The results of the latest round of the sero survey conducted in Delhi confirm what many health experts and scientists have been saying for some time now: That the prevalence of COVID-19 in the population has reached a level where the dynamics of “herd immunity” can be said to start playing a role. The results indicate that in at least some areas of Delhi, the disease prevalence could have gone beyond 50 per cent. So far, only Pune has reported similar numbers in a sero survey conducted way back in September.
In a situation where a vast majority of infected people exhibit few or only mild symptoms, a high disease prevalence rate can be reassuring. This could be one of the more important reasons why the spread of the disease has slowed down considerably in the last few months, leading to a steady decline in the number of cases that are detected. But it is important to remember that these results do not give any reason to believe that the epidemic is over, or that the whole population has achieved enduring immunity. And that is because one key detail is still missing: How long does the immunity, gained through natural infection or through vaccination, last? So, just like the start of the vaccination drive must not bring complacence, and prompt the abandoning of masks or norms of physical distancing, so must not the results of a sero survey, however reassuring they may be.
At the same time, the government must undertake more sero surveys in many more places. This is the only way to know what might be happening on the ground, beyond the numbers revealed by testing. The last round of the country-wide sero survey by the Indian Council of Medical Research was conducted in July and August. After that, individual research groups have been conducting it in some major cities. In the absence of such data, the reasons for the spectacular decline in the number of cases that has happened since the middle of September will not be understood, nor the bewildering numbers that continue to be reported from Kerala for the last three months. It will not be known either why the trajectory of the epidemic in India has been so different from that in many other countries in the world. Reliable and transparently-obtained data can help scientists and health experts to assess the situation better, and improve preparedness for the future.