Washington, Apr 1 (IANS): A Phase 2 clinical trial evaluating various additional Covid-19 booster shots has begun enrolling adult participants in the US, the country's National Institutes of Health (NIH) announced.

The trial, sponsored by the NIH, aims to understand if different vaccine regimens can broaden immune responses in adults who already have received a primary vaccination series and a first booster shot, Xinhua news agency quoted the Institutes as saying on Thursday.

"We are looking beyond the Omicron variant to determine the best strategy to protect against future variants," said Anthony Fauci, Director of the National Institute of Allergy and Infectious Diseases (NIAID), part of the NIH.

"This trial will help us understand if we can use prototype and variant vaccines alone or together to shift immune responses to cover existing and emerging Covid-19 variants," the country's top infectious disease expert added.

Despite waning protection against infection and mild illness during the Omicron wave, vaccines available in the US so far have maintained durable protection against severe Covid-19, said the NIH.

However, the NIAID is preparing for the possibility of future variants evading protection against currently available Covid-19 vaccines.