The Covid-19 vaccines are already easing UK's coronavirus spread with early data showing it is definitely reducing cases.
Research due to be published within days will provide real-world evidence that Britons are being protected, including suggestions that the effect is still building a month after one dose, said Anthony Harnden, deputy chairman of the Joint Committee on Vaccination and Immunisation (JCVI), reports The Sunday Times.
With fears about an EU export ban on vaccines, Professor Harnden said plans were in place for people to be able to mix and match vaccines if second doses of the Pfizer jab were unavailable.
Decisions on vaccinating teachers and other key workers in the second phase are likely to involve political and economic input because clear evidence that the vaccines block transmission of the virus is unlikely to be available for months, he said.
Official figures on Saturday showed that another 414,419 people had been given a first dose, taking that total to 7.9 million, suggesting the NHS is more than halfway towards its target of immunising the four most vulnerable groups by the middle of next month.
Protection is thought to take at least two weeks to build but Professor Harnden said it was clear that mass vaccination was working to bring down cases.
"The preliminary data indicate a vaccine effect from the first dose in both younger adults and in older adults over 80. The effect seems to increase over time," he said.
"It is possible that we may get stronger and better long-term protection by a delayed second dose."
Initial vaccination focused on the over-80s and NHS and care staff and has since widened out to younger people and the clinically vulnerable. Infection rates in the over-80s have fallen by 36 per cent this month. Other age groups have seen similar falls, while the biggest drop is in twenty-somethings whose rates have halved.
However, Public Health England has access to infection data linked to vaccination records, and its researchers can compare the chance of catching the virus between those who have had the jab and those of similar ages who have not. Initial findings from their study are due out next week.
Harnden said: "The data we have is still is very early because it only reflects approximately three or four weeks of the programme and it's mainly based on the Pfizer vaccine."
The data does not yet show an effect on hospital admissions or deaths but scientists are optimistic given that case rates in the over-60s have been one of the most reliable predictors of impending hospital admissions.
UK ministers are likely to use the results as vindication of the strategy of delaying second doses beyond the three or four-week window specified by manufacturers.
Chris Whitty, the chief medical officer for England, has said that it is likely to save lives by doubling the number protected.
The delay has raised concerns about whether disruptions to supply will prevent people getting their second doses within the 12-week window.
Professor Harnden said: "Our position is you should receive the same second dose of vaccine as you did the first dose, unless there were exceptional circumstances and those exceptional circumstances may be supply constraints. In which case it's better to have a second dose of a different vaccine than no second dose at all."
"There aren't results from studies on mixing vaccines at the moment, so we haven't got evidence but there's no theoretical evidence why you shouldn't mix vaccines."
The JCVI is also due to set out guidance for the second stage of the vaccines programme within weeks, amid pressure for teachers and other key workers to be given priority over other younger adults. The list includes everyone over 50 and younger people with chronic conditions. Given that younger people are at much lower risk, ministers are keen to prioritise frontline workers.