LONDON -- Britain's medicines regulator on Thursday said there had been 168 major blood clots following a dose of AstraZeneca's COVID-19 vaccine, a rate of 7.9 clots per million doses, a jump in incidence from the previous week's figure.

This was up from the 100 cases reported last week, when the overall case incidence was 4.9 per million doses.

There has been scrutiny of the AstraZeneca vaccine on the issue of the very rare clots and some countries, including Britain, have recommended that only people over a certain age get the shot.

Adam Finn, Professor of Paediatrics at Bristol University, said the jump in reported cases was expected.

"Cases are being reported reliably and quickly but there are also cases that occurred previously now being recognized and reported as well," Finn said.

"I would expect the true number of cases per million doses of vaccine to become clear fairly soon as these reports stabilize but it is already clear that it is going to remain a very rare event."

There have been 21.2 million first doses of AstraZeneca's shot given in Britain's rollout, with all except one of the side effect case reports coming after a first dose. A total was not given for second doses administered.

There were 32 reported deaths from clots in total, compared to 22 reported last week, but the fatality rate of the reported clots dropped to 19% from 22%.

Britain has advised that under-30s receive an alternative to the AstraZeneca-Oxford vaccine, after the MHRA medicine regulator found there was evidence of a link to rare clots with low platelet levels.

Officials have emphasized the side effect is "vanishingly" rare and advised that most people still get the shots. The preference for under-30s to get a different shot is mainly informed by that age group's low risk from COVID-19, combined with Britain's low prevalence of infection.

That differs from several European countries such as France, which has decided to restrict use of AstraZeneca's shot to people over 55.

"On the basis of this ongoing review, the advice remains that the benefits of the vaccine outweigh the risks in the majority of people," the MHRA said on Thursday, reiterating its advice for the shot.

(Reporting by Alistair Smout Editing by Frances Kerry)