People may have Omicron covid variant even with a negative lateral flow test when they first start showing symptoms.
Speaking at the science and technology committee in parliament on Tuesday, Dr Angelique Coetzee, the South African doctor who first detected the Omicorn variant, was addressing members of her findings.
During the meeting, she said people who first showed signs of Omicron symptoms such as a headache may test negative on a lateral flow test.
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Dr Coetzee said: "The three major complaints are myalgia which is what we call sore muscles or body aches and pains.
"Headache as well as a little bit of fatigue for a day two.
"Normally the patients come, because we run a campaign where we ask people please get tested even if you wake up with a slight headache and not feeling well please let us come and double check.
"Consequently we have noticed if you wake up this morning and you have this headache and myalgia, it would be better just to wait 24 hours.
"Especailly for the rapid test as we have seen the rapid test can be false negative, especially in the first period, the first less than 24 hours.
"But after 24 hours and up to five to six days later, the rapid test is more than sufficient to detect cases."
Dr Susan Hopkins, chief medical adviser at the UK Health Security Agency, said people should take a lateral flow test before they socialise.
She told the science and technology Committee of MPs: “Lateral flow devices will detect in asymptomatic as well as symptomatic infection.
“They’ve been used very effectively now for almost one year in the UK population.
“What we know is that overall it will detect about 50% of cases compared to PCR, but it will detect about 80% or even more than that of people who have high amounts of virus and therefore are at the highest risk of transmitting to others.
“Our strong public health recommendation is if people are going out to venues to socialise, they should do lateral flow before they go to reduce the risk of going into that venue with asymptomatic infection and therefore transmitting to others.”
She added: “Again, the public health advices to take a lateral flow before you go out to socialise at the moment, we would recommend that.
“The Government certification policy, which has been discussed for many months includes certification or a lateral flow test, but it does not preclude people doing a lateral flow test in addition to being vaccinated.”
Daily testing for anyone double-jabbed who comes into contact with a coronavirus case has started.
Fully vaccinated people who are 'pinged' by the Track and Trace app should take a daily lateral flow test for seven days to slow the spread of the virus, instead of isolating - as announced by the Prime Minister last week.
Any unvaccinated adults must continue to self-isolate for 10 days if they come into contact with the virus as well as anyone who tests positive themselves or develops symptoms.
Dr Jenny Harries, chief executive of the UK Health Security Agency, said: "If you are identified as a contact of someone with COVID-19, taking a rapid daily test - and only needing to isolate if it is positive - will help reduce the spread of the virus and minimise its impact on our everyday lives over the coming weeks and months."
