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AS NEWS EMERGED this week that an investigational covid-19 vaccine has proven effective in late-stage clinical trials, hopes that the days of the pandemic might be numbered are high (see article). But, even with the best of luck, it will be months before a vaccine begins to make a difference on the ground in countries that receive the first supplies, let alone those at the end of the line. In the meantime, the heavy weight of the pandemic continues.
To try to slow it down, many countries are starting to roll out tests which, at some cost in precision, provide their results much faster than the polymerase chain reaction (PCR) tests that were common at the start of the pandemic. These rapid tests will detect and quarantine more infected people than ever before before they can spread the contagion. They are therefore increasingly used to detect the presence of SARS–VSOV-2, the virus that causes covid-19, in settings ranging from airports to retirement homes. In Europe, in fact, they are sometimes used to blitz entire neighborhoods, towns and even small countries, such as Slovakia. But will they change the course of the pandemic?
Smaller, faster, cheaper
PCR tests look for the genetic sequence of the virus in nose and throat swabs. These swabs must be processed in laboratories and require machines that take hours to produce a result. They are extremely precise. But the delay involved can hamper testing and traceability systems.
Rapid tests, on the other hand, are designed to detect certain proteins that SARS–VSOV-2 casts when it replicates during infection. These proteins, called antigens, cause the immune system to make other proteins, called antibodies, that turn off the virus. Antigen testing does not require laboratory backup and can report a result within 15 to 20 minutes. They work by dipping the swab into a vial containing a solution that extracts the antigen of interest. A few drops of the mixture are then applied to a test strip containing antibodies that recognize this antigen. The test strip displays 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 or another, according to a catalog compiled by the Foundation for New Innovative Diagnostics (FIND), an association in Geneva which supports the World Health Organization (WHO) with research on diagnostic tools. So far, only two of them have obtained provisional approval (“emergency use”) by the WHOand seven by the US federal regulator, the Food and Drug Administration. But further approvals are expected to be obtained in the coming weeks as FIND and other organizations perform validation studies that test the tests under the real conditions in which they are likely to be used.
The early antigen tests weren’t very good, but many of the newer ones are extremely accurate. If a PCR test is negative, a modern antigen test on the same individual will agree with this test more than 97% of the time, a value called its specificity. The story becomes more complicated, however, when the virus is actually present. If someone tests positive for covid-19 in a PCR test, the best antigen tests will be more than 90% successful if the test is done within a week or two of symptom onset, a value called sensitivity. But the match rate decreases if the antigen test is done at the beginning or the end of an infection, when the amount of virus in the nose and throat is considerably lower. This means that diagnoses based on antigen tests are unreliable during these times.
Fortunately, from a public health standpoint, it may not matter. The relationship between viral load and contagiousness is not fully understood, but current thinking is that higher loads make people more contagious. Since those with higher loads are the most likely to test positive in an antigen test and therefore be asked to isolate, the transmission breakage value of the new tests should not be too severe. compromised.
In theory, this all sounds good. But the reality is more messy. Even a very accurate test will produce fewer true positives than false positives if the people tested are unlikely to be infected in the first place (see graph). This would be the kind of problem that arises with mass testing in places that are not covid-19 hotspots. For example, the UK Office for National Statistics estimates that as of October 28, 0.82% of people living in private households in London were infected. If everyone in London that day has received a test whose minimum “acceptable” accuracy for rapid tests is 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.
That’s why deciding whether to trust the result of a flawed rapid test – or, indeed, whether it is really worth using the test – depends on who is tested and why. A positive result is more credible for a person with symptoms, or who is in close contact with an infected person, and may be living 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 likely to be a waste. A positive result in this case will be suspect.
Try this at home
Doctors are used to making such decisions when testing things like 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 are changing rapidly. To deal with this, some test developers are pairing their products with ‘digital envelopes’ such as applications in which such decision-making algorithms are fed with up-to-date data on things such as local prevalence trends. of covid-19 and the weight of various personal risk factors derived from various analyzes. Some of these apps issue a time-limited barcode to those who test negative, to be used when proof of a negative test may be required.
At this time, rapid tests are allowed to be used only by healthcare professionals. The regulatory bar for self-testing at home is high. They need to be 99% accurate and pass extensive usability testing to make sure people are using them correctly. It would be easier if the secretion being tested was saliva, which is freely accessible, rather than material found high in the nose or back of the throat. Saliva works reliably in some PCR tests but no one has yet developed a good antigen test that uses it.
At the current rate of progress, however, that may soon change. Bruce Tromberg of America’s National Institutes of Health (NIH) thinks an over-the-counter rapid test could be available in America as early as next summer. Rapid antigenic testing is therefore likely to become an important part of countries’ covid-19 testing strategies. In particular, they will be used for tests at home, in medical offices and in remote places where PCR laboratories are not available. They will be especially useful for mass testing in places prone to epidemics, such as prisons and student dormitories.
As faster tests are developed and demand increases, competition and large-scale manufacturing will make them cheaper. Stand-alone antigen tests are now available for as little as $ 5 each, but prices could potentially drop closer to $ 1, which equates to the cost of a rapid malaria test. Tests that use small machines cost around $ 10 to $ 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 come in bulk on a fixed schedule, such as samples from universities or workplaces.
Even though antigen testing is cheap, some people fear that rich countries will corner the market until production has increased enough, leaving poorer regions in short supply. To prevent this, the Bill and Melinda Gates Foundation, a large charity, has partnered with the WHO to place an order for 120 million rapid tests that will go to 133 developing countries over the next six months.
Dr Tromberg, who is leading a project at NIH which invests in new covid-19 testing technologies that can be rapidly scaled up for mass production, estimates that the 22 products in its pipeline that are already in the manufacturing stage will add 2.5 million tests per day by the end of this year – thus helping to increase the total to 6m-7m. Around the world, several manufacturers of rapid covid-19 tests have said they have the capacity to test in the tens or hundreds of millions per year. This seems plausible, given that 400 million malaria test kits are made each year. But the expansion into the billions is unknown land. Although new production lines can be built and existing ones put into service around the clock, carrying out tests requires skilled workers, whose supply is limited.
Whether rapid tests change the course of the pandemic and end the need for containment until a vaccine can also be manufactured and widely distributed will depend on whether those that are available are being used properly. carefully. Ultimately, such a vaccine will significantly reduce the demand for testing. But, for now, the world needs it.■
This article was published in the Science and Technology section of the print edition under the title “Test match”
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