The deputy chair of the Joint Committee on Vaccination and Immunisation, Professor Anthony Harnden said that this was a better strategy than giving first jabs to younger people, as some local leaders have demanded.
But he confirmed that earlier doses for over-50s will make the vaccine less effective for these individuals over the long term.
“Leaving the second dose for a longer period of up to 12 weeks does give you better and longer term protection,” Prof Harnden told BBC Radio 4’s Today programme. There’s data on that now.
“This is a bit of a trade-off, and that we think that actually protecting vulnerable people with their second dose earlier might not give them better longer-term protection but it will give them better short-term protection, in this rather urgent situation with a highly transmissible virus.”
Prof Harnden said that the B1.617.2 strain of coronavirus was “clearly more transmissible” than variants previously present in the UK and may reduce the effectiveness of vaccines in preventing mild disease and stemming transmission of the virus.
But he said it appeared the vaccines remain equally effective in preventing severe illness from the new variant.
With the mutant strain likely to spread quickly, the priority was to protect unvaccinated members of vulnerable groups, like the elderly and those with underlying health conditions, with their first jab or by bringing forward the second dose, he said.
It was possible to “cope” with higher levels of infection causing mild illness in the community, so long as hospitals do not become overwhelmed, he said.
“We believe that vaccinating those in at-risk groups who are currently unvaccinated and bringing forward that second dose for the over-50s by four weeks is a better strategy,” said Prof Harnden.
“If we immunise 18-29 year-olds, for instance, in these areas, we’ll be taking vaccines from somebody else in the country. The vaccines may be less effective against transmission as immunity takes a number of weeks to develops. It’s not a very good strategy for preventing transmission.
“From a vaccination strategy, it just won’t help mass-vaccinating a number of young people at the expense of older people who haven’t been vaccinated.”
One dose of vaccine gives good protection against coronavirus, but not as good as two doses,” said Prof Harnden.
“There is a trade-off here,” he said. “But we feel that that’s a much better strategy. There’s lots of unvaccinated people still in these at-risk groups in some of these areas. What we want to do is prevent hospitalisations and people dying, because we can cope with infection rates in the community providing the hospitals don’t get overwhelmed.”