The second round of the serological survey conducted by the Delhi government concluded on Friday. The results of the first survey had shown that nearly a fourth of the respondents had developed antibodies against the novel coronavirus. The Delhi government intends to use the results of its latest survey “to tailor its (COVID-19) strategy to the changing circumstances”.
It plans to make such studies a monthly affair. The health authorities of Mumbai and Ahmedabad have also conducted serological surveys. These are much-needed endeavours given that several facets of the coronavirus are still in the realm of the unknown. Experts, however, rightly counsel that the data generated should only be used to draw estimates about the spread of the virus, and not reach conclusions about immunity against the pathogen.
After the results of Delhi’s first serological survey were announced, Delhi Chief Minister Arvind Kejriwal suggested that the capital was on the road to herd immunity. Such a sweeping inference is problematic. The threshold of herd immunity with respect to COVID-19 is uncertain. We do not know what percentage of infected people will ensure immunity to a community.
It would also be wrong to extrapolate the data from one pocket onto a broader community. A person starts developing an immune response in about two weeks of contracting the contagion, but we do not know how many antibodies are required to stave off a re-infection. The jury is still out on whether antibodies offer lasting immunity against the virus. In any case, a positive serology report does not tell if a person has an adequate number of antibodies to repel the pathogen. Experts reckon that an antibody positive person needs to be tracked for several months to ascertain if she has developed an adaptive immune response to repeated attacks by the virus.
The WHO cautions against the use of serological tests for granting immunity passports. But it also recommends such tests for academic purposes that could guide nuanced public health responses. For example, they can help health authorities to ascertain vulnerabilities on the basis of socio-economic status, geographic location, age-group or gender. For that to happen, the data has to be disaggregated to account for such variables. The Delhi government’s decision to collect samples across distinct age groups in its second serological survey is, therefore, a move in the right direction.