The dose of AstraZeneca and Oxford University’s Covid-19 vaccine that was shown to be 90 per cent effective was only tested in people aged 55 and under, according to a US official.
Moncef Slaoui, the scientific head of the US’s Operation Warp Speed – the programme to supply America with vaccines – told US reporters that the half-dose regime, which was discovered by accident, was only given to younger age groups.
Scientists across the globe are hoping to find vaccines that work in older people, who are most at risk from Covid-19.
Oxford’s overall efficacy from two different dosing regimes in a phase three trial was announced last week at a combined figure of 70 per cent.
A half dose followed by a full dose was found to be 90 per cent effective according to a subset of data, but efficacy was 62 per cent for people given two full doses.
Results from an earlier phase two study of the vaccine published in The Lancet medical journal showed the vaccine produced a strong response in all age groups.
Professor Andrew Pollard, director of the Oxford Vaccine Group, told reporters at a UK-based press briefing last week that the 90 per cent effectiveness finding had already met the “necessary statistical evidence as required by regulators”.
He said further evidence will probably be available next month but it is “a highly significant result even with the numbers that we have”.
The 90 per cent efficacy was based on a dosing regime given to 2,741 people.
The other regime (two doses) involved 8,895 people.
Neither AstraZeneca nor Oxford disclosed at the briefing that the 90 per cent figure was based on adults aged 55 and under.