“Preliminary data indicate that about 40% of the Omicron variant positives may be negative in rapid tests”: after this bombshell launched yesterday by Guido Rasi, consultant to General Figliuolo and scientific director of Consulcesi, the experts are working to better understand if Omicron really risks sending antigenic swabs to the attic. With all the catastrophic consequences on the ability to contain the pandemic: given that up to now the rapid tests have been – albeit not very precise – essential for their operational usefulness to decide access to places or the possibility of traveling. “The real usefulness of the antigen test is not so much in knowing if a person is infected – because to have this information reliably a molecular test would be needed – as if it is infectious, that is able to infect others because it has a high viral load “explains the virologist Carlo Perno, director of the microbiology department of the Bambino Gesù Pediatric Hospital. “So if I want to prevent a person capable of infecting others from entering a closed environment like a theater, then the rapid test greatly reduces this possibility.” Precisely for this particularity, today it is accepted that antigenic tests have a sensitivity, or an ability to recognize Sars-CoV-2 positives, which does not exceed 75%, as reported by a recent study by researchers at Johns Hopkins University on Frontiers in Microbiology. A percentage that, scientists suspect, can be greatly reduced with Omicron.
Negative result: is it to be trusted?
“It must be said that rapid antigen tests have always been less sensitive and precise than molecular tests, but equally useful in many circumstances. With the Omicron variant the sensitivity problems of these tests are further accentuated because the antigens of this variant are highly mutated. and therefore they are less well recognized “explains the virologist Paolo Luxury, head of the viral pathogenesis laboratory at the National Institutes of Health in Washington and a collaborator of Anthony Fauci. “Today we cannot say that these tests have become totally useless, but it is certainly not prudent to trust 100% of a negative result: confirmation with the molecular test becomes necessary every time a person has symptoms or has had high-risk contacts. “.
With Omicron reduced sensitivity
Just a few days ago, on December 30, the FDA and the National Institutes of Health stated that antigen tests detect the Omicron variant, but it may have reduced sensitivity. Several hypotheses have been made to explain this reduced sensitivity, such as the fact that the Omicron variant appears to replicate more rapidly in the bronchi than in the nose, causing the rapid swab not to find enough viral load to recognize a positive. “The – for now only suspected – drop in sensitivity to Omicron of the rapid tests could be linked to the genetic characteristics of Omicron” explains Perno. “Omicron is very different from all the variants known so far. Alpha, Beta, Gamma, Delta were evolutions of each other, with all in all similar characteristics. And this diversity can deceive the tests. To simplify, it is as if the rapid test it was a key built to fit into the “Delta” lock. If suddenly the lock changes, and becomes “Omicron”, the key no longer works as it did before. ” In short, the antigenic tests were calibrated on the previous variants. “Today the question is:” Do we have to recalibrate the antigen tests on Omicron? “Explains Perno.” Many are working to resolve the doubt: even our group to the Child Jesus. And in a few days – we are completing the experiments – we will have the answer “.
Quick swab: take it 48 to 24 hours?
In addition to updating the tests, one measure that could prove useful in the event that Omicron actually escapes the rapid swab to a worrying extent is the reduction of the validity of the swab from 48 to 24 hours. “The rapid diagnostic test, when it goes well, covers for 24 hours: certainly not for 48 hours. And if we add to this simple reality the fact that these tests can see Omicron less well, we understand that this can become a problem,” explains Perno. “Having said that, I find it wrong – as well as disturbing – to send the message” Warning! Omicron fails the diagnostic tests! The truth is, we need to collect more data before judging. What is certain is that from a virological point of view the fact that the rapid test gives a “license” of 48 hours is nonsense. We see patients subjected to repeated rapid tests who, although negative the day before, are strongly positivized the next day. Rapid tests do not give us an indication of “non-infection”, but they do give a reasonable indication of “non-infectivity” within 24 hours. But they don’t tell us anything about the 48 hours. “
Negative rapid test? Here’s how to behave
When an asymptomatic person tests negative on the rapid test, what should they think? “If the asymptomatic has no reason to believe a possible contagion, because he is certain that he has not been in contact with positive people for Covid, then it is reasonable that, after the negative rapid test, he considers himself not infected” explains Perno. “But if, while remaining asymptomatic, he has been in contact with three or four positive people, then it would be advisable to repeat the swab, because the first result could have been a” false negative. ” more important to repeat the swab “. Although there is one thing to specify about the symptoms: “Due to the season today, unlike months ago, other respiratory viruses are also circulating, such as the other four human coronaviruses, the cold virus, the flu virus and parainfluenza, then enteroviruses and bocaviruses … “explains Perno. “While a certain symptomatology months ago meant:” It’s Covid “, today it means more than anything else:” It could be, let’s go and see “”
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