Pfizer says a booster dose of its COVID-19 vaccine may offer important protection against the new Omicron variant even though the initial two doses appear significantly less effective.

Pfizer and its partner BioNTech said that while two doses may not be protective enough to prevent infection, lab tests showed a booster increased by 25-fold people's levels of virus-fighting antibodies.

Blood samples taken a month after a booster showed people harboured levels of Omicron-neutralising antibodies that were similar to amounts proven protective against earlier variants after two doses.

Scientists don't yet know how big a threat the Omicron variant really is.

Currently the Delta variant is responsible for most of the COVID-19 cases around the world.

But the Omicron variant, discovered late last month, carries an unusually large number of mutations and scientists are racing to learn how easily it spreads, whether it causes illness that is more serious or milder than other coronavirus types - and how much it might evade the protection of prior vaccinations.

Pfizer's findings, announced in a press release, are preliminary and have not yet undergone scientific review.

But they are the first from a vaccine maker examining whether the booster doses that health authorities are urging people to get may indeed make an important difference.

Scientists have speculated that the high jump in antibodies that comes with a third dose of COVID-19 vaccines might be enough to counter any decrease in effectiveness.