Delhi: What the sero survey reveals

More than half the population of Delhi (56%) appears to have been exposed to Sars-cov-2, according to the fifth round of serological survey conducted in mid-january. This is a jump of over 30 percentage points over the last sero survey from October. In parts of Delhi, this number is as high as 62%, hovering close to what experts believe is the herd immunity threshold. Herd immunity is when a large enough proportion of the population get infected (and thus get immunity) and, so, a disease runs out of vulnerable people to spread to.

So what does this mean for Delhi? The answer is not clear. The third sero survey showed the presence of antibodies drop four percentage points, which can only be explained by faults in the sampling design or of the quality of kits used. In an outbreak, antibodies are only expected to grow within a population, not contract.

Then there’s the debate on the concept of herd immunity. Sweden, which relied on this, admitted in December that despite a large proportion of the population getting infected, they saw no signs of community immunity. In the Brazilian city of Manaus, sero surveys in August showed that 75% of people had antibodies, yet the city saw cases rising in October, and by January, hospitals were overflowing with patients. So reliance on herd immunity is hardly an answer, especially when a far more effective strategy (vaccinations) is already underway. Social distancing, mask and hand hygiene until everyone is vaccinated remains the best chance at fighting off this disease.