Sero-survey numbers shouldn’t dull Covid-19 fight

The fifth sero-survey in Delhi has reported prevalence of antibodies against SARS-CoV-2 in 56.13% of the 28,000 subjects analysed—indicating that nearly every second person in the national capital has been exposed to the virus and developed some measure of immune response. With daily new infections in Delhi below 200, it would be easy to conflate this with some sort of ‘herd immunity’, given most experts hold that the herd immunity threshold for the virus is likely 60-80%—though the emergence of variants with higher transmissibility and the likelihood that there could be further such mutations queers any herd immunity estimate. However, such conflation will not just be unproductive for Covid-19 control efforts, but also dangerous.

Indeed, epidemiological experts say that seroprevalence numbers are more indicative than exact, and that is a fact that needs to be taken into consideration while making inferences. The example of Manaus in Brazil shows how seroprevalence numbers and a popular misconception of herd immunity can cause a dropping of the guard with rather devastating consequences for Covid-19 control.

In Manaus, a 76% seroprevalence was reported in a study published in Science in December. Barely weeks after, this city in northwestern Brazil, reported a second wave, with healthcare facilities overwhelmed and Covid-19 deaths contributing to sombre visuals. A paper in Lancet published late last month, while mentioning deviations inferences from seroprevalence reports had with later infection, offered four “non-mutually exclusive” reasons—first, the seroprevalence data led to an overestimation of herd immunity, and the population remained under this till December 2020; second, the immunity developed against the infection may have already started to wane; third, certain viral lineages might have evolved that evade the immune response to the earlier circulating variant; and fourth, that certain variants might have evolved to become highly transmissible though they are checked by the antibodies developed in response to the earlier circulating variant.

In the second scenario, it is unlikely that the high exposure to the virus that Delhi is reporting now could be of much use in the long run, though, at the population level such high levels of exposure could offer some hurdles to the virus. The third scenario has probably the most dangerous implication, indicating that naturally-induced immunity would simply not be enough against reinfections; as per Brazilian government data, though, the overwhelming majority of the cases in the second wave in Manaus are not reinfections.

Against such a backdrop, vaccines and therapeutics, as and when these are developed, and non-pharmacological measures such as distancing, wearing masks, hand sanitation, etc, remain important. Vaccines, even if they don’t prove to be failsafe in individual cases, elicit a far stronger immune response than casual natural exposure to the virus, and can prevent a serious onset of Covid-19 even if they don’t manage to prevent it in the off-chance.

Herd immunity, bear in mind, acts at the population-level, lowering the chances of transmission in the population, while uninfected individuals—assuming reinfections remain negligible—remain vulnerable. To that end, Delhi health minister Satyendar Jain did well to warn the public that the usual anti-Covid-19 measures need to be strictly followed; this is more important in the present context since meaningful vaccine coverage is still some time away.