In a trial of six vaccines, the two mRNA jabs topped the list with both triggering the biggest rise in Covid antibodies.
Does the Covid booster vaccine protect against the Omicron variant?
In the study, six vaccines were tested as a third dose, they were:
Professor Saul Faust, trial lead and director of the NIHR Clinical Research Facility at University Hospital Southampton NHS Foundation Trust, said the study had shown that those vaccines are safe and effective boosters for people who had already had two doses of AstraZeneca or Pfizer.
“All of the vaccines in our study do show a statistically significant boost… RNA (Pfizer and Moderna) very high, but very effective boosts from Novavax, Janssen and AstraZeneca as well,” Prof Faust said.
Although Omicron was not tested specifically in the study, he added that the vaccines worked well against existing variants.
Asked specifically about the new strain, he said: “Our hope as scientists is that protection against hospitalisation and death will remain intact.”
Samples from the study have now been passed to the UK Health Security Agency (UKHSA) to look at how well the Omicron variant can be neutralised by vaccines.
What is the latest news on the Omicron variant?
The latest data from South Africa - where the new variant was first detected - suggests that Omicron appears to be able to get around some immunity but vaccines still offer protection against severe disease.
Omicron is also fast becoming the dominant variant in the country, overtaking Delta, with the daily number of reported Covid cases doubling to 8,561.
It is not known how many of these new cases were due to the new variant but health officials said it was "rapidly becoming the dominant variant".
The latest report also suggests Omicron is now in five out of nine South African provinces but is likely to be present across the country.
Will the UK offer annual booster vaccines?
The trial results come as the UK has ordered an extra 114 million doses of Moderna and Pfizer vaccines which will be rolled out over 2022 and 2023.
All adults aged over 18 will be offered a third dose by the end of January but the purchase of the additional vaccines suggests a fourth vaccine could be rolled out.
The head of Pfizer earlier this week claimed that booster jabs - which can cut the risk of infection by more than 93% - could become an annual event.
What else did the study find out about booster vaccines?
The data showed that all vaccines tested posed no safety concerns with fatigue, headache and sore arm the most commonly reported issues.
When looking at antibody levels in the trial, people who had received two doses of AstraZeneca initially had booster responses that were between 1.8 times higher to 32.3 times higher depending on the booster vaccine used.
After two doses of Pfizer, the range was 1.3 times higher to 11.5 times higher.
More information will be published early next year which will look at whether a longer period between second and third doses improves the response.
Prof Faust said: “It’s really encouraging that a wide range of vaccines, using different technologies, show benefits as a third dose to either AstraZeneca or Pfizer/BioNTech.
“That gives confidence and flexibility in developing booster programmes here in the UK and globally, with other factors like supply chain and logistics also in play.”
The new CovBoost trial, published in The Lancet, involved 2,878 people aged 30 or over receiving a booster 10 to 12 weeks after their initial two-dose vaccination.
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