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.
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.
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.
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