Rapid tests would have stemmed the pandemic within weeks

The usefulness of a screening test would not be in its accuracy, but in its speed and frequency of execution, according to a new study published in the journal Science Advances. Researchers from the TH Chan School of Public Health at Harvard and Boulder University in Colorado say this after the analysis of a predictive mathematical model, which correlates the influence of tests on the course of the COVID -19 pandemic. “Mass screening measures must prioritize accessibility, frequency and speed of the result, not test accuracy” conclude the researchers.

When speed rhymes with efficiency

In fact, according to one of their scenarios, in a large city of about eight million inhabitants, a rapid mass screening test performed twice a week would reduce the R0 (or baseline reproduction rate, which corresponds to the average number of people). that a patient can infect) with the COVID-19 virus 80%. The use, en masse, of a classic antigenic PCR test – which recognizes or does not recognize the presence of viral particles, in more or less 48 hours after a nasopharyngeal sampling – at the same frequency would reduce this ratio by only 58%. According to the researchers, this disparity stems from the fact, in particular, that an infected person can see their viral load go from 5,000 to one million viral particles in 18-24 hours. This explains how, for example, two thirds of asymptomatic patients would continue to transmit their disease while awaiting the results of a classic test. Conversely, a proven positive rapid test would allow, despite its relative inaccuracy, to isolate the patient before the virus spreads.

Rapid COVID-19 screening tests

Rapid detection devices for the SARS-CoV-2 coronavirus have been available for several months at some doctors, pharmacies or testing laboratories. Like the classic tests, these can be distinguished by two types: the PCR antigen test (which detects the number of RNA copies, corresponding to the viral particles found in a nasopharyngeal sample taken with a swab), relating to a patient’s contagiousness, and the serological test (which evaluates the number of specifically anti-COVID-19 antibodies identified in the blood), allowing you to advance whether a person is or has been infected in the previous weeks. These tests are usually done within 48 hours. Quick tests can deliver a result in about thirty minutes. They are also distinguished by their imprecision: they do not allow to give a positive result only if the viral load is very high. They may therefore not notice the presence of a small amount of the virus and give a “false positive”. “

Case-by-case isolation rather than collective confinement?

In a more precise scenario, in which 4% of the inhabitants would have tested positive, the establishment of rapid tests en masse (in at least three quarters of the population) every three days reduce the number of infected people by 88% in six weeks, “Enough to bring the local epidemic to its complete containment. “ In short, according to Professor Daniel Larremore of Boulder University in Colorado, “It is better to rely on a less accurate test, but one that gives a result during the day, rather than a more accurate test that would give a result the next day.” “ In a press release issued by Science notice, believes that, in order to contain the pandemic rapidly, the authorities could, by applying this strategy, “It only orders people who test positive to stay at home on a case-by-case basis, rather than preventing the entire population from doing their business, to make sure no sick people pass. “