(Reuters) - Moderna Inc (MRNA.O) said on Wednesday it had started a mid-stage study, testing a booster dose of its COVID-19 vaccine specifically designed to target the Omicron coronavirus variant, a day after rival Pfizer Inc (PFE.N) launched a similar trial.

The company said while a third shot of its original coronavirus vaccine increased neutralizing antibodies against the variant at the lower dose, their levels declined six months after the booster dose was administered.

However, neutralizing antibodies remained detectable in all participants, Moderna said.

While studies have indicated that Omicron results in a less severe COVID-19 than seen during previous waves, the variant has quickly become dominant in many parts of the world, driving up infections and straining healthcare systems.

The variant currently accounts for 99.9% of the COVID-19 cases in the Unites States.

Pfizer and its partner BioNTech started a clinical trial on Tuesday to test a new version of their vaccine specifically tailored to the Omicron variant. read more

Moderna said it would study its Omicron-specific booster in adults aged 18 years and older.

It would test the booster in individuals who received only the two-dose primary series of Moderna's original vaccine, mRNA-1273, and also in those who received the primary series and a booster dose of the same vaccine, the company said.

Moderna plans to enroll about 300 participants in each of the two groups in the study.

Three studies, led by the U.S. Centers for Disease Control and Prevention have shown that a third dose of an mRNA vaccine, such as those from Pfizer and Moderna, is key to fighting the Omicron variant. read more

Some countries have already started offering additional booster doses, but a recent study from Israel showed that while a fourth dose of an mRNA vaccine boosted antibodies, the level was not high enough to prevent an Omicron infection.