NEW YORK (BLOOMBERG) - Moderna has enrolled the first participant in a clinical trial of its Omicron-specific Covid-19 vaccine booster, another step towards fighting the highly transmissible variant that has spread around the world in a matter of weeks.

Moderna and rival Pfizer are each rushing to test Omicron-specific vaccines in case they are needed as the variant continues to pummel the globe.

Moderna's statement on the new trial comes just one day after Pfizer said it had begun a 1,400-person study of a messenger RNA shot targeted against Omicron.

The Moderna study will test an Omicron-specific booster called mRNA-1273.529 in 600 adults who have received its existing vaccine, including 300 who have received just two doses and another 300 who have also received a third booster shot.

The trial will examine both the safety and the immune response generated by the Omicron-specific shot.

Vaccine makers are grappling with what kinds of shots may be most effective as the coronavirus continues to mutate.

Omicron-specific boosters are one possibility; another is a multivalent vaccine that would induce protection against multiple strains. Other strategies include broad-acting vaccines that would work against even more mutations.

Moderna is testing the targeted booster because of "the long-term threat demonstrated by Omicron's immune escape", chief executive officer Stephane Bancel said in the statement.

"We are also evaluating whether to include this Omicron-specific candidate in our multivalent booster programme."

In the same statement, the company said it was reassured by a report in the New England Journal of Medicine showing that patients who had received three doses of Moderna's existing vaccine still had neutralising antibodies against Omicron six months after the third shot.

However, after the booster, levels of Omicron-fighting antibodies declined faster than those against the original virus strain.