One of the co-developers of AstraZeneca's Vaxzevria COVID-19 vaccine, Oxford scientist Adrian Hill, has defended the vaccine amid concerns of a possible link to very rare side effects.

Hill, who is director of the Oxford University's Jenner Institute, said the extremely rare side effects, such as the reported blood clots, would not have emerged even in large-scale trials involving tens of thousands of participants.

"Nobody can detect something that occurs in one in 300,000 people, and maybe causes deaths in about one in a million, if you're doing studies of the order of 10,000, 20,000 vaccinees," Hill told Reuters.

"You might see one, but you wouldn't know what to make of it," he added.

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Hill added that teams around the world were working to pin down any potential mechanism for what might be causing the blood clots, using real-world data now that so many shots have been administered.

He agreed with medicines regulators in the UK and Europe, and with World Health Organization experts, that the overall benefit of this vaccine massively outweighs the risks in essentially all age groups.

Suspensions and bans

More than a dozen European countries had suspended use of the Vaxzevria jab amid reports of cases of the clots, combined with low platelets in a very small number of people who had received it.

Many countries have resumed using the shot, but with some restrictions. Last week, Denmark became the first country in the world to permanently ban use of the jab.

"Our view is that individual national authorities are there to try and do the best for that particular country, that may well be the different regimens and different vaccines on different schedules and age groups are different in different places," Hill said.

After reviewing the safety reports, the European Medicines Agency regulator said that while there was a possible link, a direct causal link had not been established and the vaccine's benefits in preventing severe COVID-19 disease were substantial.

"The overriding point is that the world doesn't have enough COVID-19 vaccines altogether," Hill argued.

"If some countries choose to use one vaccine, there'll be more of the other vaccine for other countries. So we don't see this as a huge issue as long as we can distribute the doses that we have and, as things stand, the last time I looked there were 115 countries distributing the AstraZeneca/Oxford vaccine, and we'd like to have even more as soon as possible to maximize global usage," he added.