LONDON -- Long lines formed Monday at vaccination centers across England as people heeded the government's call for all adults to get booster shots to protect themselves against the omicron variant and as the U.K. recorded its first death of a patient infected with omicron.
In a televised announcement late Sunday, Prime Minister Boris Johnson said everyone 18 and older would be offered a third vaccine dose by Dec. 31, a month earlier than the previous target. Johnson said boosters would "reinforce our wall of vaccine protection" against an anticipated "tidal wave of omicron."
U.K. health authorities say omicron cases are doubling every two to three days in Britain, and that the variant will soon replace delta as the dominant coronavirus strain. Health Secretary Sajid Javid told lawmakers Monday that omicron will be dominant in London "within 48 hours."
While omicron is acknowledged to be much more transmissible than previous variants, it's unclear both how virulent it is and whether the expected wave of infections will inundate the country's state-funded health care system.
Barely two weeks after it was identified in South Africa, 10 people are in British hospitals with omicron-related covid-19. The British government raised the country's threat level Sunday, warning that the rapid spread of omicron "adds an additional and rapidly increasing risk to the public and health care services."
Scientists in South Africa say the variant may cause less severe disease than the delta variant but caution that it's too soon to be certain. Health authorities around the world are watching Britain closely to see what an omicron surge looks like in a country with an older, more highly vaccinated population than South Africa's.
"The idea that this is somehow a milder version of the virus, I think that's something we need to set on one side and just recognize the sheer pace at which it accelerates through the population," Johnson said as he visited a vaccination center in London. "So the best thing we can do is all get our boosters."
The U.K. Health Security Agency says existing vaccines appear less effective in preventing symptomatic infections in people exposed to omicron, though that effectiveness appears to rise to between 70% and 75% after a third dose.
More than 80% of people age 12 and older in Britain have received two doses, and 40% of adults have had three. But the acceleration of the booster program will be a huge challenge, requiring almost 1 million doses given out each day -- more than the previous high of about 850,000 a day. Some 750 soldiers and thousands of volunteer vaccinators will be drafted to give the shots at doctors' offices, hospitals, pharmacies and pop-up vaccination centers.
Many routine procedures will be postponed as Britain's National Health Service swings into high gear for the boosters.
While the online booster appointment system will not be open to under-30s until Wednesday, adults could -- and did -- show up at walk-in centers to get a booster starting Monday.
At St. Thomas' Hospital, on the south bank of the River Thames in London, the lines of people waiting for shots stretched across Westminster Bridge toward Parliament. At the Gordon Hospital walk-in clinic in central London, most of those lining up were in their 20s and 30s.
Sam Collins, 30, said he was "not especially" worried about omicron, "but I'd just prefer to be triple-vaxxed."
"Also, my partner has just had a baby and she's not vaccinated, so if I can be extra-vaccinated, then that will help," he said.
The government's appointment-booking website struggled to keep up with demand, and also ran out of rapid at-home virus test kits, which have been distributed free to households through the pandemic.
The British government's Dec. 31 booster 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.
While omicron is spreading around the world, Britain may be especially affected because it ordinarily has high levels of travel to South Africa. The omicron outbreak is also more visible in Britain because the U.K. is also a world leader in genomic sequencing, which identifies and tracks new variants.
Researchers in the U.K. have sequenced about 13.3% of all positive cases, compared with 3.8% in the U.S., according to GISAID, which promotes rapid sharing of data on covid-19 and the flu. While Iceland and Denmark have sequenced a greater percentage of their positive cases, the size of the U.K.'s population and the scope of its outbreak mean that Britain has sequenced many more cases.
This surveillance provided key evidence that Johnson and his chief medical officers used in deciding to tighten pandemic restrictions and ramp up the U.K.'s vaccination program.
Johnson's Conservative government is requiring vaccine certificates to enter nightclubs and reintroducing restrictions that were lifted almost six months ago. Masks must be worn again in most indoor settings, and as of Monday people were urged to work from home if possible.
Many scientists say those measures are unlikely to be enough and are calling for tougher ones. But cafes, pubs and shops in city centers fear that plummeting numbers of commuters will hammer their Christmas business.
Johnson is facing a major rebellion from unhappy Conservative lawmakers when Parliament votes on the new restrictions, although the measures are still likely to pass with support from the opposition Labour Party.
Robert Read, a professor of infectious diseases at the University of Southampton, said it was still unclear how severe cases of covid-19 from omicron are, but "omicron probably requires much larger amounts of antibody in the blood in order to thwart the virus as much as possible."
"We need to get those third doses into as many adults as we possibly can, just in case this virus turns out to be a raging bull rather than a pussycat," Read told radio station LBC.
Information for this article was contributed by Pan Pylas of The Associated Press.