The trials include a total of 600 adults, half of whom already received two doses of Moderna's vaccine at least six months ago, and the other half of whom received not only these initial two doses but also, at least three months ago, the booster dose.

This specific booster dose against Omicron will thus be evaluated as a third or fourth dose of the Covid-19 vaccine.

The US company also revealed results on the efficacy of the third dose of its Omicron vaccine.

According to Moderna, six months after the booster dose, antibody levels against Omicron were reduced six-fold from the peak observed 29 days after inoculation.

However, the antibodies remain detectable in all participants, he assured.

These data were obtained by analysing the blood of 20 people who received the 50 microgram booster dose (half of the first two injections).

"We are reassured by the persistence of antibodies against Omicron, six months after the third dose currently authorised," Moderna CEO Stéphane Bancel said, quoted in a press release.

"However, due to the long-term threat posed by the spread of Ómicron, we are moving forward with the specific booster dose against this variant," he added.

The Pfizer-BioNTech alliance began recruitment for adult clinical trials on the safety and immune response of the Covid-19 vaccine targeting the Ómicron variant specifically.

But Pfizer's formulation is being studied not only as a booster, but also for first inoculations against Covid-19 for people not yet vaccinated.

Based on RNA technology, it makes it relatively easy to modify and update these two vaccines, to follow the evolution of specific mutations into new variants.

Several countries, including the United States, are already experiencing a drop in the number of infections due to the Omicron wave, despite the increase in new cases worldwide.