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AS NEWS EMERSE this week as an experimental covid-19 vaccine has proven effective in late stage clinical trials, hopes that the days of the pandemic can be numbered are running out (see article) But, even with the best of luck, it will be months before a vaccine starts making a difference in the field in those countries that receive the first supplies, not to mention those at the bottom of the line. Meanwhile, the pandemic behemoth continues.
To try and slow it down, many countries are starting to implement tests that, at some cost in terms of accuracy, deliver their results much faster than the polymerase chain reaction (PCR) tests that were on the agenda at the start of the pandemic. These rapid tests will enable more infected people to be detected and quarantined than possible before they can spread the infection. They are therefore used in increasing numbers to select people for the presence of SARS–C.ORV.-2, the virus that causes covid-19, in contexts ranging from airports to nursing homes. In Europe, in fact, entire neighborhoods, cities and even small towns, such as Slovakia, are sometimes used to blitz. But will they change the course of the pandemic?
Smaller, faster, cheaper
PCR the tests look for the genetic sequence of the virus in nasal and throat swabs. These swabs must be processed in laboratories and require machines that take hours to obtain a result. They are extremely accurate. But the delay involved can hinder the evidence and trace systems.
Rapid tests, by contrast, are designed to detect certain proteins that SARS–C.ORV.-2 breaks free when replicating during an infection. These proteins, known as antigens, stimulate the immune system to make other proteins, called antibodies, which disable the virus. Antigen tests require no laboratory backup and can report a result in 15-20 minutes. They work by immersing the swab in a vial containing a solution that extracts the antigen of interest. A few drops of the mixture are then applied to a test strip laced with antibodies that recognize that antigen. The test strip shows the results like a home pregnancy test.
The speed with which these tests were 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 obtained provisional approval (“emergency use”) since WHOand seven by the US federal regulator, the Food and Drug Administration. But further approvals are expected in the coming weeks such as FIND and other organizations complete validation studies that test the tests in 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. The story gets complicated, however, when the virus is actually around. If someone tests positive for covid-19 in a PCR tests, the best antigen tests will agree in more than 90% of cases if the test is done within a week or so of symptom onset, a value called sensitivity. But the concordance rate decreases if the antigen test is performed at the beginning or at the end of an infection, when the amount of virus present in the nose and throat is significantly less. This means that diagnoses that rely on antigen tests are unreliable during those times.
Fortunately, from a public health point of view this may not matter. The relationship between viral load and contagiousness is not fully understood, but the current thinking is that higher loads make people more contagious. Since those with higher loads are more likely to show up as positive on an antigen test and then be asked to isolate themselves, the transmission breakdown value of the new tests should not be too compromised.
In theory, then, this all sounds great. But the reality is more complicated. Even an extremely accurate test will produce fewer true positives than false positives if the people being tested are unlikely to be infected in the first place (see graph). This would be the kind of problem that comes with mass testing in places that aren’t covid-19 hotspots. For example, Britain’s Office for National Statistics estimates that 0.82% of people in private households in London were infected on 28 October. If all Londoners had been provided that day with a test that had the minimum “acceptable” accuracy for rapid tests established by 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 an imperfect quick test result 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 in close contact with an infected individual and possibly lives in an area with a high rate of covid-19. But testing people when there are no obvious reasons to believe they may be infected is likely to be a waste. A positive result in that case will be suspicious.
Try it at home
Doctors are used to making such decisions when testing for things like cancer, sexually transmitted infections, and so on. The guidelines they employ are based on years of research and practice. But for covid-19, things are new and changing rapidly. To address this, some test developers are pairing their products with “digital wraparounds” such as apps where those decision algorithms are fed with up-to-date data on things like trends in local covid-19 prevalence and the weight of various factors. of personal risk derived from various analyzes. Some of these apps issue a time-limited barcode to those who test negative, for use where proof of a negative test may be required.
For now, rapid tests can only be used by medical professionals. The regulatory limit for self-contained home testing is high. They need to be 99% accurate and pass thorough usability tests to ensure people are using them correctly. It would be easier if the secretion to be tested were saliva, which is freely accessible, rather than material found high in the nose or deep in the throat. Saliva works reliably in some PCR test but no one has yet come up with a good antigen test that uses it.
At the current pace of progress, however, this may soon change. Bruce Tromberg of the American National Institutes of Health (NIH) believes an over-the-counter rapid test could be available in America as early as next summer. Rapid antigen testing is therefore likely to become an important part of countries’ covid-19 testing strategies. In particular, they will be used for testing at home, in doctors’ offices and in remote places where PCR laboratories are not available. They will be particularly useful for mass testing in places prone to epidemics, such as prisons and student dormitories.
As faster tests are developed and their demand increases, competition and large-scale production will make them cheaper. Standalone antigen tests are now available for as little as $ 5 each, but prices could eventually drop closer to $ 1, which is the cost of a rapid malaria test. Tests using small machines cost around $ 10-20 each, plus a few hundred dollars for the device. A PCR the test now costs around $ 50, but will be cheaper for large-scale automated testing of samples that arrive in bulk according to a set schedule, such as samples from universities or workplaces.
Although antigen tests are cheap, however, some people fear that rich countries will block them until production is sufficiently increased, leaving poorer places with a shortage. To avoid this, the Bill and Melinda Gates Foundation, a large charity, has partnered with the WHO place an order for 120 million rapid tests that will go to 133 developing countries over the next six months.
Dr Tromberg, who leads a project at the NIH which invests in new covid-19 test technologies that can be rapidly expanded to mass production, believes the 22 products in its pipeline that are already in production will add 2.5 million tests per day by the end of this. year, helping to grow the total to 6m-7m. Worldwide, several manufacturers of covid-19 rapid tests have claimed that they have the capacity to run tests in the tens or hundreds of millions per year. This seems plausible, given that 400 million malaria test kits are produced every year. But expanding by the billions is unknown land. Although new production lines can be built and existing ones put into operation around the clock, testing requires skilled workers, whose availability is limited.
Whether rapid tests change the course of the pandemic and end the need for lockdowns until a vaccine can be made and distributed on a large scale will depend on whether those available are used wisely. Eventually, such a vaccine will drastically reduce the demand for testing. But, for now, the world needs them.■
This article appeared in the Science and Technology section of the print edition under the title “Test match”
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