Mia Malan and Aisha Abdool Karim from News24
South Africa needs to test 12 times more people per day for the new coronavirus than it currently does. Our testing plan can only do this if we use a wider range of tests – including controversial ones.
1. What are rapid antibody tests and why do we need them?
Rapid antibody tests differ from the coronavirus lab tests, which use polymerase chain reaction (PCR) technology, that South Africa uses. PCR tests detect the new coronavirus, known as SARS-CoV-2, itself. Rapid antibody tests, on the other hand, detect the body’s response, in the form of SARS-Cov-2 antibodies, to the virus.
The SARS-CoV-2 antibody tests that South Africa are looking to procure are rapid tests. That means a health worker is able to give someone their results on the spot, or “at the point of care”. It’s, therefore, not necessary to send samples to a lab, as is the case with the current PCR tests.
Also, unlike PCR tests, which require specially-trained lab technicians to interpret the results, antibody tests can be administered by nurses.
Antibody tests work a little bit like pregnancy tests: a health worker will prick your finger to get blood, in which a testing stick is then dipped; if the SARS-CoV-2 antibodies are present in your blood, a little strip will change colour, which will indicate a positive result.
South Africa’s new SARS-Cov-2 testing plan aims to test 36 000 people per day by the end of April, according to a National Health Laboratory press release. Currently, we don’t come close to that number. Last week we tested an average of 3 140 people per day (and this number includes tests conducted in both public and private labs).
This means that, within less than a month, the country would need to test 12 times more people per day than what it currently does. Achieving this by only using PCR tests, which are time consuming and expensive, will be close to impossible.
And, even if South Africa can do that many PCR tests per day, it is unlikely to procure enough of the chemicals needed to conduct the tests, because of a global shortage: all countries need them.
The new testing plan, therefore, incorporates two new kinds of tests that will be used in conjunction with traditional PCR tests: a much faster PCR test using GeneXpert machines, which the country currently uses to test for tuberculosis, and rapid antibody tests.
2. How accurate are rapid antibody tests?
When researchers determine how accurate a medical test is, they look at its sensitivity (how often the test correctly generates a positive result for people who have the condition that’s being tested for) and specificity (the test’s ability to correctly generate a negative result for people who don’t have the condition that’s being tested for).
Because the body takes time to produce antibodies, between fiveto 14 days after the onset of symptoms, SARS-Cov-2 antibody tests are not useful early in the infection cycle, as there are no antibodies to detect.
There are many SARS-Cov-2 rapid antibody tests, but South Africa will only be procuring tests that have been approved by regulatory bodies recognised by our own regulator, the South African Health Products Regulatory Authority (SAHPRA). These include kits being used in Australia, Europe, Canada, Brazil, Japan and the United States of America.
The exact sensitivity and specificity levels will differ, depending on which test is used. As an example, one antibody test developed in China showed a sensitivity of 88.66% and specificity of 90.63%. How this compares to PCR tests in South Africa is also unknown, as there is not enough data on test accuracy levels yet.
There have been media reports about inaccurate rapid antibody testing kits, but these have mostly been home testing kits. The tests that the FDA has registered are “strictly for medical professional use and not intended for personal use” and have to be administered by a trained health professional.
Community health workers will also not be permitted to do the tests; they will screen people and refer them to a health facility for a test, if needed.
Before the tests can be used in South Africa, they will have to be approved by SAHPRA.
While, ordinarily, this may take some time, SAHPRA will be speeding up the process for licensing medical devices, such as these test kits, to respond to the Covid-19 pandemic timeously.
3. Will SA use antibody tests to diagnose people with Covid-19?
This answer isn’t simple — mainly because we still know so little about the novel coronavirus and because rapid antibody tests for SARS-Cov-2 are so new.
PCR tests are really good at detecting SARS-Cov-2 in patients with clear symptoms — doctors call this ‘acute infection’ — but not so great at picking up the virus in those in the very early or very late stages of infection, when there isn’t enough virus present in someone’s system for the test to detect it.
Although antibody tests are also bad at diagnosing people in the early stages of infection, because the body hasn’t yet produced antibodies, they could potentially be useful to pick up those with mild or no symptoms — which is 81% of people with the new coronavirus, according to a Chinese Centre for Disease Control and Prevention study that was published in the The Journal of the American Medical Association in February.
People with mild or no symptoms are rarely aware that they have contracted the new coronavirus, so they mostly don’t go for PCR testing. The problem, of course, is that they’re still infectious and spreading the virus without knowing it.
Also, because there is a global shortage of PCR tests, South Africa can’t use them to test everyone who wants to be tested — the world simply doesn’t have enough PCR tests available. We’ll also be putting tremendous pressure on our labs.
In the process, we’ve missed many cases with mild or no symptoms.
On April 2, South Africa, therefore, relaxed its testing criteria, so that a wider range of people could qualify for testing. Many of them will be tested for SARS-Cov-2 with the help of antibody tests, once they have been approved by SAHPRA.
To ensure accurate results and to compensate for any shortcomings in the tests, it would be ideal to do more than one antibody test at a time, says Glenda Gray, president of the SA Medical Research Council and chairperson of the subcommittee for research on the ministerial committee that advises the president and health minister on its Covid-19 response.
Depending on which rapid tests South Africa can source, Gray says this might involve running two antibody tests simultaneously or running one test and then returning after a few days to do the second one.
For instance, if someone tests negative, they will have to return for a second antibody test seven days later, in case the first test was administered during the early stages of infection when their body had not yet manufactured SARS-CoV-2 antibodies. During that period, they will be asked to self-isolate, in case they are indeed infected and then spread the virus.
Most antibody tests measure two types of antibodies: the first, IgM antibodies, are short-lived and usually indicate that the virus is still present or was recently present. They normally disappear a couple of weeks after infection. In their place a second more durable type, IgG antibodies, emerge. In the case of SARS-Cov-2, no one knows yet how long these will last and provide resistance to reinfection; it could be anything from a few months to a few years.
So, if someone tests positive using an antibody test, the result will tell a health worker whether the person is infected or whether they have been exposed to the virus previously, and have since recovered.
Someone who tests positive for IgM antibodies may need to quarantine themselves for 14 days. If they had tested negative initially, and had been self-isolating for seven days between the two tests, and then test positive with the second test, they will be asked to continue self-isolating for another seven days.
There is, however, no evidence that doing two rapid tests, one week apart, works — or how accurate that is compared to PCR tests. It simply hasn’t been done before.
South Africa is, therefore, going to, at least initially, check the accuracy of rapid antibody test results with PCR tests. This process (it will be done for about 10 days) will allow researchers to collect data that can be used to establish how accurate the diagnostic results of antibody tests are.
If the data shows that the accuracy of rapid tests is adequate, PCR tests will be dropped and only rapid tests will be used in communities where testing is done on a wide scale.
If antibody tests turn out to not be accurate enough, the tests will be done away with and the testing team will investigate other rapid test tests, such as a new test provided by the Abbott pharmaceutical company, using molecular technology similar to PCR methods. It delivers positive results within five minutes and uses a test machine that is small enough to be moved around.
4. How will antibody tests help us to map the outbreak?
Antibody tests give us information PCR tests can’t: They tell us whether someone has been exposed to the virus, even if they were not infected at the time of taking the test, because these tests detect the presence of both short-lived IgM and more durable IgG antibodies.
PCR tests can’t do this; when someone who was infected with SARS-CoV-2 has recovered, there will no longer be any virus left to detect and a PCR test will indicate negative.
One of the most useful aspects of rapid antibody tests is that they can be used to ‘map’ infections, essentially identifying ‘hot spots’ — in so doing, indicating where investigations of viral spread need to be undertaken.
At the beginning of February, “rapid point of care diagnostics for use at a community level” was identified as a priority in a World Health Organisation report.