What Delhi sero survey means

Herd immunity is getting closer, but don’t lower the guard

Delhi’s second serological survey, which was conducted between August 1 and 7 and sampled 15,000 people, found that 29.1% of the city’s residents have developed antibodies against Covid-19. Extrapolating the findings to Delhi’s population would mean that around 5.8 million people in the city have been infected with Sars-cov-2 at some point. This is a 6.2 percentage point increase from the first sero survey conducted around a month ago, which showed 22.86% people had developed antibodies. This proportion of infected has been extremely varied in the different sero surveys conducted across the country so far. Much of this variation may be because of sample sizes and sampling region within these cities. In Pune (sample size: 1,664), officials found that 51% of people had the antibodies earlier this week. In Mumbai (sample size: 6,936), this number was 40% at the end of July. In Ahmedabad (sample size: 30,054), 17.61% people exhibited seroprevalence at the end of July.

Here is the good news. One, the findings show deaths have been lower than feared. As on August 7 (the last day of the sampling), Delhi had 4,082 deaths due to Covid-19. If this is pitted against the estimated 5.8 million infected, the infection fatality rate is 0.07%. Two, herd immunity is around the corner. Scientists say herd immunity occurs when a large proportion of the population contracts an infectious disease and build natural immunity to it. Once a certain percentage becomes immune, the disease starts running out of people to infect, and so the chain of transmission slows down, or halts entirely. With the vaccine still far off, herd immunity offers hope even though scientists remain divided on where this herd immunity threshold exactly lies.

Now, the not-so-good news — testing has been woefully inadequate. By the government’s own numbers, there were 142,723 confirmed cases on the day the sero survey ended. But if 5.8 million people had been infected by August 7, then the testing strategy managed to only identify less than 2.5% of those infected. This means that it missed over 5.6 million infections, most of whom were probably asymptomatic. So, while the results show that Delhi is inching closer to herd immunity (and getting there faster than thought), it should also serve as a wake-up call to not let down one’s guard, particularly on the testing front.