New York, March 4 : Covid-19 vaccines and boosters are effective against the highly transmissible sub-lineages of Omicron -- XBB and its subvariant XBB.1.5 -- the US Centers for Disease Control and Prevention (CDC) has said. First seen in August, XBB.1.5 now accounts for nearly 90 per cent of all Covid-19 cases reported in the US, the CDC said. According to officials, a "bivalent" booster that targets both the initial strain of the virus and the original Omicron variant, would be more protective than the original "monovalent" vaccine as the virus continues to evolve, the USA Today reported. The latest CDC study found that, despite the mismatch in variants, the booster remains protective against XBB. "There is incremental or additional protection from getting the bivalent on top of those past monovalent doses," Ruth Link-Gelles, the paper's first author, was quoted as saying to the media. The prevalence of XBB.1.5 has kept increasing since late last year, and it is estimated to account for 89.2 per cent of new Covid-19 cases in the US this week, up from 85.4 per cent last week and 79.7 per cent two weeks prior, Xinhua news agency reported. Early study suggests XBB.1.5 has a couple of concerning mutations that suggest it is even more contagious than other strains, possibly the most transmissible one so far. While XBB.1.5 is spreading easier, scientists said it does not seem to cause more severe disease. However, a separate study, which hasn't yet been peer-reviewed, suggests that protection against Covid fades within about three months against XBB, faster than against other variants. Being vaccinated can reduce the risk of death 13-fold compared to being unvaccinated, the CDC officials noted. They added that taking boosters provides a two-fold reduction in risk. /IANS