In this strange beginning of summer, with Italy split in two by the weather, Covid seems to be rearing its head again. An increase in cases which, according to the latest official data, concerns most regions and is particularly evident in Lazio. Ancient questions are therefore relevant again: to take the test or not? And if so, when? Is the rapid test reliable? And what to do if you test positive? In the USA, a study by the University of Colorado (Cu) in Boulder, published in ‘Science Advances’, seeks to clarify thanks to a new mathematical model that offers useful indications not only for Sars-CoV-2 infection, but also for other communicable diseases such as influenza or respiratory syncytial virus Rsv. And there is one piece of advice that catches the eye: in case of suspicious symptoms, it is better to wait 2 days before taking the Covid test.
Covid test, when to do it?
“For Covid,” says Casey Middleton, the first author of the paper, “we found that if you only do one test, it’s best to wait 2 days after symptoms appear before using it. Because before that, the virus is unlikely to be detectable.” In other words, the test will likely not produce a reliable result. On the contrary, “for influenza and RSV, it’s best to do the rapid test when you first feel symptoms,” adds Middleton, a doctoral student in the Department of Computer Science at CU Boulder’s IQ Bio program, who developed the new model with senior author Daniel Larremore, a professor of Computer Science at the university’s BioFrontiers Institute.
Covid test yes or no?
To those who ask ‘Covid test, yes or no?’, the latter responds with conviction: “If you want to go to the book club or bingo night with your grandparents, getting tested is a really good idea.” As if to say that even today, once the pandemic emergency is over, it is always better to know that you have been infected before coming into contact with fragile people. Having said this, “Covid has changed – specifies Larremore – each variant behaves differently and the way in which it interacts with the tests could be different”.
And in fact, when together with Middleton he inserted information on the now ubiquitous Omicron variants, on patient behavior and other factors into the new computational model, it was seen that if an infected Covid person is subjected to a rapid test immediately, at the first symptoms, the false negative is almost certain: the infection escapes in a percentage of cases of up to 92%. By waiting 2 days the false negatives drop to 70%. If there is the possibility of doing a second test on the third day, they drop to 66%: one infection in 3 is detected. It may seem like little, but in practice it is enough: “Diagnosing a third of infections – assures Larremore – can still substantially reduce the transmission” of Covid. The rapid tests were in fact designed to intercept the positives with the highest viral load. The most contagious.
The new variants of Sars-CoV-2 – explain the authors – also considering that most people have now developed a certain degree of immunity to Covid, replicate a little more slowly than the original new coronavirus. It therefore happens that “the symptoms appear earlier, but it takes longer for the body to reach a viral load that can be detected” with the rapid test, points out Middleton. For RSV infection and influenza, however, the virus multiplies so rapidly that already at the first symptoms there is enough of it for the test to be positive. Another dilemma therefore arises, observes Larremore: “If you test immediately for everything”, i.e. for Sars-CoV-2, influenza viruses and syncytial virus, “it might be fine for influenza and RSV, but it may be too early for Covid”; on the contrary, “if you wait a few days it could be the right time for Covid, but too late for flu and RSV”.
What to do in case of a positive test?
Assuming you have hit the ideal window and have a positive Covid test in hand, what should you do? Isolating yourself would be a responsible gesture, but for how long? “Assuming we have enough swabs available to repeat at home”, the authors of the study suggest that using the old “exit test” strategy, which involves “performing it again to decide whether to return to work and social life , can prevent more infections and with less inconvenience” compared to the “5 days of isolation” approach which until March was the standard advice of the American CDC, the Centers for Disease Control and Prevention. “This policy has forced people to isolate for too long in most cases,” says Middleton. The “test-to-exit” strategy is better, which “first releases those who will no longer transmit the virus, keeping only those with a high viral load at home”.
Larremore and Middleton’s hope is that their mathematical model can help companies develop better tests for Covid, influenza and RSV, doctors to better advise patients and health authorities – in the event of a new pandemic – to adopt a policy of agile and evidence-based testing.
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