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AS NEWS ARISING This week, when an experimental vaccine against covid-19 has been shown to be effective in late-stage clinical trials, hopes are being exhausted that the days of the pandemic are numbered (see article). But, even with the best of luck, it will be months before a vaccine starts to make a difference on the ground in those countries getting the first supplies, let alone those at the bottom of the queue. Meanwhile, the pandemic monster continues.
To try to stop it, many countries are starting to implement tests that, with some cost in precision, deliver their results much faster than the polymerase chain reaction (PCR) tests that were common at the beginning of the pandemic. These rapid tests will allow more infected people to be detected and quarantined than was possible before they can spread the contagion. Therefore, they are used in increasing numbers to screen people for SARS–CORV-2, the virus that causes covid-19, in environments ranging from airports to nursing homes. In Europe, in fact, they are sometimes used to bomb entire neighborhoods, cities and even small countries, such as Slovakia. But will they change the course of the pandemic?
Smaller, faster, cheaper
PCR The tests look for the genetic sequence of the virus in nose and throat swabs. These swabs must be processed in labs and require machines that take hours to produce a result. They are extremely accurate. But the delay involved can hamper test and trace systems.
Rapid tests, by contrast, are designed to detect certain proteins that SARS–CORV-2 sheds when replicating during an infection. These proteins, known as antigens, stimulate the immune system to make other proteins, called antibodies, which continue to disable the virus. Antigen tests do not need laboratory support and can report a result in 15-20 minutes. They work by dipping the swab into a vial containing a solution that extracts the antigen of interest. Then a few drops of the mixture are applied to a test strip mixed with antibodies that recognize that antigen. The test strip shows the results like a home pregnancy test.
The speed with which these tests have been developed is impressive. More than 70 are now on the market in one part of the world or another, according to a catalog compiled by the Foundation for Innovative New Diagnostics (FIND), a charity in Geneva that supports the World Health Organization (who) with research on diagnostic tools. So far, only two of them have received provisional approval (“emergency use”) from the whoand seven by the US federal regulator, the Food and Drug Administration. But more approvals are expected in the coming weeks as FIND and other organizations complete validation studies that test the tests under the real-life conditions in which they are likely to be used.
The first antigen tests weren’t very good, but many of the newer ones are extremely accurate. If a PCR The test is negative, a modern antigen test on the same individual will agree with that analysis more than 97% of the time, a value called its specificity. However, the story is complicated when the virus is present. If someone tests positive for covid-19 in a PCR test, the best antigen tests will agree in more than 90% of cases if the test is carried out within about a week of the onset of symptoms, a value called sensitivity. But the rate of agreement decreases if the antigen test is done at the beginning or end of an infection, when the amount of virus present in the nose and throat is considerably less. This means that diagnoses that are based on antigen testing are not reliable during these periods.
Fortunately, from a public health point of view, this may not matter. The relationship between viral load and contagion is not fully understood, but current thinking is that higher loads make people more contagious. Since those with higher loads are more likely to test positive for an antigen and therefore be asked to isolate themselves, the transmission interruption value of new tests should not be too compromised.
So in theory this all sounds great. But the reality is messier. Even a highly accurate test will produce fewer true positives than false positives if the people tested are unlikely to be infected in the first place (see chart). That would be the kind of problem that comes up with mass testing in places other than covid-19 hotspots. For example, Britain’s Office for National Statistics estimates that on October 28, 0.82% of people in private homes in London were infected. If everyone in London that day were to undergo a test that has the minimum “acceptable” precision for rapid tests set by the who (80% sensitivity and 97% specificity) the number of those with false positive results will be 353% greater than those with true positive results.
This is why deciding whether to trust the result of an imperfect rapid test — or indeed whether the test is worth using — depends on who is being tested and why. A positive result is more credible for someone with symptoms, or who is a close contact of an infected person, and perhaps lives in an area with a high rate of covid-19. But testing people when there is no obvious reason to believe they may be infected is probably wasteful. A positive result in that case will be suspect.
Try that at home
Doctors are used to making such decisions when screening for cancer, sexually transmitted infections, etc. The guidelines they use are based on years of research and practice. But for covid-19 things are new and changing rapidly. To deal with that, some test developers are pairing their products with “digital environments,” as applications where such decision-making algorithms are fed up-to-date data on things like trends in the local prevalence of covid-19 and the weight of various personal risk factors derived from various analyzes. Some of these apps issue a time-limited barcode for those who test negative, to use when testing for a negative result is required.
For now, rapid tests are licensed for the exclusive use of medical professionals. The regulatory bar for independent home testing is very high. They must be 99% accurate and pass extensive usability tests to ensure people are using them correctly. That would be easier if the discharge being tested was saliva, which is freely accessible, rather than material found high up in the nose or deep in the throat. Saliva works reliably in some PCR tests, but no one has yet come up with a good antigen test that uses it.
However, at the current rate of progress, this may change soon. Bruce Tromberg of the United States National Institutes of Health (NIH) believes that an over-the-counter rapid test could be available in the United States starting next summer. Therefore, rapid antigen testing is likely to become an important part of countries’ covid-19 testing strategies. In particular, they will be used for testing at home, in doctor’s offices and in remote locations where PCR labs are not available. They will be especially useful for mass testing in locations prone to outbreaks, such as prisons and student residences.
As faster tests are developed and demand increases, competition and large-scale manufacturing will make them cheaper. Independent antigen tests are now available for as little as $ 5 each, but prices may eventually drop closer to $ 1, which is the cost of a rapid test for malaria. Tests using small machines cost $ 10-20 each, plus a few hundred dollars for the device. A PCR The test now costs around $ 50, but it will be cheaper for large-scale automated testing of samples that come in bulk on a set schedule, like samples from universities or workplaces.
However, while antigen testing is cheap, some people worry that rich countries will corner the market until production has increased enough, leaving poorer places in short supply. To prevent this, the Bill and Melinda Gates Foundation, a large charity, has partnered with the who to order 120 million rapid tests to be shipped to 133 developing countries over the next six months.
Dr. Tromberg, who leads a project in the NIH which invests in new covid-19 testing technologies that can be rapidly scaled to mass production, sees the 22 products in its portfolio already in the manufacturing stage to add 2.5 million tests per day by the end of this year , which will help increase America’s performance. total at 6m-7m. Around the world, several manufacturers of rapid covid-19 tests have said they have the capacity to test tens or hundreds of millions a year. This sounds plausible, given that 400 million malaria test kits are manufactured each year. But expanding into billions is unknown land. Although new production lines can be built and existing ones up and running 24 hours a day, testing requires skilled workers, whose supply is limited.
Whether rapid tests change the course of the pandemic and end the need for closed closures until a vaccine can also be manufactured and distributed on a large scale, it will depend on whether those that are available are used wisely. Eventually, such a vaccine will dramatically reduce the demand for testing. But for now, the world needs them.■
Editor’s Note: Some of our Covid-19 coverage is free to readers of The economist today, our daily newsletter. For more stories and our pandemic tracker, check out our hub
This article appeared in the Science and Technology section of the print edition under the title “Matching Evidence.”
