Lateral flow tests may be less effective at detecting Omicron even when people are at their most infectious, a US real-world study has found.
Researchers at the US Covid Sports and Society Workgroup examined lateral flow and PCR tests results from 30 people working in offices where there had been an Omicron outbreak in December.
It took an average of three days longer for people to test positive on a lateral flow test — which give results in as little as 15 minutes — after their infection had already been confirmed on a PCR test, they found.
It is not clear whether the results simply support previous findings that PCR tests are inherently more sensitive, or if the lateral flow tests are less accurate at detecting Omicron.
Neither of the lateral flow tests brands examined — Abbott BinaxNOW and Quidel QuickVue — are used in the UK. And British health chiefs have insisted the swabs dished out across the country can spot Omicron just as well as they can detect other variants.
But their US counterparts warned last week that early evidence suggests they are less sensitive to the mutant strain — meaning they may incorrectly tell more people they are not infected with Omicron compared to the number of false negatives they dished out for other strains.
Doubts over the tests come as ministers and health officials in the UK urge people to take a lateral flow test before going out to meet people in an attempt to reduce the chances of people unknowingly passing the virus on to others.
Scores of Brits complained over the Christmas period that they received a negative lateral flow result only later to test PCR positive, prompting concerns about the kits’ accuracy.
The graph shows the viral load — measured in cycle threshold (Ct) — and lateral flow test results from patients infected with Covid after they tested positive on a PCR test (day zero). Patients were tested with either an Abbott BinaxNOW test (squares) or Quidel QuickVue test (circles). In the first three days after testing positive on a PCR, patients falsely tested negative on both tests (indicated by red shapes) despite being most transmissible at that point (indicated by a low Ct value, which equates to a higher viral load and more chance of infecting others)
Lateral flow test kits, like the one pictured above are a cornerstone of the Westminster Government’s plans to keep venues like nightclubs and pubs open with ministers urging revelers to take one before heading out to meet others in case they have Covid
The US team wanted to examine whether there was a difference in the sensitivity of PCR tests that swab the throat and two lateral flow tests that use a nasal swab.
The study, which was published on preprint website Medrxiv and has not been peer-reviewed, examined swabs taken from workers as part of daily testing between December 1 and 31 at offices in New York, Los Angeles and San Francisco where there had been an Omicron outbreak.
Each worker was double-jabbed and most were boosted.
Sajid Javid ‘opposed loosening Covid travel testing regime’
Sajid Javid opposed Boris Johnson’s decision to loosen travel testing rules because of fears the move could heighten the risk of another lockdown, it was claimed today.
Mr Johnson announced earlier this week that travellers arriving in England will no longer have to take a pre-departure Covid test.
He also axed the requirement for travellers to self-isolate on arrival until they receive a negative PCR test, with people now required to just take a lateral flow test within two days instead.
The PM said he had taken the decision because the Omicron variant is now so prevalent in the UK that the border measures were only having a limited impact on the spread of the disease.
But Mr Javid is said to have strongly opposed relaxing the rules, warning his Cabinet colleagues that the move increased the risk of having to shut down the economy again.
Mr Javid told a meeting of the Cabinet’s Covid-O committee that dropping the PCR testing requirement would restrict the UK’s ability to identify new coronavirus strains, according to The Times.
The Health Secretary told his colleagues that ditching the PCR testing rule could therefore increase the risk of a dangerous new variant spreading undetected in the UK which could then force the Government to impose a lockdown.
A Government source told The Times: ‘Sajid was very unhappy about the decision to remove the requirement for PCR tests.
‘He argued they are instrumental in spotting new variants sooner. He said that by scrapping them you increase the risk of having to shut down the entire economy. But he lost the argument.’
The team found that 29 of the 30 infections were caused by Omicron and everyone developed symptoms within two days of their first positive PCR test.
Four people transmitted the virus to others when they were still testing negative on lateral flow tests but tested positive on a PCR swab.
And the PCR results showed they were very infectious at the point when they tested negative on lateral flow tests, having a viral load — which is measured in cycle thresholds (Ct) — between 23 and 28. A figure below 30 is considered to be high.
On the first two days of their infection, all lateral flow tests gave false negative results despite 28 of the 30 people having a viral load below 29.
And it took an average of three days from the first positive PCR test to then test negative on a lateral flow test.
The team said Omicron infects ‘faster and more efficiently than Delta’ in the airways and less severely in the lungs.
This causes symptoms including a sore throat and a loss of taste and smell, which are better detected by saliva, which is collected for a PCR test, than a nasal swab, which is used for the two lateral flow tests examined in the study.
Results from a sub-group of five workers, who received daily throat PCR, nasal PCR and nose swabs for lateral flow tests showed the viral load peaked in the throat swabs two days before the nasal tests.
The researchers said their findings show lateral flow tests ‘lagged in the ability to detect Covid during an early period of disease when most individuals were infectious with Omicron’.
‘Rapid antigen tests may not be as fit-for-purpose in routine workplace screening to prevent asymptomatic spread of Omicron, compared to prior variants, given the shorter time from exposure to infectiousness and lower infectious doses sufficient for transmission,’ they wrote.
‘In real-world antigen testing, the limit of detection was substantially lower than manufacturers have reported to the FDA [US Food and Drug Administration] based on laboratory validation,’ the team said.
Dr Robby Sikka, the study author, told the New York Times that US officials are ‘aware that there are flaws with antigen testing’.
The study comes as the UK Health Security Agency (UKHSA) continue to insist the lateral flow tests dished out by NHS Test and Trace are effective at detecting Omicron.
In its tests the UKHSA used the five lateral flow tests used in the UK — Acon Flow Flex, Innova Biotime, Orient Gene and two by SureScreen — on samples from patients infected with Omicron.
Record numbers of Britons have been tested for the virus in recent days, with more than 2million getting swabbed on January 4 and 5
All of the tests showed a positive result for each patient, even when their sample was diluted, showing the devices have ‘a comparable sensitivity to that observed for previous strains’ of Covid, the UKHSA said.
How to do a rapid lateral flow test
Rapid lateral flow tests are for people who do not have Covid symptoms, such as a high temperature, a cough or a loss or change to smell or taste.
The tests give a quick result using a device similar to a pregnancy test.
People with Covid symptoms should do a PCR test.
Before doing a rapid lateral flow test the NHS advises:
- try not to eat, drink, smoke or vape 30 minutes before doing the test as this may affect the result
- read the instructions carefully
- clean the surface you’re putting the test on
- check nothing in the test kit is damaged or broken
- start the test within 30 minutes of opening the test kit
If your test requires a throat swab:
- open your mouth wide and rub the swab over your tonsils (or where they would have been)
- avoid the end of the swab touching your teeth, tongue and gums
- put the same swab inside your nose (about 2.5 cm up or until you feel some resistance)
If your test requires a nose swab only:
- put the swab inside your nose (about 2.5 cm up)
Completing the test:
- put the end of the swab into the tube so it’s in the liquid
- squeeze the liquid from the tube onto the test strip
- check the waiting time in the instructions that came with your test kit
- wait for the time shown in your test kit instructions
- read your result
- report your result on the Government website
However, at the time, the UKHSA said it was monitoring how well the tests perform in the real world to ensure their sensitivity is not ‘significantly reduced’ when people swab themselves.
And the US FDA last week warned new laboratory findings indicated lateral flow tests may have reduced sensitivity against Omicron, meaning they could wrongly tell more infected people they are free of the virus.
However, it said the rapid tests could still spot the highly-infectious variant.
The FDA previously tested the accuracy of the tests using samples of dead Omicron virus and found they were just as effective.
But the lab ‘suggests that antigen tests do detect the Omicron variant but may have reduced sensitivity,’ the agency said.
However, the FDA is still advising people to use lateral flow tests, saying the findings are still at an early stage.
‘It is important to note that these laboratory data are not a replacement for clinical study evaluations using patient samples with live virus, which are ongoing,’ it said.
‘The FDA continues to authorize the use of these tests as directed in the authorized labeling and individuals should continue to use them in accordance with the instructions included with the tests.’
No data on how less sensitive lateral flow tests may be at detecting Omicron or how many samples of the virus were tested was detailed in the FDA announcement.
Lateral flow tests are a cornerstone of the UK’s plans to keep venues like nightclubs and pubs open with ministers and scientists urging Britons to take one before meeting others to reduce the spread of the virus.
But thousands have struggled to get hold of the tests in recent weeks, as pharmacies ran out and the Government’s website was unable to send them out.
The UKHSA this week announced that Covid testing rules would be eased for people who test positive on a lateral flow swab from January 11 in England.
Up to then, people who receive a positive result on the rapid tests were told to book a PCR test to confirm they were infected and to isolate until they got the second set of results.
But under the new move, people will no longer need to confirm their result.
The UK Health Security Agency said the ‘vast majority’ of people with a positive LFD result can be confident they have the virus because case rates are so high. It estimates the number ‘false positives’ are as low as one in 3,000.