There is no indication that the Oxford-AstraZeneca Covid-19 vaccine is linked to an increased risk of blood clots, the EU's medicines regulator says.
It said the number of cases in vaccinated people was no higher than in the general population.
The statement came after a number of countries, including Denmark, Norway and Iceland, suspended the use of the jab.
The suspension followed reports that a small number of people had developed clots after receiving the vaccine.
There were also reports that a 50-year-old man had died in Italy after developing deep vein thrombosis following a dose of the jab.
"There is currently no indication that vaccination has caused these conditions, which are not listed as side effects with this vaccine," the European Medicines Agency (EMA) said on Thursday.
"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," it added.
It said there had been 30 cases of "thromboembolic events" among the five million Europeans who have received the jab.
AstraZeneca said the drug's safety had been studied extensively in clinical trials. "Regulators have clear and stringent efficacy and safety standards for the approval of any new medicine," a spokesperson said.
In the UK, the Medicines and Healthcare products Regulatory Agency (MHRA) said there was no evidence the vaccine had caused problems, and people should still go and get vaccinated when asked to do so.
"Blood clots can occur naturally and are not uncommon. More than 11 million doses of the Covid-19 AstraZeneca vaccine have now been administered across the UK," Phil Bryan of the MHRA said.
The decision to temporarily suspend the use of AstraZeneca's jab has come as a setback for a European vaccination campaign that has stuttered into life, partly due to delays in delivery of the drug.
However, there was a positive development on Thursday as the European Medicines Agency approved the single-shot Johnson & Johnson (J&J) vaccine.
"More safe and effective vaccines are coming to the market," EU Commission President Ursula von der Leyen tweeted, although some reports suggest that J&J vaccine shipments may not arrive until April.
Austria earlier stopped using a batch of AstraZeneca shots while investigating a death from coagulation disorders and an illness from a pulmonary embolism.
Still, the European medicine regulator EMA said the vaccine's benefits outweighed its risks and could continue to be administered.
Europe is struggling to speed up a vaccine rollout after delivery delays from Pfizer and AstraZeneca, even as a spike in cases amid a more contagious virus variant has triggered fresh lockdowns in countries like Italy and France.
Denmark suspended the shots for two weeks after a 60-year-old woman, who was given an AstraZeneca shot from the same batch used in Austria, formed a blood clot and died, Danish health authorities said.
Their response was also prompted by reports "of possible serious side effects" from other European countries.
"It is currently not possible to conclude whether there is a link. We are acting early, it needs to be thoroughly investigated," Health Minister Magnus Heunicke said on Twitter.
The vaccine would be suspended for 14 days in Denmark.
"This is a cautionary decision," director of infection prevention and control at the Norwegian Institute of Public Health (FHI) Geir Bukholm, told a news conference.
FHI did not say how long the suspension would last.
"We ... await information to see if there is a link between the vaccination and this case with a blood clot," Bukholm said.
Iceland has suspended jabs with the vaccine as it awaited the results of an investigation by the EMA. Italy, also on Thursday, said it would suspend use of an AstraZeneca batch different to the one used in Austria.
Romanian authorities have also temporarily stopped vaccinating people with one batch of the vaccine as an "extreme precaution" while deaths in Italy are investigated, but are continuing to use other doses from the company, a health agency said.
Some health experts said there was little evidence to suggest the AstraZeneca vaccine should not be administered and that the cases of blood clots corresponded with the rate of such cases in the general population.
"The problem with spontaneous reports of suspected adverse reactions to a vaccine are the enormous difficulty of distinguishing a causal effect from a coincidence," professor of pharmacoepidemiology Stephen Evans at the London School of Hygiene and Tropical Medicine, told Reuters.
Evans added that the Covid-19 disease was very strongly associated with blood clotting.
Vaccine well-tolerated - AstraZeneca
AstraZeneca told Reuters in a written statement that the safety of its vaccine had been extensively studied in human trials and peer-reviewed data confirmed it was generally well tolerated.
The drugmaker said this week there had been "no confirmed serious adverse events associated with the vaccine". It said it was in contact with Austrian authorities and would fully support their investigation.
The European Union's drug regulator, the EMA, said on Wednesday there was no evidence so far linking AstraZeneca to the two cases in Austria.
It said the number of thromboembolic events - marked by the formation of blood clots - in people who have received the AstraZeneca vaccine was no higher than that seen in the general population, with 22 cases reported among the 3 million people who have received the shot as of 9 March.
EMA said it understood the decision by Denmark and Norway was taken as a precaution.
Four other countries - Estonia, Lithuania, Luxembourg and Latvia - have stopped inoculations from the batch while investigations continue, the EMA said.
The batch of 1 million doses went to 17 EU countries.
Swedish authorities said they did not find sufficient evidence to stop vaccination with AstraZeneca's jab.
"There is nothing to indicate that the vaccine causes this type of blood clots," the head of drug safety at the Swedish Medical Products Agency, Veronica Arthurson, told a news conference.
The Danish Medicines Agency said it had launched an investigation into the vaccine together with corresponding agencies in other EU countries and the EMA.
So far, 138,148 Danes have received a shot with AstraZeneca's vaccine in a country of 5.8 million. The Nordic country, which also uses vaccines from Pfizer-BioNTech and Moderna, is set to receive 2.6 million doses from AstraZeneca over the coming months.
Denmark's Health Authority said the final date for when it expects all Danes to have been fully vaccinated would be pushed back by four weeks to 15 August.
Spain on Thursday said it had not registered any cases of blood clots related to AstraZeneca's vaccine so far and would continue administering the shots.
US warns EU over supplies
Washington has told the European Union that it should not expect to receive AstraZeneca Covid-19 vaccines manufactured in the United States any time soon, two EU sources said on Thursday, in a new blow to the bloc's supplies.
The US message could complicate vaccination plans in the EU, which has been grappling since January with delays in deliveries from vaccine makers.
"The US told us there was no way it would ship AstraZeneca vaccines to the EU," said a senior official directly involved in EU-US talks.
AstraZeneca told the EU earlier this year it would cut its supplies in the second quarter by at least half to less than 90 million doses, EU sources told Reuters, after a bigger reduction in the first three months of the year.
Later, however, AstraZeneca offered to partly plug the gap with vaccines produced outside Europe, including in the US.
A senior EU diplomat said the European Commission told member states' diplomats at a meeting in Brussels on Wednesday that the bloc should not expect any exports from the United States "at this point in time".
"Basically the situation is such that any exports are tricky, but there is a willingness to talk," the diplomat said.
AstraZeneca declined to comment.
White House Press Secretary Jen Psaki told reporters the US government had been clear publicly and privately that it would focus first on ensuring Americans were vaccinated, but said direct purchases would be up to countries and companies.
"Our first focus is in ensuring the American people are vaccinated," she said. "Of course any country can purchase vaccines from these manufacturers directly."
It is not known what is behind the possible move on exports. It is unclear whether AstraZeneca is producing much vaccine in the US or if the US would apply a restrictive measure on trade. AstraZeneca's vaccine has not yet been approved for use in the United States.
The US stance could jeopardise AstraZeneca's attempts to bring deliveries closer to its contractual obligation with the EU of 180 million doses in the second quarter.
AstraZeneca's Covid-19 vaccines are produced in the United States in a plant near Baltimore run by Catalent, which is listed in the EU supply contract with AstraZeneca as a "back-up supply site" and has been authorised by the EU drugs regulator as a manufacturer of vaccine ingredients.
- Reuters / BBC