Pfizer has begun a study comparing its original Covid-19 vaccine with doses specially tweaked to match the hugely contagious Omicron variant.

Pfizer and its partner BioNTech announced the study on Tuesday.

Vaccine makers have been updating their jabs to better match Omicron in case global health authorities decide the change is needed.

While Omicron is more likely than previous variants to cause infection even in people who have been vaccinated, it is not yet clear that a change to the vaccine recipe is needed.

The original jabs still offer good protection against severe illness and death. Studies in the US and elsewhere have made clear that adding a booster dose strengthens that protection and improves the chances of avoiding a milder infection.

“We recognise the need to be prepared in the event this protection wanes over time and to potentially help address Omicron and new variants in the future,” said Kathrin Jansen, Pfizer’s vaccine research chief.

The new US study is enrolling up to 1,420 healthy adults, aged 18 to 55, to test the updated jabs for use as a booster or for primary vaccinations.

Researchers will examine the tweaked vaccine’s safety and how it revs up the immune system in comparison to the original jabs.

Full study results will take many months as volunteers receive multiple vaccine doses — and as researchers measure how long virus-fighting antibodies remain at high levels after an Omicron-adapted dose versus the regular booster.

Pfizer’s chief executive told CNBC earlier this month that the company could have some Omicron-matched doses ready as early as March, but doing what the company calls “at-risk” manufacturing does not mean those doses will be rolled out to the public.

Pfizer and other vaccine makers also have brewed and tested experimental doses to match previous variants, changes that ultimately were not needed but offered practice in tweaking the recipe.

In one study group, about 600 volunteers who received two doses of the current Pfizer vaccine three to six months ago will receive either one or two Omicron-based jabs as boosters.

Another 600 who have already had three regular doses of the Pfizer vaccine will be given a fourth dose of either the regular vaccine or the Omicron-matched version.

The study also will enrol some unvaccinated volunteers who will receive three doses of the Omicron-based vaccine.