Better speed or accuracy? When it comes to Covid tampons, high specificity is not always the best choice. A group of researchers used Indian contagion data to do a computational analysis of the tests to find the right tracking strategy, minimize the number of infections in a population, and stem outbreaks on a budget.
And the result speaks for itself: testing a considerable number of people with the faster and cheaper antigen tests is just as effective as using PCR tests, which are certainly more specific but take longer to give the result and have a much higher cost.
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The study conducted by Philip Cherian e Gautam Menon of Ashoka University in Sonipat e Sandeep Krishna of the National Center for Biological Sciences Tifr of Bangalore was published on Plos Computational Biology. “Also in India different types of tests are used to track Covid: the most common is the high sensitivity PCR test, then there is a quick test of the less specific antigen that gives a result in 30 minutes. But until now it was not clear. what was the precise mix of tests needed to optimize the results taking into account cost constraints “. Cherian and colleagues therefore hypothesized a series of scenarios based on the transmissibility of Sars-Cov-2 to find the right compromise between effectiveness and cost.
Their analyzes show that “the use of rapid antigen tests alone could obtain similar results, in terms of total infections, despite the lower sensitivity of the PCR test, provided that the number of people tested is sufficiently high”. This suggests that “governments in low- and middle-income countries may be able to achieve optimal results by focusing on accelerating tests by using less sensitive tests that provide immediate results at a lower cost.”
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If the budget allows, the best option remains to use both tests, as well as those to monitor variants. But fortunately, “rapid tests are continually improving and trade-offs are in their favor even though they will continue to be less sensitive than PCR,” says computational biology professor. Gautam Menon.
“Modeling the effects on the use of different combinations of tests, taking into account their costs, can suggest specific policy changes that will have a substantial effect on the progress of the epidemic.” This is especially true when it comes to Delta and other high transmissibility variants.
Since the beginning of the pandemic, the entire scientific community has focused almost exclusively on the sensitivity of the test, or on how well a single swab could detect Sars-Cov-2 viral proteins or RNA molecules. The rapid tests were received with distrust, precisely because of their non-specificity and low quality of some brands.
However, as screening expands, or needs an immediate response, the context changes. As well as the choices of tracing, from the analysis only of the symptomatic, of the exposed people or of the sample.
“The study carried out by Indian colleagues wants to send a strong message: doing more tests is the best way we can go”, he comments Lorenzo Richiardi, professor of medical statistics at the University of Turin. “Their model is very sophisticated even if it depends on the parameters used, some obtained on the basis of the literature of the first wave, but beyond this they underline something that we already understood, namely that even rapid swabs have a relevant importance in identifying cases, even more so in the case of limited budgets. If there were no spending and feasibility limits, the best thing in my opinion would remain to do a large number of accurate tests, but we must not forget the time issue: there are situations in which which is better to have immediate results, especially when there is no certainty that a potentially positive person will remain isolated for the time it takes to get the PCR test results. ”
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For future containment strategies “the possibility of using simpler test methods, such as salivaries, should not be overlooked – continues Richiardi – if the sampling is not invasive, it can be done more often, for example, for screening purposes. in some specific populations. As far as sample tests are concerned, I think they can be very useful in closed communities, such as in schools, using a rotation system, while I do not believe that random controls on the whole population, such as those imagined in the model, are effective. simulation in India “.
The key will therefore be the right combination of accuracy, completeness and convenience. “Accurate, comprehensive and cost-effective testing would clearly be preferable – write the researchers – however, what is possible in reality is often only a reasonable triangulation of these three factors. Understanding which regimes are optimal requires reasonable estimates of how much the number of tests will be able to do. be increased, at what cost, as well as with what index of containment of new infections “.
As for the Delta variant, it will once again be the number of tests that will make the difference. “If the infection is very contagious – concludes Richiardi -, even just a missed positive case can create an outbreak, and this can happen both due to the lack of sensitivity of a test as well as the lack of tracking. Symptoms should be tested, too. if vaccinated. And the significance of a negative test result carried out in an asymptomatic person, for example in view of participation in an event, changes not only according to the sensitivity of the test, but also how high the circulation of the virus is “.
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