CHICAGO, Jan 21 (Reuters) - Three U.S. studies show that a third dose of an mRNA vaccine is key to fighting the Omicron coronavirus variant, providing 90% protection against hospitalization due to COVID-19, the U.S. Centers for Disease Control and Prevention (CDC) said on Friday. 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% of all new COVID 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 SE with Pfizer Inc (PFE.N) or Moderna Inc (MRNA.O). "Protection against infection and hospitalization 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 hospitalization, 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% in people who got their second shot at least six months earlier. Among those who received a booster, protection from hospitalization and urgent care visits was 90%. In another study published in the JAMA medical journal, researchers reviewed data on 23,391 COVID-19 cases caused by either the Delta or Omicron variant among people seeking testing between Dec. 10, 2021, and Jan. 1, 2022. They found that among people seeking testing for COVID-like symptoms, those who had received three doses of an mRNA vaccine had the highest protection from infection compared with those who got two doses or were unvaccinated. Because of the timing of U.S. booster recommendations, most people in the study had been vaccinated within a month of seeking testing, which likely contributed to the benefit. Data from the UK has shown that the increased antibody protection gained from boosters wanes after 10 months, so it is unclear how long that benefit would hold. 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 infection by the Omicron variant. Early enthusiasm for boosters in the United States has been lackluster, fueled in part by rapidly shifting public health messaging and concerns among some experts over a lack of U.S. data demonstrating their benefit. There is also the misguided perception among some Americans that if you can still get infected, why bother with a booster? read more According to the CDC, only 82.5 million, or 39.3%, of fully vaccinated Americans have received a COVID-19 booster dose.