UK medical regulators have advised against the roll-out of the AstraZeneca/Oxford vaccine to people aged under 30 after reports of rare cases of blood clots while India has reported a spike in new coronavirus cases.
The European Union regulator also said on Wednesday it had found possible links between the AstraZeneca jab and very rare cases of unusual clotting but reaffirmed the vaccine's importance in protecting people against COVID-19.
A UK government advisory group said the vaccine should not be given to the under 30s where possible, though an official said this was "really out of the utmost caution rather than because we have any serious safety concerns".
More than a dozen countries have at one time suspended use of the vaccine, which has been given to tens of millions across Europe.
But most have resumed and some including France, the Netherlands and Germany have set a minimum age.
The European Medicines Agency (EMA) received reports of 169 cases of the rare brain blood clot by early April after 34 million doses had been administered in the European Economic Area, according to Sabine Straus, chair of the EMA's safety committee.
The EEA comprises the 27 EU countries plus Iceland, Norway and Liechtenstein.
In its statement, the EMA said it was reminding health professionals and recipients to remain aware of "the possibility of very rare cases of blood clots combined with low levels of blood platelets occurring within 2 weeks of vaccination".
"So far, most of the cases reported have occurred in women under 60 years of age within 2 weeks of vaccination," it added, without issuing new guidelines.
EU health ministers began meeting soon after the EMA's statement.
AstraZeneca's shot is by far the cheapest and most high-volume launched so far and has none of the extreme refrigeration requirements of some other COVID-19 vaccines.
After extensive use in the UK and mainland Europe, it is set to be the mainstay of vaccination programs in much of the developing world.
The World Health Organisation's advisory vaccine safety panel said a causal link to blood clots with low platelets "is considered plausible but is not confirmed".
Experts say that, even if a causal link is proved, the risks to the general population of getting a serious clot are vanishingly small compared to the risks from possible COVID-19 infection, which can also cause similar clots, or from many other widely used drugs such as the birth control pill.
"The risk of mortality from COVID is much greater than the risk of mortality from these rare side-effects," EMA's executive director Emer Cooke said.
The head of UK's medicines regulator, June Raine, said the benefits outweighed the risks for the vast majority but were more balanced for younger people - for whom the risks from coronavirus infection are, on average, lower.
Wei Shen Lim, COVID-19 chair for UK's advisory Joint Committee on Vaccines and Immunisation, said it was preferable for adults under 30 with no underlying health conditions to be offered another vaccine.
Meanwhile, India reported 115,736 new coronavirus cases on Wednesday - a record spike for a second day this week - as a new wave of the pandemic continued to spread rapidly.
The next four weeks were going to be critical, COVID-19 task force chief Vinod Kumar Paul said at a health ministry briefing.
The single-day rise in cases within 24 hours reported on Wednesday topped that of 103,558 reported on Monday.
On Tuesday the daily tally was 96,982.
India has the world's third-largest caseload of coronavirus cases, after the United States and Brazil.
It has registered 166,177 total deaths, with 630 new ones reported on Wednesday.
The country had a consistent downward trend of COVID-19 cases between October and mid-February.
Since then, daily infections have jumped from about 9000 to more than 100,000.