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The Omicron variant of coronavirus surprised many Britons in recent weeks by turning up at parties undetected by rapid Covid-19 tests, prompting questions about whether the devices provide true confidence for people to live without fear of infection.

The UK Health Security Agency has said initial investigations show the rapid tests work just as well against Omicron as against Delta, the previously dominant strain. Experts believe the UK tests are likely to continue to work because they detect a different protein from the much-mutated spike — and the variant’s potentially far higher viral load means they could even be more effective.

But rapid lateral flow tests have always been imperfect, detecting an average of 72 per cent of symptomatic cases, and 58 per cent of asymptomatic ones, according to a meta-analysis of studies by the medical database the Cochrane Library.

Tim Peto, a professor of medicine at Oxford, who has worked on validating tests’ accuracy, said the scientific community had “very poor methods” of telling if people were infectious. “The LFTs are the best of a bad lot,” he said.

The government is maintaining faith in its pioneering rapid testing programme to control the spread of infection while it resists more drastic measures. It has introduced, for England, negative tests as an option to vaccination certificates to access large events and nightclubs, and asked the vaccinated to test every day for a week after they come into contact with Covid-19, rather than isolating.

“We have always been clear that rapid testing is a vital tool in curbing the spread of Covid-19 because it helps to rapidly uncover hidden cases of the virus in people without symptoms,” the UKHSA said.

Their use has been intensely debated. Supporters point to research that shows they are more effective at identifying people at their most infectious, while detractors worry they offer a false sense of security.

Lateral flow tests return results faster than PCRs and are far cheaper — but less accurate. The rapid tests contain a strip of antibodies that turns red if it reacts with the plentiful protein that makes up the virus’ shell. PCR tests detect the presence of virus earlier and in smaller quantities because they amplify the sample and pick out genetic signatures. In the UK, people are advised to have a PCR test to confirm a positive result.

The sheer number of cases of Covid-19 in the UK — up 44 per cent to 534,415 in the past seven days means that LFTs will pick up more cases, but they will also miss more people who could spread the disease.

Tom Lewis, a medical microbiologist at North Devon district hospital, said the tests were most useful if you understand how likely it is that you are infected — and the probability someone is carrying the virus has changed “overnight”. 

If you assume 1 in 100 people is now infected, he said, there will be an average of four people at risk of spreading the virus at a 400-person event. Based on experience, lateral flow tests will pick up only two.

“That doesn’t sound much, but two people are now moving around in that room with Covid and infectious . . . they have about a 50 per cent chance of transmitting, probably higher now with the Omicron variant. It’s definitely a transmission event,” he said.

But he said the tests make people feel safe. “The lateral flows are a massive confidence trick,” he added.

The tests are most useful if taken just before attending an event or visiting a vulnerable person. Catherine Moore, a consultant clinical scientist at Public Health Wales, said she had seen people post on social media that they were shocked when a test taken in the morning was negative and one in the afternoon was positive.

“That’s just showing you that the viral load increases over the day, which you would expect to see,” she said.

Viral loads increase in bursts, often about 48 to 72 hours later than the initial infection, she said. “The big downside of lateral flow devices is probably their sensitivity in the early parts of the infection when you may not have lots of virus on board straightaway. It could be six to eight hours later there’s another surge in virus that makes the test positive,” she said.

Repeating tests can make the results more robust, so many experts endorse the government recommendation for vaccinated people exposed to the virus to test for seven days straight.

Chart showing that in the early days after exposure to the virus, a negative lateral flow result is by no means a guarantee that you are virus-free. Negative results are expected in the first couple of days after exposure, but a positive could be just hours away. Just before the infectious, period, the lower sensitivity of lateral flow tests means they could miss an infection that a PCR would pick up. It’s possible to test positive by PCR several days after lateral flows would no longer detect the virus

On a population level, a new Liverpool university study shows that using the tests to prompt people to isolate reduces the burden on the healthcare system. When the Liverpool City region rolled out asymptomatic testing before it was available nationally, it reduced hospital admissions by 32 per cent compared with other areas. It also saved more than 8,300 key worker days by allowing staff to test, rather than isolate, after a contact.

Iain Buchan, the university’s chair of public health, said lateral flow testing should no longer be “divisive” when the evidence is clear it is a “very important public health tool” to balance risks while avoiding lockdowns.

“We know the damage that has been done to local economies and social wellbeing,” he said.

But the whole NHS Test and Trace programme has come at a financial cost that the House of Commons public accounts committee described as “eye-watering”, with a budget of £37bn for two years. A report published by the committee in October said only 14 per cent of the rapid tests had been registered, making it hard to know if people isolated after positive results.

Allyson Pollock, clinical professor of public health at Newcastle University, said it would have been far more cost-effective to do traditional contact tracing to break chains of transmission.

“We’ve spent billions of pounds with no good evidence for the effectiveness of lateral flow tests,” she said. “If the purpose is to give confidence, it’s a very expensive way to give people confidence.” 

The utility of the tests in the current wave will depend on how ill people become. While early studies suggested Omicron causes milder symptoms in vaccinated or previously infected populations, more recent research — such as that undertaken at Imperial College London — has found this is unlikely to be caused by the virus’s mutations causing it to be less innately severe.

If the effects for most people are milder, then even the self-described “great test advocate” Peto worries we could be just “locking people up and causing anxiety”.

“It’s not entirely clear that optimism is justified, it is a bit premature at the moment,” he said. “In two weeks’ time it will be obvious if I’m right or wrong.” 

Additional reporting by Oliver Barnes

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