UK Edition
Official advice should be updated to urge people to take lateral flow tests "just before" meeting others, because of how quickly omicron patients appear to become infectious, a senior scientist has warned.
Irene Petersen, a professor of epidemiology at University College London, said those who initially appear not to have Covid-19 "may switch from being non-infectious to infectious within hours".
That means that a negative test carried out in the morning could be overtaken by a positive result in the afternoon, she said.
Prof Petersen called for ministers to urgently update official advice, including guidance that simply recommends taking a test "if you will be in a high risk situation that day".
Under the NHS Covid pass requirement for venues such as nightclubs, a negative test result will be valid for 48 hours.
The warning came as scientists urged ministers to update the official list of Covid-19 symptoms to state that any "cold-like symptoms" could be a sign of omicron and should therefore prompt someone to get a test and self-isolate.
Tim Spector, the lead scientist on the Zoe Covid Study app, which is funded by the Government, suggested that the current official symptoms - fever, cough and loss of sense of smell or taste - were outdated, because they applied to the alpha variant of coronavirus.
Early data showed that a runny nose, headache and fatigue were the most common symptoms of omicron, he said.
"We need to educate people, go back to the basics, and say that if you’ve got cold-like symptoms keep away from people," said Prof Spector. "You shouldn’t be waiting for the three classic symptoms.”
Asked about current government guidance on taking a test on the day, or within 48 hours, of attending an event, 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 as well. It is hours that we are talking about now."
Omicron: ‘Don’t just take one test’
Earlier this month, Sajid Javid, the Health Secretary, warned that the “window between infection and infectiousness may be shorter” for omicron, compared to the delta variant of Covid-19.
Prof Petersen said: "We have now seen that people have gone to events and they say: 'I'm double vaccinated and I've taken a test.'
“But that doesn't help if you took that test yesterday morning, because in the meantime you may have become infectious."
The public should be aware that a negative result "expires very quickly and therefore we need to take the test just before meeting up with other people - or on the doorstep," Prof Petersen said.
Prof Petersen agreed with Prof Spector on the need to update the official symptoms list for Covid-19, so that anyone with "cold-like symptoms" gets tested.
However, she added that the system should be overhauled so those with symptoms can use lateral flow tests instead of PCRs.
"We do not have the capacity to PCR test this many people, and lateral flow tests are much quicker," she said.
"I would also say, don't just take one test. Take another one again 12 hours later, because what we may see now is that people start to get symptoms before they become infectious, because we have a vaccinated population and it is your immune system that induces some of the symptoms. It may take time before you become infectious."