The U.K. government ramped up its response to COVID-19 Monday, asking citizens to cut all unnecessary contact with other people, after a British research team warned its earlier strategy would lead to hundreds of thousands of deaths. But those researchers also gave a troubling warning for countries around the world implementing lockdown measures: in order to be effective, they would need to last 12 to 18 months.

On Monday evening, Prime Minister Boris Johnson announced what many saw as an abrupt shift in his government’s strategy for combating the new coronavirus. Where before authorities told Brits to continue living more or less as normal and to self-isolate for a week if they displayed symptoms, the entire population is now asked to stop seeing friends and family, avoid public spaces like pubs and restaurants, and self-isolate for 14 days if anyone in their home has a cough or a fever. A member of the White House task force also cited modeling by British researchers, apparently the Imperial report, as the U.S. announced a strengthening of its own social distancing rules on Monday.

Though the measures stop short of the obligatory quarantine imposed in Italy, France and Spain, Johnson’s announcement brought the U.K. closer in line with the international coronavirus response after a week in which the country seemed to be opting for a unique strategy. On Friday, the government’s chief scientific adviser said that by not immediately introducing restrictions on travel and socializing, the virus would be allowed to circulate among healthier members of the population; its impact would be reduced by isolating those with symptoms, allowing the British population to develop some “herd immunity.” That sparked a backlash from some immunologists and epidemiologists, who argued the strategy would have “severe” consequences.

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Those concerns were born out in new modeling on different social distancing approaches in both the U.K. and the U.S., published Monday by London’s Imperial College COVID-19 Response Team, which advises the British government. In both countries, the scientists found that the kinds of limited social distancing that the U.K. had been implementing – a “mitigation” approach – would exceed health care systems’ maximum capacities “many times over.” In the U.K., roughly 250,000 people would die from the virus. In the U.S., the figure would be between 1.1 and 1.2 million.

The report concludes that a “suppression” approach, which includes strict social distancing for everyone and quarantining for households with victims of the virus, “is the only viable strategy at the current time.” That’s the approach that China and a growing crowd of European countries are already taking.

But the report’s authors also warn of a “major challenge” associated with the suppression approach. It “will need to be maintained — at least intermittently — for as long as the virus is circulating in the human population, or until a vaccine becomes available. In the case of COVID-19, it will be at least 12-18 months before a vaccine is available.” A trial of a vaccine began its first stage on Monday in the U.S., but those leading the work reiterated that 12-18 months is the likely timeframe for public use, if all goes well.

Experts say governments are only now beginning to acknowledge the length of time needed for lockdown measures to work. “This has been very unspoken about in countries like Italy for example,” says Elisabetta Gropelli, a virologist and lecturer in global health at St. George’s University of London. “Once you start isolation, how do you decide when to stop? Once you go in, when can you come out?”

So far, many of the social distancing measures that countries have announced are slated for an initial period of several weeks. In Italy, schools and universities will be closed and travel without a special permit will be banned until April 3. In the Netherlands, schools, restaurants and gyms are all closed until April 6. The president of the European Union is proposing a 30-day ban on non-essential travel into the bloc. It is highly likely that at least some of these measures will be extended. “People should be thinking of a minimum of weeks to months and, depending how it goes, it may be longer,” the U.K.’s chief medical officer said of his country’s new restrictions.

In China, where the coronavirus began in late December, strict quarantine measures across cities have allowed the country to dramatically reduce the spread, with the number of new daily cases falling from over 3,000 in February to just 16 on Monday. But health officials fear that once most people begin to leave their homes and start mixing as normal, the virus will quickly begin to spread again.

The Imperial report warns that if “intensive” strategies aimed at suppression are not maintained in the U.K. and the U.S., “transmission will rapidly rebound, potentially producing an epidemic comparable in scale to what would have been seen had no interventions been adopted.” The report’s authors say that social distancing measures could be relaxed for short periods of time but would need to be reintroduced “if or when case numbers rebound.”

Nevertheless, there’s reason to hope that the measures won’t have to be so tough for that entire period. The report says that suppression strategies could evolve over time if the virus can be controlled. For example, if a country manages to reduce the spread of the virus and the caseload with lockdown measures, it could set up sophisticated contact tracing and quarantine measures, like those currently being used in South Korea, where more than 270,000 people have been tested.

South Korea, which in early March had the worst outbreak outside of China, has managed to slow the rate of new infections without introducing lockdown-style measures. Instead, the country carried out testing on a massive scale and aggressively traced patients’ contacts by tracking their movements through their credit card transactions and cell phone use — as well as widespread surveillance cameras — enabling them to isolate those who were at risk. A transparent public information campaign also helped people to stay away from high risk neighborhoods and take other action to protect themselves.

Aris Katzourakis, a Professor of Evolution and Genomics at the University of Oxford, says that aggressive social distancing measures implemented now could “buy time” for the U.K. and other countries to eventually implement high-tech tracking and surveillance measures.

First, though, he says countries would need to slow the spread of the virus, reducing the basic reproductive number of the pandemic — the number of people that each person who has the virus will infect— from its current estimated average of between 2 and 2.5 down to just 1.

“Once you get things under control, with a lot of targeted surveillance, there are ways of locally relaxing restrictions and returning to some level of activity,” Katzourakis says. “[But] it will never be the same until there is a cure, or a vaccine. ‘Returning to normal’ simply isn’t something we should expect to see for a very, very long time.”

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Write to Ciara Nugent at ciara.nugent@time.com.