Science

COVID: rapid and frequent tests could strongly curb the pandemic in a few weeks

In addition to wearing a mask, confinement and social distancing, screening is also an integral part of the measures to combat COVID-19. In many countries, the population has access to testing centers in order to be tested. However, the high sensitivity of the tests requires a relatively long waiting time before obtaining a result. A new study shows that speed and frequency of testing are more important than sensitivity to significantly curb the pandemic (and this within weeks).

Testing half the population every week with inexpensive, quick-results COVID-19 tests would drive the virus to eradication within weeks, even though these tests are significantly less sensitive than benchmark clinical tests, new study finds published yesterday by the University of Colorado.

Such a strategy could lead to “custom lockdown rules” without shutting down restaurants, bars, retail stores and schools. ” Our general conclusion is that in public health it is better to have a less sensitive test with results today than a more sensitive test with results tomorrow. Says lead author Daniel Larremore, professor at UC.

Rather than telling everyone to stay home to make sure someone who is sick doesn’t pass it on, we could order people with infectious disease to stay home only so everyone can live their lives. », He adds.

Speed ​​and frequency of tests: two more important parameters than sensitivity

For the study, published in the journal Science Advances, the authors sought to find out which factors between sensitivity, frequency, or time to complete tests are most important in stopping the spread of COVID-19. Researchers scanned the available literature on how the viral load rises and falls during infection, when people tend to have symptoms, and when they become contagious.

Graph showing the infectivity suppression rate in the population as a function of the frequency of testing. Credits: Daniel B. Larremore et al. 2020

They then used mathematical modeling to predict the impact of screening with different types of tests on three hypothetical scenarios: in 10,000 individuals; in a university-type setting of 20,000 people; and in a city of 8.4 million inhabitants. When it comes to reducing propagation, they found that frequency and turnaround time are much more important than sensitivity of tests.

80% reduction in infectivity

For example, in a scenario involving a large city, widespread testing twice a week with a rapid but less sensitive test reduced the degree of infectivity (or R0) of the virus by 80%. But twice-weekly testing with a more sensitive PCR (polymerase chain reaction) test, which takes up to 48 hours to return results, reduced infectivity by just 58%.

In other scenarios, when the amount of tests was the same, the rapid test still reduced infectivity better than the slower, more sensitive PCR test. Indeed, about two-thirds of those infected show no symptoms and, while awaiting their results, they continue to spread the virus.

Infection reduction frequency test chart
Graph showing reduction in infectivity as a function of frequency of testing. Credits: Daniel B. Larremore et al. 2020

This article is one of the first to show that we should care less about the sensitivity of testing and, in public health, prioritize frequency and turnaround time. », Says Roy Parker, director of BioFrontiers Institute.

The study also demonstrates the power of frequent testing to shorten the pandemic and save lives. In a scenario in which 4% of people in a city were already infected, rapid testing of three out of four people every three days reduced the number of infected by 88% and was “enough to drive the epidemic to extinction. within six weeks ”.

Great variability in sensitivity

The study comes as companies and university research centers develop rapid, low-cost tests that could be deployed in large public spaces or marketed for home use.

Levels of sensitivity vary widely. Antigen testing requires a relatively high viral load – about 1,000 times more virus than PCR testing – to detect infection. Another test, known as the RT lamp (Isothermal Reverse Transcription Loop Mediated Amplification), can detect the virus at about 100 times the viral load compared to PCR.

The standard PCR test usually provided by healthcare professionals only requires 5,000 to 10,000 copies of viral RNA per milliliter of sample, which means it can detect the virus very early or very late.

Fast and efficient contagiousness tests

In the past, federal regulators and the public have been reluctant to adopt rapid tests for fear of missing cases early in the infection. But, in reality, an infected person can go from 5,000 particles to 1 million copies of viral RNA in 18 to 24 hours. ” There is a very short window, at the onset of infection, in which PCR will detect the virus. But something like an antigen test or a LAMP test won’t do Parker explains.

And during this time, the person is often not contagious. The authors recently used these results to call for a change in the way we use test sensitivity. An article on this subject had been published in the New England Journal of Medicine.

These rapid tests are contagiousness tests. They are extremely effective at detecting COVID-19 when people are contagious », Says Michael Mina, professor of epidemiology at Harvard. They are also affordable. Rapid tests can cost as little as $ 1 each and return results in 15 minutes. Some PCR tests can take several days.

Inexpensive and Fast Home COVID Testing

Mina envisions a day when the government sends simple, inexpensive home tests to every home in the United States. Even if half of Americans tested themselves weekly and self-isolate if they tested positive, the result would be significant. ” In a matter of weeks, we could see this outbreak go from a huge number of cases to very manageable levels. Mina says.

Rapid testing could also be the key to reviving old and widespread threats such as football stadiums, concert halls and airports, with customers testing themselves along the way and still wearing masks as a precaution, a Larremore said. The authors say they are encouraged to see that several countries have already started testing all of their citizens and hope the new US administration will make rapid testing a priority. Tuesday, the Food and Drug Administration approved the first rapid home test.

Sources: Science Advances

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