Medical experts in the United States are trying to assuage fears that Covid-19 vaccines may be unsafe after several European countries suspended AstraZeneca‘s shot following reports of blood clots among some recipients.

On Tuesday, Sweden, Latvia and Lithuania became the latest countries to join a growing list of nations suspending the use of the AstraZeneca-Oxford shot over blood clot concerns. Germany, France, Italy and Spain all said on Monday said they would also stop administering the shot.

The European Medicines Agency, which evaluates drug safety for the EU, called a meeting Thursday to review the findings. So far, it’s maintained that the benefits of the shot when it comes to preventing hospitalizations and deaths still “outweigh the risks of side effects.” The World Health Organization agreed, urging countries on Wednesday to continue using AstraZeneca’s shots.

Without the results from the European Medicines Agency’s forthcoming meeting, it’s hard to say whether the vaccines are causing the reported blood clots, medical experts in the U.S. told CNBC, but the pharmaceutical giant already has a public relations mess on its hands. Some doctors in the U.S. are worried that the European nations are prematurely responding to political pressure and safety fears, and it will take extensive efforts to rebuild trust in the vaccine if allowed back online.

“There’s now been a pall over this vaccine,” Dr. William Schaffner, an epidemiologist and professor of preventive medicine at Vanderbilt University, told CNBC in a phone interview.

“I think if the vaccine is cleared — not guilty — there will have to be a substantial public relations effort made in Europe and around the world in order to restore confidence in this vaccine,” he said.

No red flags in U.S.

While the AstraZeneca vaccine hasn’t been authorized for use in the U.S. just yet, White House Chief Medical Officer Dr. Anthony Fauci told lawmakers Wednesday that there will likely be enough safety and efficacy data to grant the doses authorization in April.

When asked whether AstraZeneca’s suspension in European countries could stoke fear among Americans taking other vaccines, Fauci reiterated that the shots undergo rigorous clinical trials and are reviewed by an independent safety monitoring board before they’re widely distributed.

“The entire process is both transparent and independent, and we explain that to people and take the time to address their hesitancy without being confrontative,” Fauci told lawmakers during a hearing with the House Committee on Energy and Commerce.

This isn’t the first time Fauci has stressed the safety of the current vaccines amid AstraZeneca’s suspension. The infectious disease expert told MSNBC in an interview Tuesday that scientists in the U.S. carefully evaluate for adverse reactions among vaccine recipients, even after they’re authorized and deployed.

For instance, medical experts were concerned about reports of severe allergic reactions — or anaphylaxis — occurring among people who were vaccinated with Pfizer and Moderna’s jab. However, those cases appear to be rare, he said, even as the nation has distributed at least one shot to 73 million adult Americans — more than 28% of the population.

“Thus far, and you have to keep following these things very carefully, there are no safety signals that turn out to be red flags,” Fauci said regarding the currently deployed vaccines in the U.S.

Dr. Francis Collins, director of the National Institutes of Health, told Reuters in an interview published Monday that he has been “pretty reassured” by statements from European regulators that the problems could be occurring by chance.

“I was a bit surprised that so many countries decided to put pause on the administration of the vaccine, especially at a time where the disease itself is so incredibly threatening in most of those countries,” Collins later told CNN on Wednesday, adding that he doesn’t have access to the “primary data that might have caused them to be alarmed.”

More data needed

Adverse medical problems like blood clots happen whether people are vaccinated or not. The problem scientists are now trying to determine is whether the vaccines were the culprit, Schaffner said.

“We knew in the beginning as we started to vaccinate, given the fact that we are targeting older adults, medical events occur in that population just every day, even without vaccines,” Schaffner told CNBC.

“It’s possible that if you get vaccinated on Monday, certain medical events will occur on Tuesday, Wednesday, Thursday and Friday,” he said. “The question is: Did the vaccine accelerate, precipitate or cause these events?”

For its part, AstraZeneca said in a response statement on Sunday that of the more than 17 million people in the EU and the U.K. who have received a dose of the Oxford-AstraZeneca vaccine, fewer than 40 cases of blood clots had been reported as of last week.

The pharmaceutical giant said that across the EU and U.K. there had been 15 events of deep vein thrombosis and 22 events of pulmonary embolism reported among those vaccinated. Those figures would suggest that the adverse events are occurring at a lower rate than what would be expected in the general population, not higher.

“I don’t think this is real, but I’m very concerned because this is the vaccine that we were all counting on globally,” Dr. Carlos del Rio, a professor of medicine at the Emory University School of Medicine, told CNBC in a phone interview, adding that the shot costs less than its competitors. Del Rio noted that without the data, however, it’s hard to determine whether the suspensions are appropriate.

“This will require major damage control,” del Rio said.

Politics could be the problem

There are some concerns that the problem with AstraZeneca’s vaccine may be more political. It also comes at a dangerous time: Some European nations are battling yet another wave of new Covid-19 infections even as vaccines are deployed.

So far, the E.U.’s vaccine rollout has been sluggish compared with other countries, like the U.S. and the U.K.

“It is a big worry that Europe just doesn’t have that many people vaccinated,”  Dr. Ezekiel Emanuel, a former Covid advisor to President Joe Biden, told CNBC on Tuesday. “It’s another reason that we have to be worried about the situation of Covid in other countries, not just in the United States. 

The suspensions follow a public dispute between the EU and AstraZeneca in January when the pharmaceutical company said it was forced to cut its initial supply of doses to the bloc short. Several European countries also initially declined to recommend the shot to residents over-65s, saying there was insufficient evidence to show it was effective, before reversing that decision.

“It may be that … the governments are trying to respond to people’s worries about the vaccine and not necessarily the data,” said Emanuel, a bioethicist and oncologist who serves as vice provost for global initiatives at the University of Pennsylvania.

“Actions don’t necessarily follow the data. They follow more emotional responses to these kind of things,” he said.

— CNBC’s Sam Meredith, Holly Ellyatt and Silvia Amaro contributed to this report.