However, the rate at which several nations discontinued use of the vaccine reflected a reluctance about its safety and effectiveness that contrasts sharply with the confidence shown in other vaccines. Confidence in the vaccine has fallen in Germany, France, Italy, Spain and, to a lesser extent, the UK, according to surveys.

Participants who received the vaccine in the study did not have an increased risk of blood clots or related diseases. And a specific search did not find any cases of cerebral venous sinus thrombosis – blood clots in the brain that can cause dangerous bleeding – which raise some of the most serious concerns in Europe.

Michael Head, Senior Research Fellow on Global Health at the University of Southampton in the UK, said the results could allay concerns not only in Europe but around the world. He said he had received messages in the past few days from colleagues in Ghana resenting how he could explain safety concerns to people who had just celebrated the arrival of the vaccine weeks earlier.

“Given the light raids the AstraZeneca vaccine has suffered in the past few weeks – and particularly in the last week or two in Europe – there is new data to show that it is safe and effective, if you excuse the phrase , a good shot of the arm, ”he said. “The publication of these results is actually quite well timed as the vaccine may be hesitant.”

The US study also found no cases of serious neurological problems. This was a cause for concern last summer after two volunteers at AstraZeneca’s trial in the UK became ill with neurological problems. Although these cases forced a seven-week suspension of the clinical trial in the US, the researchers ultimately concluded that the diseases could not be linked to the vaccine. Still, the delay has been a key factor in why AstraZeneca has so far lagged behind three other vaccine manufacturers that have received emergency clearance in the United States.

According to AstraZeneca, the vaccine worked well across all races and ages. The vaccine was 80 percent effective in approximately 6,000 subjects over 65 years of age – results that likely allay concerns about insufficient clinical trial data on how well the vaccine works in the elderly. Some countries temporarily limited vaccinations with the shot to younger people before reversing course to allow use in all age groups after data from the vaccine’s launch in the UK showed the vaccine offered strong protection in the elderly.

Even if the vaccine is not used in the US, obtaining emergency clearance from the FDA – whose rigorous review process is considered the global gold standard – would be a major milestone for AstraZeneca. Some countries that have not yet approved the vaccine may try to follow the FDA’s lead.