Coronavirus: What are the serosurveys not telling us?
Recently, several surveys have shown that the number of people who have been exposed to the coronavirus is higher than the number of "cases" reported. The latest such seroprevalence survey conducted in Mumbai found that 57% of the slum population had antibodies against the virus. But what do the results of such surveys mean and do they even tell us the full story?
What is a serosurvey?
A serosurvey involves testing the blood serum of a person to see if they have been infected with COVID-19 or if they have had an infection in the past. This differs from the RT-PCR diagnostic test, which detects active infections. If a person catches COVID-19, doesn't get tested, and recovers, RT-PCR would show no infection, but a serological test could detect antibodies.
What are the limitations of antibody tests?
However, with such surveys reporting a higher prevalence of infections compared to diagnostic tests, it's pertinent to be mindful of the limitations. Firstly, there is the question of what kind of antibody tests are being used and their quality. Ideally, the tests should have high sensitivity (ability to detect true positives) and specificity (ability to rule out false positives), but there are still roadblocks.
Even accurate tests could generate significant error
Even the more accurate antibody tests do not have 100% sensitivity and specificity. But let's say if a test has 95% sensitivity and specificity, it would still generate a significant rate of error in populations where the prevalence of the disease is low.
Here's how antibody tests could fail
If in a population of 1,000, 5% is infected with COVID-19, a test with 95% sensitivity will detect 48 of the 50 true positives. But due to its 95% specificity, the test would detect 48 false positives out of the 950 people who are truly negative. In this scenario, even though the number of true positives is 50, the test reports double the number.
Other issues with antibody surveys
There are also concerns pertaining to what mechanism researchers have used to test the population sample. Further, researchers are yet to establish what level of antibodies in the blood serum could actually offer protection against the coronavirus, i.e., lead to "immunity," how long that immunity would last, and how much percentage of the population needs to be immune for herd immunity to kick in.
When does herd immunity kick in?
Herd immunity is said to have been achieved when enough people in a population have immunity against a pathogen (either through natural infection or through a vaccine), which reduces the number of carriers, thereby, reducing the likelihood of new infections. Some experts say herd immunity is achieved when 60-70% of the population is immune, but evidently, we are still far from that.
