Mixing and matching first and second Covid vaccines may work better, according to a study that questions Britain's original jabbing strategy.
All 46million fully-inoculated adults have so far received the same vaccine, made by either AstraZeneca, Pfizer or Moderna. But new research suggests giving a different jab for a second dose triggers an even stronger immune response.
A first dose of AstraZeneca followed by Moderna or Novavax offered higher levels of antibodies and T-cells than two of the Oxford-made injections. And those who had Pfizer then Moderna had a more robust immune response than those who had two Pfizer jabs.
However, Pfizer followed by Novavax produced a less robust response, according to the results published in the Lancet.
Oxford University researchers carried out analysis of six different combinations. They didn't measure vaccine effectiveness in the real world but looked at antibody and T cells levels in lab trials — the key indicators of immunity.
Chief investigator Professor Matthew Snape said the findings show there is a 'great deal of flexibility' around which vaccines are used and people are not 'locked into' having the same second dose.
It comes with the UK facing huge pressure to achieve a successful booster drive to protect against an expected Omicron wave, with immunity from two doses waning after several months. Experts believe top-up jabs will have a huge effect on the impeding resurgence.
Ministers have already expanded the third dose rollout to all over-18s and halved the waiting time between second and third jabs to three months.
So far 20.5million have received an extra dose of Pfizer or Moderna, or AstraZeneca if they cannot have the mRNA vaccines, regardless of which jab they had for their first two doses.
Extra vaccine sites, volunteers and the military are being brought in to help with the rollout and health chiefs are easing NHS staff workload so they can help.
Professor Snape said there is 'no signal' yet that Omicron causes more severe illness than previous variants, but if scientists confirm it is truly more infectious, it will cause more hospitalisations due to larger numbers of people catching the virus.
The graph shows the different combinations of first and second doses and the antibody (left) and T cell response they produced (right). Those who had Pfizer then Moderna had a more robust immune response than those who had two Pfizer jabs. However, Pfizer followed by Novavax produced a less robust response
Overall, Pfizer (left) then Moderna (right) gave the best antibody levels, while AstraZeneca then Moderna triggered the most T cells
The above chart shows the number of vaccine doses ordered by the UK, and which orders have been donated or cancelled. It includes the latest orders of 54million more Pfizer doses and 60million Moderna dose
Real-world data suggests the highly-evolved variant is three-and-a-half times more likely to infect people than Delta because of its combination of vaccine resistance, increased infectiousness and antibody escape.
Dr Simon Clarke, a microbiologist at the University of Reading, said it was 'entirely possible' that Omicron could trigger a wave of hospital admissions on par with the peak in January 2021 — even if it is milder than Delta.
He told MailOnline: 'It's not uncommon for a more transmissible but less disease-causing pathogen to cause a bigger problem than a virus that is less lethal. If it infects a very large number but only hospitalises a small percentage, we could still end up with an awful lot of people in hospital.'
There have been only 246 official Omicron cases confirmed in the UK so far, but there are likely more than a thousand already, according to Professor Paul Hunter, an epidemiologist at the University of East Anglia.
Professor Hunter said he expected it to become the dominant variant 'probably within the next weeks or a month', based on how rapidly it is outpacing Delta in the South African epicentre. He claimed while that timeline mean there is little need for more curbs at Christmas, it does not rule out more restrictions being needed at some point in the New Year.
But Boris Johnson today refused to rule out tougher Covid curbs over the festive period, merely insisting that Christmas will be 'better' than last year. He is due to review the current measures in two weeks' time. Mr Johnson said on a trip to Merseyside: 'We're still waiting to see exactly how dangerous it is, what sort of effect it has in terms of deaths and hospitalisations.'
In the UK, people either received two doses of Pfizer, AstraZeneca or Moderna and only received different first and second jabs if they were participating in a trial.
Ministers last week ordered another 54million Pfizer and 60million Moderna jabs to 'future proof' the vaccination scheme.
The new study is one of six launched this year at the request of the UK Government's vaccine taskforce, which have compared different booster jabs and whether Covid injections and flu shots can be given at the same time.
The most recent trial enrolled 1,072 participants who had been given either Pfizer or AstraZeneca for their first Covid jab between January and March.
For their second dose around nine weeks later, the participants were given either the same vaccine, Moderna or Novavax.
Novavax is not yet approved in the UK, but data on the jab — which was found to be 96.4 per cent effective against mild, moderate and severe disease — was submitted to the medicine's watchdog in October.
The researchers aimed to find out whether the mix-and-match approach triggered just as many binding antibodies, neutralising antibodies and T cells as as two doses of the same vaccine.
Antibodies attack the virus and stop it attaching to and infecting the body's cells.
Binding antibodies stick to the virus cell and alert the body to the virus, while neutralising antibodies stop the virus in its tracks.
If the virus manages to breach that defence, T cell protection