According to a new real-world study, it may take an average of three days for people to test positive on a nasal-based lateral flow test (LFT) after their first positive PCR result.
This is likely because the virus grows quicker in the throat than the nose, and is therefore more easily detectable, experts say. Whereas some LFTs only require a nasal swab, all PCR tests involve taking samples from both the nose and the back of throat.
The research, which is based in the US and has yet to be peer-reviewed, suggests most Omicron cases were infectious “for several days” before being detectable by a rapid at-home test.
The study tested 30 people during Omicron outbreaks at five different workplaces across the country. Participants were fully vaccinated and given both throat-based PCR tests and nasal-based LFTs during the course of the research.
“We found that rapid antigen tests [LFTs] lagged in the ability to detect Covid-19 during an early period of disease when most individuals were infectious with Omicron,” the authors of the paper wrote.
At the beginning of the study, 28 out of 30 people returned false negative results via the LFTs, despite having a substantial infectious viral load, before eventually testing positive.
On average, it took three days for people to test positive on the lateral flow after their first positive PCR result.
In four cases, people transmitted the virus to others while testing negative with a LFT, according to the study.
“The policy implication is that rapid antigen tests [LFTs] 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,” the authors of the paper write.
The research was based on two at-home tests that are widely used in the US: Abbott BinaxNOW and Quidel QuickVue. Neither are available in the UK.
It did not comment on whether the infections were missed because the lateral flow tests are less sensitive to Omicron, or because the throat-based tests are better at detecting the new variant.
Analysis from the UK Health Security Agency (UKHSA) has found that LFTs are just as effective in detecting Omicron compared to other coronavirus variants.
However, this was based on laboratory testing only and does not include information on the time taken to test positive for Omicron using a nasal-based lateral flow compared to a PCR - an issue that the UKHSA is currently assessing.
Mounting evidence suggests that Omicron is more likely to infect and grow in the throat than the lungs, which may explain why it is quicker to detect the virus here compared with the nose.
The authors behind the US study said: “Omicron has been shown to infect faster and more efficiently than Delta in human bronchus, but with less severe infection in lung[s], translating to symptom increase of sore throats and decrease of loss of taste and smell, better detected by saliva than nasal swabs.”
Billy Quilty, a research fellow at the London School of Hygiene and Tropical Medicine, said the research shows that “we should at least be swabbing our throats when doing LFTs (assuming saliva and throat swabs are similar) as viral load appears to rise there sooner and is likely readily emitted when speaking”.
Dr Alexander Edwards, an associate professor in biomedical technology at the University of Reading, said data from the study was “important and notably” but warned against jumping to conclusions due to “several reasons”.
“There is a lot of variation between the course of infection between individuals; there can be quite a lot of variation in the amount of virus captured by a swab, even from the same individual; and different test products have different analytical sensitivity (i.e. detect more or less virus).”