Prime Minister Boris Johnson warned Sunday that Britain faces a “tidal wave” of infections from the omicron coronavirus variant, and announced a huge increase in booster vaccinations to strengthen defenses against it. In a televised statement, Johnson said everyone age 18 and older will be offered a third shot of vaccine by the end of this month in response to the omicron “emergency.” The previous target was the end of January. He said cases of the highly transmissible variant are doubling every two to three days in Britain, and “there is a tidal wave of omicron coming.” Read: Pfizer says COVID booster offers protection against omicron ”And I’m afraid it is now clear that two doses of vaccine are simply not enough to give the level of protection we all need,” Johnson said. “But the good news is that our scientists are confident that with a third dose – a booster dose – we can all bring our level of protection back up.” He announced a “national mission” to deliver booster vaccines, with pop-up vaccination centers and seven-day-a-week getting extra support from teams of military planners and thousands of volunteer vaccinators. Johnson’s Dec. 31 target applies to England. The other parts of the U.K. — Scotland, Wales and Northern Ireland — are also expected to speed up their vaccination campaigns. The U.K. Health Security Agency says existing vaccines appear less effective in preventing symptomatic infections in people exposed to omicron, though preliminary data show that effectiveness appears to rise to between 70% and 75% after a third vaccine dose. More than 80% of people age 12 and up in Britain have received two doses of vaccine, and 40% of adults have had three doses. Giving the rest a booster in the next three weeks will be a huge challenge, requiring almost 1 million doses delivered a day. Johnson acknowledged that many routine medical procedures would have to be postponed to meet the goal. Johnson's announcement came hours after the government raised the country’s official coronavirus threat level, warning the rapid spread of the omicron variant had pushed the U.K. into risky territory. The chief medical officers of England, Scotland, Wales and Northern Ireland said the 1of the highly transmissible new strain “adds additional and rapidly increasing risk to the public and health care services” at a time when COVID-19 is already widespread. They recommended raising the alert level from 3 to 4 on a 5-point scale. The top level, 5, indicates authorities think the health care system is about to be overwhelmed. The doctors said early evidence shows omicron is spreading much faster than the currently dominant delta variant, and that vaccines offer less protection against it. British officials say omicron is likely to replace delta as the dominant strain in the U.K. within days. “Data on severity will become clearer over the coming weeks but hospitalizations from omicron are already occurring and these are likely to increase rapidly,” they said. Concerns about the new variant led Johnson’s Conservative government to reintroduce restrictions that were lifted almost six months ago. Masks must be worn in most indoor settings, COVID-19 certificates must be shown to enter nightclubs and people are being urged to work from home if possible. Read: Scientist behind UK vaccine says next pandemic may be worse Many scientists say that’s unlikely to be enough, however, and are calling for tougher measures, which the government so far has resisted. Scientists in South Africa, where omicron was first identified, say they see signs it may cause less severe disease than delta, but caution that it is too soon to be certain.