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It is proving to be one of the most open fronts in the fight against the pandemic. Rapid antigen tests have long been pointed to as a weak link in the chain of countermeasures to stem the circulation of the SARS-CoV-2 virus. In fact, they cannot see the virus except when it is present in massive quantities, with the result that in 1 out of 2 cases the result they give is a false negative. «They are tests with an extremely low sensitivity, so much so that positive cases are currently detected by 0.2% of rapid tests and by 6% of molecular ones. We also have over 50% false negatives », observes virologist Francesco Broccolo, of the University of Milan Bicocca.
The time lag
One of the reasons why there are so many false negatives is that “when a subject becomes infected, the infection shows up on the test after 48 hours, while we know that the infection must take hold and that the virus needs time to replicate. For this – says the expert – it is not necessary to take the test immediately after having had contact ». When, 48 hours after the infection, «the virus begins to replicate, after 48 hours it becomes visible to the molecular test, which is able to take a very detailed photograph; in comparison the quick test gives a grainy image. In fact, he can see the virus only if the viral load is at least 1 million copies per milliliter of biological fluid taken with the swab ». This, he adds, is “a great critical issue”.
Undifferentiated the vaccinated and not
The other, according to Broccolo, is in the fact that “with the current rules for the Green pass, those who are vaccinated are not distinguished from those who have taken the rapid test and both categories are exposed to the same events, forgetting that those who are not vaccinated he therefore has a greater risk of getting sick ». A limit is also in the validity times of the Green pass: “In the ideal world, the quick test should be done every day because, if I get infected today, you will not be able to see the infection with any test for 48 hours”. Even the 72 hours of validity of the molecular test are theoretical “because, even if this is ultra-sensitive, it does not exempt itself from the possibility that the infection is contracted after a few hours from the test”.
Even “fake” test cases
The rapid test for Covid-19 has its limits in identifying with certainty the negativity to the virus also according to the virologist Fabrizio Pregliasco. “And then the application also counts, how the tampon is made. If you pretend them, and I have seen them, it is clear that the result is false ». To give guarantees is only «the molecular swab made in the laboratory which however – underlines the virologist – does not have that practicality for mass use, because then the times are also lengthened». So would you rule out a tightening that allows the Green pass only with the molecular swab? «It would be expensive, the molecular costs around 50 euros. And it should be done every 48 hours because from a health point of view the more often it is done the better. Then it is clear – he concludes – that practical application is a political mediation ».