The global rollout of the Oxford-AstraZeneca Covid-19 vaccine has encountered another hurdle as a handful of countries paused their use of the shot following reports it could be connected to blood clots, despite no clear evidence of a link.

AstraZeneca has robustly defended its vaccine, saying Friday there was “no evidence of an increased risk” of blood clots, and European and UK medicines regulators have each said the link between the vaccine and blood clots has not been confirmed and that rollouts should continue.

After a group of European countries — including Denmark, Norway and Iceland — suspended use of the vaccine on Thursday, Thailand's Prime Minister, Prayut Chan-o-cha, cancelled plans to publicly get the AstraZeneca shot on Friday and the country also delayed its rollout.

“When there is an adverse event, we don't need to be in rush,” said Piyasakol Sakolsatayadorn, a senior member of the country's vaccination committee.

Bulgaria has become the latest country to suspend use of the vaccine on Friday pending investigations into safety. Prime Minister Boyko Borissov ordered a halt to all inoculation using the AstraZeneca Covid-19 vaccine until the European Medicines Agency “rejects all doubts” about the vaccine's safety, according to a government statement.

The moves came in response to reports of blood clots in a few inoculated people in Denmark, including one fatality. Denmark was the first country to take the precautionary measure, announcing a 14-day break while authorities investigated further.

Norway and Iceland soon followed. The Norwegian Institute of Public Health said the country has also reported cases of blood clots shortly after receiving a Covid-19 vaccination in Norway but “mainly in the elderly where there is often another underlying disease as well.”

Other countries, including Austria and Italy, have suspended specific batches of the vaccine.

But a number of nations — including Germany, France, the UK, the Netherlands, Mexico and Nigeria — stood by the shot and reassured citizens of its safety.

Health agencies tell countries to keep rolling out

The European Medicines Agency (EMA) said Thursday that it did not recommend suspending the use of the AstraZeneca vaccine, stating that there is “no indication” the vaccine caused the blood clots in the people who received the vaccine. The agency told countries they could keep rolling out the shot while investigations take place.

“The vaccine's benefits continue to outweigh its risks and the vaccine can continue to be administered while investigation of cases of thromboembolic events is ongoing,” the agency said.

The UK's medicine regulator — the Medicines and Healthcare products Regulatory Agency (MHRA) — also issued a statement Thursday reassuring the public that the vaccine is still safe and that “people should still go and get their COVID-19 vaccine.”

The episode nonetheless poses another headache for the pharmaceutical giant, whose vaccine has been beset by political disputes, delivery delays and other concerns.

But the tone across most of Europe was one of calm, as other governments sought to put the reports into context.

“Investigations are carried out systematically each time serious adverse effects are declared,” France's health minister Olivier Véran said in his weekly briefing on Thursday. “But what are we talking about? About 30 people out of more than five million Europeans having received an injection.”

The UK's MHRA, where 11 million doses of the vaccine have been administered, said blood clots “can occur naturally and are not uncommon.” Its vaccines safety lead Phil Bryan added that the reports of blood clots “are not greater than the number that would have occurred naturally in the vaccinated population.”

And Germany confirmed late on Thursday that it would stick to its rollout plans. “We are planning to continue vaccinating with AstraZeneca, just like an overall majority of other European countries,” German health minister Jens Spahn said.

‘No evidence’ of clotting link

AstraZeneca said Friday that its analysis not only shows “no evidence of an increased risk” of blood clots in vaccine recipients, but demonstrates a lower number than in the general population.

“An analysis of our safety data of more than 10 million records has shown no evidence of an increased risk of pulmonary embolism or deep vein thrombosis in any defined age group, gender, batch or in any particular country with COVID-19 Vaccine AstraZeneca,” the company said in a statement.

“In fact, the observed number of these types of events are significantly lower in those vaccinated than would be expected among the general population,” it added.

Denmark's health leaders had stressed their decision to announce a two-week pause on its use of the vaccine was a precautionary one, reminding people there is “good evidence that the vaccine is both safe and effective” but saying they would “act early” to investigate the reports of clotting.

Spain delayed giving the jab to those aged between 55 and 65 until a review is conducted, but Dutch health minister Hugo de Jonge said Thursday there is “no cause for concern,” and no reason to stop using the AstraZeneca vaccine.

Earlier this week, a number of EU nations paused the use of doses that came from a particular batch of AstraZeneca vaccine, after a 49-year-old woman in Austria died of multiple thrombosis on Sunday. The EMA said Wednesday there was “no indication” that vaccination had been behind the cases of clotting or death.

And on Thursday, Italian medicines agency AIFA also banned use of another batch of AstraZeneca vaccines. The agency said it was responding to “some serious adverse events” taking place around the time of vaccinations from one specific batch. It did not say what the events were and said no causal link between the events and the vaccine had been established.

Nigeria, which has started receiving millions of vaccine doses through the COVAX program, defended the vaccine Thursday, saying it is satisfied with the clinical evidence indicating the shot is “safe and effective” and urging its citizens to continue to participate in the rollout.

Real-world data has also shown that the vaccine is having a significant impact in reducing Covid-19 hospitalizations.

A single dose of the vaccine reduces the risk of hospitalization from Covid-19 by more than 80% in people aged over 80, data from Public Health England showed earlier this month. The vaccine is given in two doses, though countries differ in how far apart they are spreading those shots.

Concerns about the vaccine's safety nonetheless come at an awkward time for AstraZeneca, with disputes over its supplies to the EU still unresolved. After announcing it would pause the vaccine, Denmark's health authority said Thursday the country would now receive approximately 900,000 fewer doses of the shot.

“The fact that AstraZeneca is once again downgrading the number of doses delivered to the EU and thus Denmark is, of course, both unsatisfactory and a serious challenge,” Ole Jensen, deputy director at the Statens Serum Institut, said.

Polish officials also announced on Friday that AstraZeneca will reduce the number of coronavirus vaccine doses delivered there.