The top medic said younger adults would simply receive different vaccines instead. The AstraZeneca vaccine has been approved for emergency use in over 100 countries including Britain and the European Union.
Labour leader Sir Keir Starmer also urged people to "trust in our doctors and scientists" and said he was looking forward to receiving his second AstraZeneca dose.
The UK has handed out more than over 37 million COVID-19 vaccine doses including BioNTech-Pfizer and AstraZeneca's jabs (31.6 million first doses and almost 5.5 million second shots).
Belgium will stop giving the Oxford/AstraZeneca vaccine to people under age 56 for the next four weeks amid blood clot concerns, national and regional health ministers agreed today.
While some experts suggest that different vaccine combinations could work together to enforce the body's immune response to the virus, there is now no evidence it will be as effective.
As reported by the New York Times, Britain made a change to its vaccine guidelines in January so that if a second dose of one vaccine was not available, patients could be given a different one. Research into this is now ongoing.
The EU's medicines regulator said on Wednesday that blood clots should be listed as a rare side effect of the AstraZeneca vaccine but the benefits of the jab continue to outweigh risks, as several countries battle fresh virus surges amid vaccine shortfalls.
The World Health Organization said the link between the vaccine and blood clots was "plausible" but not confirmed, adding that the clotting incidents were "very rare" among almost 200 million people who have received the jab worldwide.
It said most of the cases occurred in women under the age of 60 and while they have advised against placing restrictions on the vaccine's rollout, it will provide further food for thought for the Morrison government which has already faced heavy scrutiny over a botched rollout so far.
However, based on the now available evidence, specific risk factors have not yet been confirmed. This means that, for the moment, the EMA's guidance on the vaccine remains unchanged. But the European regulator has said that may be because countries like Germany began by vaccinating communities like health-care workers, which include more women.
Several European countries are considering mixing COVID-19 vaccines for citizens who received a first dose of AstraZeneca's shot, an unprecedented move highlighting the challenge for governments struggling to tame rising infections.
Health Secretary Matt Hancock said the UK's review confirmed that the Oxford-AstraZeneca jab is "safe, effective and the benefits far outweigh the risks for the vast majority of adults". It costs much less than other vaccines.
June Raine, chief executive of the MHRA, said the side-effects of the AstraZeneca vaccine were "extremely rare" - and more work was going to identify if the vaccine was definitely causing the clots. Regulators have said it is occurring among those vaccinated at a rate above what they'd expect to see in the normal population.
The EMA will continue to look into the matter and will update information as required, the agency added.