Omicron is expected to be the dominant variant in London in the next 48 hours, the Secretary of State for Health and Social Care, Sajid Javid, warned in a statement to Parliament Monday.

Omicron represents 20% of England's cases and 44% of London's cases, Javid said.

"No variant of COVID-19 has spread this fast," Javid said.

Ten people with omicron are in U.K. hospitals.

"Hospitalisations and deaths lag infections by around two weeks," Javid said, "So we can expect those numbers to dramatically increase in the days and weeks that lie ahead."

"In preparation, the UK’s four Chief Medical Officers raised the COVID Alert level to 4, its second-highest level, this was done over the weekend. And NHS England has just announced it will return to its highest level of emergency preparedness: Level 4 National Incident," Javid said. "This means the NHS response to Omicron will be a coordinated as a national effort, rather than led by individual trusts."

Javid also urged people to get booster doses, stating that 40% of adults in the U.K. have gotten boosters so far.

-ABC News' Zoe Magee

Massachusetts officials have purchased 2.1 million at-home COVID-19 tests to give for free to 102 towns with the highest percentage of people below the poverty level, Gov. Charlie Baker said.

The tests come in packs of two and can be completed at home in 15 minutes.

"Each city or town will be able to determine how best to distribute tests within their community," Baker's office said in a statement Monday.

Philadelphia restaurants will require proof of vaccination beginning on Jan. 3, city officials announced.

The mandate applies to anywhere selling food, including bars, movie theaters and sports venues.

"New cases and hospitalizations are rising, and we must do more to protect our residents," Mayor Jim Kenney tweeted.

From Jan. 3 to Jan. 17 there will be a grace period, during which patrons can show proof of a negative test. But after Jan. 17, all patrons must show their vaccination status, officials said.

The omicron variant appears to have a "growth advantage" over the delta variant, the World Health Organization said in a technical brief released Sunday.

"It is spreading faster than the delta variant in South Africa where delta circulation was low, but also appears to spread more quickly than the delta variant in other countries where the incidence of delta is high, such as in the United Kingdom," the WHO said in the brief, which was dated Friday. "Whether omicron's observed rapid growth rate in countries with high levels of population immunity is related to immune evasion, intrinsic increased transmissibility, or a combination of both remains uncertain. However, given the current available data, it is likely that omicron will outpace the delta variant where community transmission occurs."

Meanwhile, preliminary findings from South Africa suggest omicron may cause less severe illness than delta, and all cases of omicron reported in Europe to date have been mild or asymptomatic. But the WHO said "it remains unclear to what extent omicron may be inherently less virulent" and that "more data are needed to understand the severity profile."

The WHO also noted that "there are limited available data, and no peer-reviewed evidence, on vaccine efficacy or effectiveness to date for omicron." However, preliminary evidence, and the considerably altered antigenic profile of the variant's spike protein, suggests a reduction in vaccine efficacy against infection and transmission associated with omicron.

"There is some preliminary evidence that the incidence of reinfection has increased in South Africa, which may be associated with humoral (antibody-mediated) immune evasion," the WHO said.

The diagnostic accuracy of routinely used polymerase chain reaction (PCR) test and antigen-based rapid diagnostic test (Ag-RDT) assays does not appear to be influenced by omicron. Therapeutic interventions for the management of severe or critical COVID-19 symptoms associated with omicron are also expected to remain effective, according to the WHO.

"However, monoclonal antibodies will need to be tested individually, for their antigen binding and virus neutralization and these studies should be prioritized," the WHO added.

-ABC News' Christine Theodorou

France announced Thursday that it will ban non-essential travel to and from the United Kingdom due to the country's surge in cases of the omicron variant.

Starting Saturday, France will require people to have "a compelling reason" to travel between the two countries. Travel for tourism or work will not be allowed. French citizens, however, can return to France, according to a statement from the French prime minister's office.

All travelers from the U.K. will be required to show proof of a negative COVID-19 test taken less than 24 hours before departure. Upon arrival in France, they must self-isolate for a week, but that period can be ended after 48 hours if they test negative for COVID-19 again.

The new rules apply to people regardless of their vaccination status.

"Faced with the extremely rapid spread of the Omicron variant in the United Kingdom, the Government has chosen to reinstate the need for an essential reason for travel from and to the United Kingdom, and to strengthen the requirement for tests on departure and arrival," the French prime minister's office said in the statement Thursday. "The Government is also calling on travelers who had planned to visit the United Kingdom to postpone their trip."