Why Delhi sero survey findings do not mean you’re safe

Delhi health minister Satyendar Jain said the findings indicate the city was inching towards the herd immunity threshold, but cautioned that a resurgence of the coronavirus disease cannot be ruled out.

Close to 1.2 million people more may have silently contracted the Sars-Cov2 virus between July and August, if the results from Delhi’s second round of antibody surveys are extrapolated. This suggests that while the city has turned a corner on its peak, and the impact of the disease appears to be less dire than previously thought, the outbreak is far from under control, and there is little scope for rules to be relaxed or people to become less cautious.

Migrant workers arriving from other states sit for rapid antigen tests for Covid-19 at Anand Vihar bus terminal in New Delhi on Tuesday.(Amal KS/HT PHOTO)

Data released by the Delhi government showed 29.1% of the 15,000 people sampled in the beginning of August appeared to have antibodies for the virus, which means that they likely recovered from the disease and are immune to it for now. This proportion was 22.9% in the beginning of July.

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Delhi health minister Satyendar Jain said the findings indicate the city was inching towards the herd immunity threshold, but cautioned that a resurgence of the virus cannot be ruled out. “It is relieving to know that around 29% of the city’s population has recovered... but 71% of people in Delhi have not developed antibodies, which means that they are still vulnerable... We cannot rule out a second wave because only yesterday India saw the highest ever spike in daily fresh Covid-19 cases,” he said.

The herd immunity threshold is a point in infection prevalence where the spread of a virus slows down as the proportion of immune population increases, but experts cautioned against pinning hopes on this just yet.

“The concept of herd immunity does not hold when you talk about natural infection. Herd immunity is calculated as 1-1/R0 only in the case of vaccine-induced immunity. In case of a natural infection, anyone who is infected also passes it on to others, which is not the case with vaccinations. So talking about herd immunity in this case, when we don’t have a vaccine yet, will be a mistake,” said T Jacob John, former head of clinical virology at Christian Medical College, Vellore.

R0 is the basic reproduction number of a virus, or the number of people each infected person can infect. This number can vary based on how people reduce or increase social contact. This fluctuation is noted as the effective reproduction number, or Rt, which for Delhi was estimated to be below 1 for over a month till August 10.

If Rt falls below 1, it means not every infected person is passing the virus on, which effectively begins to reduce the size of the outbreak. “As more and more people are getting infected, assuming you cannot get reinfected, the number of susceptible people is going down. So, if an infected person goes out, their chance of meeting someone who already had it is increasing, so it is now indeed harder to spread the disease than in March. But we cannot let the virus run through like wildfire in a forest, we have to let the slow burning continue,” said Bhramar Mukherjee, professor of epidemiology and the head of biostatistics at University of Michigan, which tracks state-wise Rt numbers for India.

“Herd immunity may be achieved but cannot be actively pursued,” Mukherjee added.

The numbers also underscore the scale of silent spread, which makes infection control harder. In the time when roughly 1.2 million silent infections took place, only 33,583 – or 2.79% -- were officially recorded.

This, Mukherjee added, makes it important for people to continue with precautionary measures, which are likely getting harder as fatigue sets in. “This is not going away very soon,” she said, adding that the public and authorities need to focus on a “survival and prevention strategy”.

For this, she suggests, people could carry out some social contact while being careful. “Think of outdoor evening gatherings while maintaining social distancing, forming pods with families and friends who are self isolated and meeting them exclusively, washing hands, wearing masks, etc. All of this is part of our defence mechanism,” she said, adding that the stigma around a Covid diagnosis should also be fought so that people can be tested and treated.