his week coronavirus screening tests for students are being rolled out across UK universities. Some institutions are already inaccurately claiming that if you test negative, you will know it is safe to return home. The matter is very important for many families, who don’t want their children stranded while self-isolating at university over Christmas, but are also worried about them bringing home coronavirus to elderly or vulnerable relatives. In my job as a professor of population health, I help national governments and policymakers decide whether to run screening programmes like those being undertaken at universities. Here’s what we do and don’t know about university screening. A preliminary Public Health England (PHE) evaluation found that the new lateral flow test that is to used by universities – delivering a result within 30 minutes – had previously missed cases of coronavirus in between a quarter to a half of people it tested, by giving a negative reading. It’s not simply a problem with the particular manufacturer the government chose: most rapid lateral flow type coronavirus tests seem to miss a lot of people with coronavirus. Many students recently infected (in the few days before the test) will also be missed: this is because coronavirus only becomes detectable a few days after infection has occurred. So students testing negative on two tests a few days apart, and self-isolating in between, have the lowest chance of having coronavirus, but because the test does miss cases they still may be infected. Many universities are not advocating self-isolation between tests, so many students infected in the time between the two tests will also be missed. The existing test-and-trace programme should detect people with symptoms and their contacts. The idea behind the rapid university testing is to find students who have coronavirus, but have no symptoms (probably about one in five infected people), so that they can self-isolate instead of bringing the infection home. For this to work you need two things. Firstly, to accurately detect coronavirus in students without symptoms. The instructions for use of the lateral flow test actually say it is for people with symptoms – not the students without symptoms, whom it is primarily being used for here. Secondly, for the process to work, we need the students and their contacts to self-isolate if coronavirus is detected. We don’t know if they will do this, but evidence from test and trace suggests many people do not. So, ultimately, university testing may not reduce transmission as much as hoped. The thing about screening programmes is that they often don’t quite pan out as intended. There are some obvious ways in which the university screening programme could actually increase the spread of coronavirus. The real danger is that students will mistakenly believe that a negative test means they don’t have coronavirus. It would be hard to blame students for such a misunderstanding when a health minister, Lord Bethell, announced that evaluations are “already demonstrating that lateral flow tests can be the reliable, highly sensitive technology we need”, and the health secretary, Matt Hancock, said he “didn’t recognise” PHE’s figures showing the test missing cases. Given the limitations of this rapid test, some students will go straight home after a negative result and either be infectious straight away or become infectious within days. This would happen whether they had been tested or not. The problem comes if they mistakenly believe they are not infectious because they had a negative test, and visit an elderly or vulnerable relative under that misapprehension. There are other potential unintended consequences: some students, for example, may misinterpret a negative test result as a free pass to party, which could also increase transmission. There isn’t very much evidence about whether university screening will do more good than harm, or whether overall it will decrease or increase infections. We don’t yet know how many students will take the tests, and how they will react to positive or negative results, among many other things. In a pandemic, decision-making is more urgent, and there is little time to gather research evidence. However in the UK we have fantastic researchers, dedicated funding streams and organised systems for pulling together this evidence very quickly. We aren’t rolling out a vaccine before finding out whether it is safe and effective: we are waiting for the randomised trial evidence. However, we do seem to be diving headlong into mass screening. We in the research field have investigated many other screening programmes that seem like good common sense – but when we do a randomised trial it often turns out they do quite the opposite of what was intended. Another lesson learned from decades of running successful screening programmes in the UK is the importance of honesty. Authorities need to be honest with people about the good and bad parts of screening, and let them make their own informed choice whether to take part. Families have got used to balancing risks this year: we should tell them openly that these tests can detect some asymptomatic cases but they miss some, too, and trust people to make sensible choices for their own families. Such an approach will help avoid the worst possible outcome from screening students for coronavirus: that young people returning home end up infecting elderly or vulnerable relatives based on a misunderstanding of what negative results mean. That would not only cause completely avoidable tragedy for families but people may also lose faith in the government on the next initiative – the critically important rollout of vaccines. • Dr Sian Taylor-Phillips is professor of population health at the University of Warwick, specialising in evaluating population-screening programmes