The Centers for Disease Control and Prevention had in December advised the public to steer clear of the vaccine, which was authorised as a single shot.

Even people who had mild cases that did not need hospitalisation had a higher danger of pulmonary embolism and deep vein thrombosis.

It says the 16-year-old had a prior blood clot condition not related to the vaccine.

It says increased risks associated with the jabs are short-term only, while virus infection is associated with increased risk for the whole of the 28-day period examined.

This comes as several countries limit the use of jabs like AstraZeneca and Johnson & Johnson and opt instead for vaccines like Pfizer and Moderna.

Australia now recommends that the AstraZeneca vaccine should not be given to people under 60.

Marco Cavaleri says the vaccine 'maintains a favourable benefit risk profile in all ages but particularly in the elderly above 60'.

But the EMA's head of vaccine strategy acknowledges that incidents of blood clots are very rare, and that the vaccination programme in the UK is going well.

The shot will be administered only to people aged 45 and over, as a 'preventative and proactive measure' pending further investigation.

A group of German scientists claim to have found the cause of the rare blood clots.

Scientists and drug regulators have been searching for an explanation for what is causing the rare but potentially deadly side effect.

Cyprus is also investigating four other cases of 'mild' blood clotting, three after an AstraZeneca jab and one after a Pfizer shot.