AstraZeneca says Covid vaccine 76% effective in updated US trial data

British-Swedish drugmaker AstraZeneca has revised down the efficacy figure for its two-dose Covid vaccine following a clinical trial held across the United States, Peru and Chile.

The company now says its vaccine is 76 per cent effective rather than 79 per cent effective at preventing any kind of symptomatic Covid. The vaccine has been dogged by questions over its effectiveness and possible side-effects.

This month, more than a dozen countries temporarily suspended giving out the vaccine after reports linked it to a rare blood clotting disorder in a very small number of people. The European Union's drug regulator, however, said last week the vaccine was clearly safe, but people remain sceptical about its safety.

The updated 76 per cent efficacy rate compares with rates of about 95 per cent for vaccines from Pfizer/BioNTech and Moderna.

US health officials earlier in the week publically rebuked the drugmaker for using "outdated information" when calculating that the vaccine was 79 per cent effective.

AstraZeneca reiterated on Wednesday that the shot, developed with Oxford University, was 100 per cent effective against severe or critical forms of the disease. It also said the vaccine showed 86 per cent efficacy in adults 65 years and older.

The latest trial data was based on 190 infections and 32,449 participants in the United States, Chile and Peru, as compared to the earlier interim data -- 141 infections through February 17.

The AstraZeneca vaccine is, however, seen as crucial in tackling the spread of COVID-19 across the globe, not just due to limited vaccine supply but also because it is easier and cheaper to transport than rival shots. It has been granted conditional marketing or emergency use authorisation in more than 70 countries.