Drug Administration approved a highly effective vaccine for Lyme disease in 1998, media reports and a class-action lawsuit gave fuel to complaints from people who said they had developed arthritis after being vaccinated. In 2001, the F.D.A. reconvened a panel of experts to review the vaccine’s safety data — clinical trials had turned up no difference in the frequency of long-term joint symptoms between the vaccine and placebo groups — and concluded that the vaccine should stay on the market.

But by then, public perception of the vaccine was too far gone: A steep drop-off in sales spurred the vaccine’s maker, GlaxoSmithKline, to pull the vaccine from the market in 2002.

While more than 70 countries have authorized the vaccine, the United States has not. AstraZeneca has not yet applied to the Food and Drug Administration for authorization and is waiting for results from its U.S. trial that enrolled more than 32,000 participants.

told Reuters on Monday.

Strong safety and efficacy results from that trial could go a long way in reassuring people about AstraZeneca’s vaccine, said Geoffrey Porges, an analyst for the investment bank SVB Leerink.

“But the longer this hangs out there, without a comprehensive review and without the U.S. Phase 3 results, the more people are going to be concerned,” Mr. Porges said.

View Source