By Sophie Putka, MedPage Today
In the early days of the pandemic, public health officials scrambled to get a read on the new viral outbreak. The CDC covered all its bases, recommending specimens be sent to them in three ways for each person: from the back of their throat (oral-pharyngeal), deep into their nasal passage (nasopharyngeal), and from sputum.
Not a month later, the CDC announced a change -- only the nasopharyngeal test was required. Later, shallower nasal swabs known as mid-turbinate or anterior nare swabs became more common, and the CDC reversed course on its preference for the nasopharyngeal test.
A handful of variants later, scientists are asking once again: Is nasal swabbing really the best way to detect Omicron quickly and accurately?
What's Known So Far
Two preprint studies, not yet peer reviewed and done with small sample sizes, made a preliminary case for testing with saliva by swabbing the mouth or back of the throat.
A preprint from scientists with the COVID-19 Sports and Society Working Group compared nasal rapid antigen tests to saliva PCR tests. Among 30 people with active Omicron infections tested daily with a nasal swab and saliva rapid antigen test, the nasal rapid antigen test did not pick up positive infections until around a day after a saliva PCR test did.
Notably, viral load in the saliva peaked in a subset of five patients in the first 1-2 days -- before the nasal rapid antigen tests even showed a positive test.
Although it has already been established that PCR tests detect COVID-19 at a lower threshold than a rapid antigen test, this preprint suggests that nasal rapid antigen tests may not detect Omicron at its most infectious point.
"We observed people being infectious and confirmed that they were transmitting to other people during that high early saliva viral load" period, before a nasal swab detected the virus, said study author Blythe Adamson, PhD, MPH.
She said that while researchers did not do a direct comparison in this study, and can't recommend off-label use of a rapid antigen test (i.e., with saliva), manufacturers are likely to soon begin comparing rapid antigen tests via nasal or saliva swabs.
Another recent preprint from researchers in South Africa also suggested, based on 67 Omicron and Delta cases, that positive saliva tests were more reliable than positive nasal mid-turbinate swabs for Omicron. This is the opposite of Delta, where nasal swab results were more accurate than saliva swabs. Omicron had relatively more viral shedding in the mouth than in the nose compared with Delta.
For the saliva test, patients coughed three to five times, swabbed the inside of their cheeks, above and below the tongue, and on the gums and hard palate. The standard for comparison was a composite wherein infection was considered present if either the saliva or mid-turbinate swab came up positive.
But this is preliminary data, cautioned Benjamin tenOever, PhD, a microbiologist at NYU Grossman School of Medicine.
And it doesn't show a dramatic difference, he said. Looking over the figure in the study, he noted that "for Omicron, there is the same amount in saliva as it is for the [nasal] swab... I don't really know how you take that result and justify needing to ever do saliva or a mid-turbinate. It's really saying that you could do either."
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