Rapid nasal tests have limited success with omicron, spit tests are better, new US study finds

A new US study has shown that people with Covid-19 may not test positive on rapid tests until after they’ve become infectious to others, providing unclear guidance on whether it’s safe to join gatherings.

Rapid tests have been considered one path to handle the surge of Omicron cases without large upheavals to regular life. But pharmacies have struggled to keep the tests in stock, and new research and large amounts of anecdotal evidence now suggest that they may have limited success with the highly transmissible variant and may provide false negatives.

A small study released on Wednesday found that among 30 participants who used nasal rapid antigen tests and saliva PCR tests simultaneously, four transferred the virus despite testing negative on a rapid antigen test.

The median time between their first positive result using a PCR test and their first positive result using a rapid antigen test was three days.

“Based on viral load and transmissions confirmed through epidemiological investigation, most Omicron cases were infectious for several days before being detectable by rapid antigen tests,” the authors write.

“The policy implication is that 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 add.

Other evidence has suggested that swabs of saliva may be better for finding Omicron than nasal swabs.

Harvard epidemiologist Bill Hanage told Axios that we should “start thinking about testing saliva, although as with so many things and Omicron, we are having to make important decisions in the absence of what would count as a good evidence base”.

While there is plenty of anecdotal evidence of people testing negative on a rapid test but then going on to infect others with Covid-19 or testing positive on a PCR test, official data on the issue is limited.

“Early data suggest that antigen tests do detect the Omicron variant but may have reduced sensitivity,” the Food and Drug Administration said last month.

“We have seen far too many people who are clinically ill who are in their third and fourth day of negative antigen tests but test positive by PCR,” the Director of the Center for Infectious Disease Research and Policy at the University of Minnesota, Michael Osterholm, said according to Axios. “So the question is, how often does that happen? What does that mean?”

“That’s A plus B equals C,” Dr Osterholm said. “A, a shortage of tests. B, they may not be that accurate. C, mass confusion.”

“Despite the small numbers of individuals included in this study, the findings are uniquely valuable because of the early detection of Omicron infection in frequent workplace Covid-19 testing to prevent spread,” the authors of the study write. “In real-world antigen testing, the limit of detection was substantially lower than manufacturers have reported to the FDA.”