LONDON, Dec 12 (Reuters) - Britain raised its COVID alert level on Sunday in response to the rapid spread of the Omicron variant of the virus, as medical authorities warned that hospitalisations are likely to rise sharply over the coming weeks. The chief medical officers of England, Scotland, Wales and Northern Ireland recommended an increase to alert level 4 from level 3 on its 5-point scale, which means they judge transmission of the virus to be high. "Early evidence shows that Omicron is spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced," the medical officers said in a joint statement. "Data on severity will become clearer over the coming weeks but hospitalisations from Omicron are already occurring and these are likely to increase rapidly." Prime Minister Boris Johnson is due to address the country in a televised broadcast at 2000 GMT to talk about the campaign to provide booster vaccinations. "Both booster vaccines - Pfizer and Moderna - increase the immune response substantially and show good effectiveness although with some reduction compared to Delta," the medical officers said. Level 5 - the maximum alert level - would mean the health service is at risk of being overwhelmed