LONDON (AP) — Long lines formed at vaccination centers in Britain as people heeded the government's call for all adults to get booster shots to protect against the omicron variant of the coronavirus, which the prime minister said Monday has caused at least one death.

In a televised announcement late Sunday, Prime Minister Boris Johnson said everyone 18 and up would be offered a third vaccine dose by Dec. 31 — less than three weeks away, and a month earlier than the previous target.

"We are now facing an emergency in our battle with the new variant, omicron," Johnson said. He 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 it will replace delta as the dominant strain within days. But it's unclear whether the expected wave of infections will inundate the country's health system.

About 10 people are in U.K. hospitals with COVID-19 caused by omicron, and Johnson on Monday reported the country's first COVID-19 death involving the variant.

Scientists in South Africa, where omicron was first identified, say they see signs the variant may cause less severe disease than delta but cautioned it was 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 U.K. Health Security Agency says existing vaccines appear less effective in preventing symptomatic infections in people exposed to omicron, though 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 vaccine doses, and 40% of adults have had three. Giving the rest boosters by the end of the month will be a huge challenge, requiring almost 1 million doses administered a day. Johnson acknowledged that many routine medical procedures would have to be postponed to meet the goal.

Teams of military planners and thousands of volunteer vaccinators will help give the jabs at doctors' offices, hospitals, pharmacies and pop-up vaccination centers.

While the online appointment booking system will not be open to under-30s until Wednesday, Johnson said any adult could show up at a walk-in center to get a booster starting Monday.

Lines built up at big London vaccination clinics on Monday morning. The line for shots at St. Thomas' Hospital, on the south bank of the River Thames in London, stretched across Westminster Bridge toward Parliament.

The government's appointment-booking website struggled to keep up with demand. The National Health Service advised people to try accessing the site later in the day or on Tuesday, if they were having problems.

The government also ran out of rapid at-home virus test kits, which have been distributed free to households during the pandemic. The website where tests can be ordered said none were available on Monday. Starting Wednesday, people in England must show proof of vaccination or a negative test to ender nightclubs and other crowded venues.

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 British government raised the country's official coronavirus threat level on Sunday, warning the rapid spread of omicron "adds additional and rapidly increasing risk to the public and health care services" at a time when COVID-19 is already widespread.

Concerns about the new variant led Johnson's Conservative government to bring in vaccine certificates for nightclubs and to reintroduce restrictions that were lifted almost six months ago. Masks must once again be worn 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 other businesses located in city centers fear plummeting commuter numbers will hammer business in the usually busy pre-Christmas period.

Johnson is facing a major rebellion from unhappy Conservative lawmakers when Parliament votes on the new restrictions Tuesday. Dozens are expected to oppose the restrictions — especially vaccine passports, which they say are unfair and economically damaging.

The measures are still highly likely to pass, with support from the opposition Labour Party.

Robert Read, professor of infectious diseases at the University of Southampton, said it was still unclear how severe cases of COVID-19 from omicron would be but "the evidence is that 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 just rather than a pussy cat," Read told radio station LBC.

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