The efficacy against symptomatic Covid-19 was 94 per cent seven or more days after the second dose – very close to the 95 per cent achieved during Phase 3 clinical trials.

In England, researchers on Thursday said that people who have received two doses of the Pfizer vaccine are generating strong antibody responses as the shot is rolled out.

An Imperial College London survey showed 87.9 per cent of people over the age of 80 tested positive for antibodies after two doses of the Pfizer-BioNTech vaccine, rising to 95.5 per cent for those under the age of 60 and 100 per cent in those aged under 30.

“Although there is some fall-off in positivity with age, at all ages, we get that very good response to two doses of the vaccine,” Paul Elliott, Chair in Epidemiology and Public Health Medicine, Imperial College London, told reporters.

Pfizer’s coronavirus vaccine significantly reduces transmission, Israeli studies find

Antibody levels are only one part of the immunity picture, with vaccines also shown to generate strong T-cell protection.

Nearly 95 per cent of under-30s tested positive for antibodies 21 days after one dose, but this fell in older groups.

The research found 34.7 per cent of those 80 years or older generated antibody responses from one dose of Pfizer vaccine, but Britain’s Joint Committee on Vaccination and Immunisation (JCVI) has previously found high protection from the Pfizer vaccine after one dose, even when antibody levels are lower.

Britain has extended the gap between doses to 12 weeks, even though Pfizer has cautioned it only has data for clinical efficacy with a three-week gap between shots.

Over 154,000 participants took part Imperial’s home surveillance study for Covid-19 antibodies, which monitors antibody levels from natural infection as well as among the vaccinated, between January 26 and February 8.

The survey also looked at confidence in vaccines, and showed it was high, with 92 per cent having accepted or planning to accept a vaccine offer, though confidence was lower among black people, dropping to 72.5 per cent.

Signs of weaker vaccine response to new strains in South Africa, Brazil

More than 217 million vaccine doses have been administered globally, though the vast majority have been given in high-income countries.

Hopes are high that the inoculations will allow the world to finally emerge from a pandemic that has killed more than 2.4 million, infected 112 million, and hammered the global economy.

But health experts have warned that unless the whole world has access to vaccines, the pandemic will not end.

This came as Ghana became the first country to receive shots under the global Covax Facility scheme, paving the way for poorer nations to catch up with wealthier parts of the world. The 600,000 doses are from Oxford-AstraZeneca, and will be administered in several Ghanaian cities from Tuesday.

More upbeat data meanwhile emerged about Johnson & Johnson’s single-shot vaccine, which was shown to be highly effective against severe Covid-19, including newer variants, in detailed data released by the US regulator. The vaccine is likely to be authorised soon, making it the third available in the hardest-hit country.

US biotech firm Moderna also announced its new Covid vaccine candidate aimed at the dangerous South African coronavirus variant had been shipped to government labs for testing.

Additional reporting by Reuters