Taking a lateral flow test the night before a big event like a wedding might have helped for previous variants – but experts have warned that Omicron is so infectious, the result could have already expired by the time you need it.
People should now do their tests immediately before heading out somewhere they may be at risk of infecting others, they urged.
Government guidance currently recommends taking a test ‘if you will be in a high-risk situation that day’.
The NHS Covid pass scheme will accept negative rapid test results uploaded to their website in the previous 48 hours.
But Irene Petersen, a professor of epidemiology at University College London, said those infected with Omicron ‘may switch from being non-infectious to infectious within hours’.
Prof Petersen told The Telegraph: ‘That’s not good enough. We’re seeing so many examples now where people have taken a test a day before and then when they take one the day after they are positive.
‘Omicron is very, very fast, so the test result expires very quickly. It is hours that we are talking about now.’
Alan McNally, professor of Microbial Evolutionary Genomics at the University of Birmingham, also supported this.
He tweeted: ‘If you have symptoms and are LFD (lateral flow) positive you have Covid. Don’t wait on PCR confirm to act. If you are LFD negative you still need to exercise caution. Protect yourself and others.
‘And use LFD just before you go out/visit. Not within 24 hours. Just before.’
Evolutionary virologist Emilia Skirmuntt replied to him saying that it was ‘quite worrying how many LFDs results [are] negative in people who get positive tests just a few hours later’.
A woman living in Cardiff responded to say ‘last Tuesday at 7am I had a negative lateral flow, at 5.30pm a positive one. That’s how quick it can change.’
Yesterday, the UK recorded another 91,743 Covid-19 infections and data confirmed another grim pandemic milestone: the number of positive tests collected on December 15 passed 100,000 for the first time since the disease arrived, when including backdated results.
New restrictions before Christmas were effectively ruled out yesterday when the prime minister stopped short of introducing new measures.
Speaking after an emergency Cabinet meeting, Boris Johnson said: ‘We will have to reserve the possibility of taking further action to protect the public, and to protect public health and the NHS, and we won’t hesitate to take that action.’
He added the data is being kept under ‘constant review… hour by hour’ and warned the Government ‘won’t hesitate to take action’.
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