A new study published in the preprint open-access journal medRxiv in April 2020 assesses the value of untargeted testing for infection or immunity in COVID-19 vs. high-quality targeted testing in keeping down the number of infections while slowly relaxing lockdown measures.
As leaders over the globe prepare to ease their countries out of prolonged lockdowns, the need for an effective testing strategy has intensified. In the absence of careful execution of relaxation measures, the second wave of infection becomes significantly more likely, and this could exceed the earlier wave due to the presence of the virus in the community.
In the current study, the researchers used a modified SIR model to identify what would happen with imperfect testing on the disease dynamics in a situation when social distancing measures were being relaxed. The model was modified to include quarantined states and its performance was tested using publicly available estimates. The study explored three possible scenarios - immediately ending lockdown measures, continuing the lockdown with immunity passports issued on the basis of antibody testing, and gradual relaxation of lockdown with active viral testing.
The role of testing
The UK, in particular, faced criticism over its low testing capacity early in the pandemic. Even now, many countries seem to be prioritizing quantity over quality when it comes to testing kits. However, when only a small percentage of the overall population is infected, false-positive tests are more likely than positive tests.
This could be a significant barrier to containing the epidemic when the healthcare system is operating under great pressure due to the sheer volume of patients, as is indeed occurring with infected patients being missed.
"The quantity of tests is not a substitute for an effective strategy. Poorly targeted testing has the propensity to exacerbate the peak in infections," state the authors.
Why different tests need different quality parameters
Testing will be crucial in planning social distancing relaxations. However, all tests are not equal. Antibody tests check whether a person has some immunity against the virus - "have you had it?" while active viral tests test for people who are currently infected - "have you got it?".
The researchers state that in order to be effective in the current scenario, both tests have to focus on different characteristics. Active viral tests need to maximize their sensitivity - how likely a person who tests positive is to be actually positive, or in other words, how accurate the test is in identifying infection.
If an active viral test is inaccurate and tells people they are not infected when they are, they may behave more recklessly than they would have if they were not sure whether or not they were sick, and in the process infect more people.
Say the researchers, "The capacity for infection screening needs to be significantly increased if it is to be used to relax quarantine measures, but only if it is well-targeted, for example, through effective contact tracing. Untargeted mass screening would be ineffectual and may prolong the necessary implementation of lockdown measures."
Antibody testing, on the other hand, seeks to identify those who are not likely to get infected because they already have had the disease. The authors say this test should focus on high specificity, or how likely the person is NOT to have had the disease if testing positive. In other words, that means how often the test tells someone they have had the disease when they haven't.
A false positive here would have serious effects since it tells someone they have immunity to the virus when they don't.
"Antibody testing, with high specificity may be very useful on an individual basis, it certainly has scientific value, and could reduce the risk for key workers." the authors say. "But any belief that these tests would be useful to relax lockdown measures for the majority of the population is misguided. At best it is a distraction, at worst it could be dangerous."
The paper also pointed out that countries which carried out untargeted mass testing early in the pandemic but had much lower case fatality rates might have been reporting a large number of false positives.
In the first scenario - a sudden end to the lockdown - researchers say sensitivity and specificity of testing will have little effect on controlling the virus since the second wave of COVID-19 cases is inevitable if people are allowed to go about with a relaxation of social distancing measures.
In the second - a hypothetical situation with widespread antibody testing and allowing people to move freely on the basis of an "immunity passport" (a positive antibody test certifying that they have some level of immunity to the disease) - researchers say that here too, false negatives would allow re-entry of the virus into the population, and thus send infection rates up.
The third and most likely scenario - the incremental relaxation of social distancing measures - will probably be the most effective in suppressing infection, researchers say. While high infection rates will probably be present for a longer period, it is possible to continuously keep the virus under control.
However, the percentage of the population released in each increment of lockdown relaxation must be carefully determined, because this, in combination with the test capacity and performance parameters, and the prevalence of infection in the target population, are all factors that are crucial in deciding the expected peak of infections following relaxation. This will, therefore, shape the policies regarding the duration of mass-scale social distancing.
In contrast to the WHO's focus on "Testing, testing, testing," the researchers conclude, "A bad test is worse than no tests, but a good test is only effective in a carefully designed strategy."
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.