CHICAGO, Jan.21 (REUTERS) -- Three US studies show that a third dose of an mRNA vaccine is key to fighting the Omicron coronavirus variant, providing 90 per cent protection against hospitalisation due to COVID-19, the US Centers for Disease Control and Prevention (CDC) said on Friday (Jan 21).

The studies, led by the CDC, are among the first in the United States to look at the impact of booster doses against the fast-spreading Omicron variant, which now accounts for 99 per cent of all new COVID-19 cases.

Overall, they suggest that boosters helped protect against both infection and symptomatic disease. Adults aged 50 and older saw the most benefit from an extra dose of the vaccines made by BioNTech with Pfizer or Moderna.

"Protection against infection and hospitalisation with the Omicron variant is highest for those who are up to date with their vaccination, meaning those who are boosted when they are eligible," CDC Director Dr Rochelle Walensky said in a White House briefing on Friday.

As has been shown in other countries, vaccine boosters performed better against the Delta variant than Omicron, a highly mutated version of the SARS-CoV-2 virus that has been able to evade immunity from vaccines and prior infections.

One of the studies, published on Friday in the CDC's Morbidity and Mortality Weekly Report, looked at rates of hospitalisation, emergency department and urgent care visits in 10 states between Aug 26, 2021, and Jan 5, 2022.

It found that protection from two doses of vaccine fell to 57 per cent in people who got their second shot at least six months earlier. Among those who received a booster, protection from hospitalisation and urgent care visits was 90 per cent.