As many as 10,000 people in the UK are probably already infected with coronavirus, and many people should expect to lose loved ones, the government has said as it announced measures less stringent than those taken by other countries. Britain moved from the “contain” phase of the crisis to the “delay” phase on Thursday, when the death toll for UK citizens was at 12, two of them having died overseas, and the official number of infected people reached 590. However, speaking after Thursday’s emergency Cobra meeting, the government’s chief scientific officer, Sir Patrick Vallance, said the true total was likely to be between 5,000 and 10,000. He said 20 people were being treated for Covid-19 in intensive care units and that the UK was on a trajectory about four weeks behind that of Italy, which has had more than 1,000 deaths. Advertisement Chris Whitty, the chief medical officer, said that worst-case scenario planning projected that 80% of the country would contract the virus, with a 1% mortality rate. This equates to more than 500,000 deaths. The prime minister, Boris Johnson, said: “We have all got to be clear, this is the worst public health crisis for a generation. Some people compare it to seasonal flu. Alas, that is not right. Due to the lack of immunity this disease is more dangerous. “It is going to spread further and I must level with you, I must level with the British public: many more families are going to lose loved ones before their time.” Get Society Weekly: our newsletter for public service professionals Read more Johnson said schools would not close and neither did he join Scotland in banning gatherings of more than 500 people, though he warned that major events may be cancelled in future because of the burden they placed on public services during the outbreak. He announced new advice for people aged over 70 – that they should not go on cruises – and said schools should not take pupils on trips abroad. Health workers would no longer test people for the virus in their homes, but would continue to test people already in hospitals, where the risk of contagion is higher and the impact more serious. He said the most dangerous period was “some weeks away” and the intention with delay was to increase the ability of the emergency services and society more widely to cope. When enthusiasm for measures started to flag, people were less vigilant, he told reporters. He said people with even mild symptoms should stay at home for a week and that the advice could soon become stricter. “At some point in the next few weeks we are likely to go further, and if someone in a household has those symptoms then we will be asking everyone in that household to stay at home. I want to signal now that this is coming down the track.” He ended with an upbeat message, saying the country had faced challenges before and “will get through this”. Announcing Scotland’s decision to ban public gatherings of more than 500 people earlier on Thursday afternoon, Nicola Sturgeon said it was not based on scientific evidence, which suggests it will not have a significant impact on the spread of the virus, but would help for consistency of messaging and to alleviate pressure on the emergency services being stretched by the pandemic. Advertisement Whitty said the UK would not be introducing social distancing measures yet as it was not the right moment within the pandemic. UK coronavirus cases: find out how many are in your area Read more Ireland moved to close schools and colleges on Thursday morning to try to halt the spread of the virus. On Wednesday, Donald Trump decided to ban citizens from 26 European countries from entering the US for the next 30 days. Britain and Ireland are exempt from the ruling because they are not in the Schengen border-free travel agreement. No 10 said on Thursday the government had no plans for the UK to follow the US. “It’s not the current position of the UK, based on medical and scientific advice, that we should halt flights,” the spokesman said. Downing Street has so far judged that the disruption that would be caused by blanket school shutdowns, for example, would outweigh the benefits in terms of slowing the spread of the disease. Q&A How can I protect myself from the coronavirus outbreak? Show However, some health experts have questioned whether the government has acted aggressively enough to contain the virus. Richard Horton, the editor of the Lancet, has argued that social distancing measures should have been taken earlier, claiming the government was “playing roulette with the public”. Contingency planning is under way to enable Brexit talks with the EU to continue next week. This could involve phone or video calls. The prime minister’s spokesman said there was no change to the timetable and June was still the point at which the UK government would walk away from negotiations if it could not secure a Canada-style trade deal. Share your story Share your stories If you have been affected or have any information, we"d like to hear from you. You can get in touch by filling in the form below, anonymously if you wish or contact us via WhatsApp by clicking here or adding the contact +44(0)7867825056. Only the Guardian can see your contributions and one of our journalists may contact you to discuss further. Tell us You can’t make the news stop… … but you can support a news organisation who’ll make sense of it. This is turning into a year like no other, and the swirl of news can be overwhelming. At the Guardian, we draw on all our experience to report the facts, the context, the big picture in tones that are steady and measured. 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