The Pfizer and Moderna vaccines are the most effective Covid booster jabs. a new study has found, but all boosters could help fight the Omicron variant.

Newly published research from the ongoing CovBoost study into vaccines, shows that six different vaccines are safe and effective as booster doses for people who have already had two doses of AstraZeneca or Pfizer/BioNTech.

The latest CovBoost trial, published in the Lancet, involved 2,878 people aged 30 or over who received a booster 10 to 12 weeks after their initial two jabs.

The six vaccines tested as a third doses were AstraZeneca, Pfizer/BioNTech, Moderna, Novavax, Janssen (made by Johnson and Johnson) and CureVac (which has ceased production).

“All of the vaccines in our study do show a statistically significant boost,” says Professor Saul Faust, trial lead and director of the NIHR Clinical Research Facility at University Hospital Southampton NHS Foundation Trust.

He stressed that while the boost delivered by the two mRNA vaccines – Pfizer and Moderna – was “very high”, Novavax, Janssen and AstraZeneca also delivered “very effective boosts”.

Although the newly emerged Omicron variant was not tested in the study, the study showed that booster vaccines are working well against existing variants.

Asked specifically about Omicron, Prof Faust said: “Our hope as scientists is that protection against hospitalisation and death will remain intact.”

“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,” said Prof Faust. “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.

Additionally, researchers think that T cell immunity, which was studied alongside antibodies in the research, could play a big role in fighting off the new variant.

“Even though we don’t properly understand its relation to long-term immunity, the T cell data is showing us that it does seem to be broader against all the variant strains, which gives us hope that a variant strain of the virus might be able to be handled, certainly for hospitalisation and death if not prevention of infection, by the current vaccines,” Prof Faust said.

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.

There were 13 different groups testing the boosters or acting as controls, with controls given a meningitis vaccine. Immunity was then assessed after 28 days, with experts saying that more data will be published in the future on the immunity results three months and one year after receiving boosters.

Jonathan Ball, professor of molecular virology at the University of Nottingham, commented: “This is a fantastic study and it’s great to finally see the data that was no doubt pivotal in deciding the UK’s vaccine booster approach.

“The data clearly shows that all boosters provided a lift to at least one aspect of your Covid immunity, and that side effects were, on the whole, mild.”

He added: “The data also shows that an mRNA booster – such as Moderna or Pfizer – provided the best overall boost, irrespective of whether your first doses were mRNA or (AstraZeneca). The fact that the mRNA vaccine boosts gave a marked increase in both antibodies and T cells is great news, especially now, when our attention has been grabbed by the emergence of the Omicron variant.”

Samples from the study have now been passed to the UK Health Security Agency to look at how well the Omicron variant can be neutralised by vaccines.

More data will be published early next year looking at whether a longer period between second and third doses improves the response.