COVID-19 transmission: Here’s why India needs to conduct sero-surveys; what do they tell

By: |
August 19, 2020 12:02 PM

Six months after the Coronavirus transmission began in India, the country is betting on multiple sero-surveys to check the extent of transmission.

Case fatality rate has declined to 1.94 per cent.Sero-surveys also help examine herd immunity but they do not reveal at what point herd immunity has been achieved or will be achieved.

Testing COVID-19 impact in India: Six months after the Coronavirus transmission began in India, the country is betting on multiple sero-surveys to check the extent of transmission. In a sero-survey, liquid part of blood (called serum as well) is tested by health officials where they detect if there is an immune response against the viral material. Usually, when human body catches a viral infection, it comes up with immune responses called antibodies. These antibodies (IgG) are developed within a few days of infection and they tend to stay for a couple of months, even when the infection has gone. Now testing for these antibodies indicates whether a person has been an asymptomatic carrier of Coronavirus or not. These antibodies will not be formed within those individuals who have not come in contact with the infection.

To be sure, infection can be detected via a swab test, RT-PCR, done via nose, mouth or throat fluids. However, this will only work at the time of infection and if the infection is gone, it cannot be detected. In such cases, IgG antibodies stay in the human blood, which is being tested right now as a part of sero-survey.

A report by The Indian Express highlighted that the people chosen for sero-survey are random as everyone cannot be tested. The results can help provide an estimate of the number of people having the infection in the past, thus, portraying a big picture of how Coronavirus has spread in a community. While the survey is quite helpful in providing the estimates, the report said it does not detect ‘immunity’ to SARS-CoV-2, or simply put, it does not reveal if people are protected against the virus. It said that all immune responses are not protective. Therefore, just some antibodies are detected in sero-survey tests, which does not necessarily mean that they are the protective ones. Notably, protective antibodies are harder to test on a scale this large.

Even if antibodies are protective, it is still unclear what kind of antibodies are needed for actual protection against the novel Coronavirus. Furthermore, sero-surveys also help examine herd immunity but they do not reveal at what point herd immunity has been achieved or will be achieved. It is to note that herd immunity is a situation where a set of people in the community becomes immune or protected from the virus and person to person transmission reduces.

Meanwhile in India, as many as five sero-surveys have been conducted, starting from surveys in Delhi, Mumbai and Berhampur in Odisha, and Pune city. The earlier surveys done in Delhi showed more than 23 per cent of seropositivity whereas Mumbai showed above 40 per cent. Berhampur, on the other hand, highlighted that 31 per cent of people have been exposed to the virus. According to the report, the numbers are averages having many variations between neighbourhoods.

While these many sero-surveys have been conducted, the report asserted the need for more sero-surveys in the country. More people and localities also need to be taken into account whereas the same localities over time will also have to be tested again over time. This will help provide the footprints of the Coronavirus moving through communities. The continuous testing may also give an understanding of what actual protection would look like, the report said. Further, sero-surveys can also be used for proper planning and evaluation of vaccine trials and its deployment.

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