Under-30s in Britain will be offered an alternative to the AstraZeneca vaccine after the European Medicines Agency confirmed it has found possible links between the jab and rare cases of blood clotting.

Of the more than 20 million people in Britain who have had their first dose, 51 women and 21 men aged 18 to 79 have had blood clots, the Medicines and Healthcare products Regulatory Agency revealed. Of those patients, 19 have died.

The findings have led Belgium to impose a four-week ban on administering the AstraZeneca vaccine to all people under 56. It announced the ban on Thursday morning (Australian time).

“We will keep it this way for the next four weeks and reassess it then,” Belgian Health Minister Frank Vandenbroucke said.

Spain and Italy followed within hours with their own restrictions, saying they would limit the vaccine’s use to people over 60.

“The vaccination strategy is changing and from tomorrow only over 60-year-olds will be vaccinated with AstraZeneca,” Spanish Health Minister Carolina Darias said.

In Germany, Christian Bogdan, a member of the country’s vaccine committee, said instances of a rare clotting condition in women aged under 60 who received AstraZeneca’s vaccine were 20 times higher than would normally be expected.

He did not specify how many cases of blood clots with low blood platelet counts would be expected in a normal population but said their higher prevalence in one population group over a defined timeframe represented a “very clear risk signal”.

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 others – including France, the Netherlands and Germany – have set a minimum age.

‘Benefits outweigh risks’

Britain’s medicines regulator said 14 of the 19 deaths involved cerebral venous sinus thrombosis (CVST), a type of clot that prevents blood from draining from the brain.

Three people who died had been under the age of 30.

• Read: You’ll likely get the AstraZeneca vaccine – these are the possible side effects

Emer Cooke, executive director of the European Medicines Agency, announced on Thursday morning (Australian time) that unusual blood clotting with low blood platelets would be added as a “very rare” side effect to the AstraZeneca vaccine’s product information.

Ms Cooke said it was not yet known what was causing the blood clots, but revealed one plausible explanation.

She said the vaccine caused an adverse reaction similar to heparin-induced thrombocytopenia, which is sometimes seen in patients who receive a medication called heparin.

“This case clearly demonstrates one of the challenges posed with large-scale vaccination campaigns,” Ms Cooke said.

“When millions of people receive these vaccines, very rare events can occur that were not identified in the clinical trials.”

Physicians know how clots form. What they don’t know is why the AstraZeneca jab appears to cause them.

The EMA maintains the benefits of the shot outweigh the risks but has warned “you should still be aware of symptoms so you can get prompt medical treatment to help recovery and avoid complications”.

It flagged the chance of having a blood clot was “very low”.

By early April, the EMA had received reports of 169 cases of the rare brain blood clot after 34 million doses were administered across 27 EU countries plus Iceland, Norway and Liechtenstein.

EMA officials said most of the cases occurred in women under 60 within two weeks of being vaccinated.

“The risk of mortality from COVID is much greater than the risk of mortality from these rare side-effects,” Ms Cooke said.

Dr Wei Shen Lim, COVID-19 chair for Britain’s advisory joint committee on vaccines and immunisation, said the decision to offer an alternative was “really out of the utmost caution, rather than because we have any serious safety concerns”.

Scott Morrison holds a vial of AstraZeneca vaccine in February. Photo: AAP

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 Britain and mainland Europe, it is likely to be the mainstay of vaccination programs across the developing world.

-with AAP