Drug makers Moderna and Pfizer have begun human trials of vaccines specially tweaked to match the hugely contagious Omicron variant of COVID.

Pfizer and its partner BioNTech announced a study involving human volunteers on Tuesday, while rival Moderna said on Wednesday it had begun a similar trial.

COVID-19 vaccine makers have been updating their shots 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’s not yet clear that a change to the vaccine recipe is needed.

The original vaccines 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,” Kathrin Jansen, Pfizer’s vaccine research chief, said this week.

The US study for Pfizer and BioNTech will enrol up to 1420 healthy adults, aged 18-55, to test the updated Omicron-based shots 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 shots.

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 shots 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.

Moderna’s study will enrol about 600 people who have already received either two doses of its original shots or two and a booster. All the volunteers will receive a dose of the experimental Omicron-matched version.

Moderna also pointed to a small study published in the New England Journal of Medicine on Wednesday that showed antibodies that targeted Omicron lasted for six months after a booster vaccine.

“We are reassured by the antibody persistence against Omicron at six months after the currently authorised” booster, Moderna chief executive Stephane Bancel said.

“Nonetheless, given the long-term threat demonstrated by Omicron’s immune escape, we are advancing our Omicron-specific variant vaccine booster candidate and we are pleased to begin this part of our phase two study.”

-with AAP