What Delhi’s coronavirus outbreak can teach us about herd immunity

Almost a quarter of Delhi’s 28 million residents may have coronavirus antibodies, making the Indian capital one of the worst hit cities in the world, research has revealed.

A sample of 20,000 people carried out by India’s National Centre for Disease Control (NCDC) found 23.4% had antibodies to the virus, with the majority of infections appearing to have been asymptomatic.

The pandemic last month hit Delhi hard, “pushing the city’s healthcare system to the brink of collapse” and causing many deaths as “hospitals ran out of beds and ventilators”, The Guardian says.

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Latest figures show that India has the third highest number of Covid-19 cases in the world, with 1.2m confirmed infections and numbers rising by more than 40,000 a day.

And the “curve of infections is showing no sign of flattening”, with states across the country reintroducing local lockdowns and curfews in an effort to curb the virus’s spread, the paper adds.

The NCDC study suggests that around 180 million Indians, representing almost 15% of the population, may already have developed Covid-19 antibodies. Experts have heralded the discovery as “proof that India is hurtling towards herd immunity”, India Today says.

“Only around six per cent of Delhi's population tested positive for novel coronavirus, but one-fourth of Delhi got immunity. This is herd immunity. These people have immunity even without getting a vaccine,” Dr SK Sareen, director of the New Delhi-based Institute of Liver and Biliary Sciences, told India Today.

“Even as these self-acquired antibodies are yet to be titrated [measured], their presence means that these people were infected and managed to clear the virus,” Sareen added.

A severe lack of testing across India has been blamed for the wide discrepancy between reported cases and the number of people thought to have antibodies, The Telegraph says.

A senior Indian epidemiologist said the findings were “encouraging and could see its cities reach herd immunity by the end of August”.

“In a few weeks, Delhi will attain herd immunity,” Dr Jayaprakash Muliyil told The Hindu. “There are some questions on how long the immunity lasts. I think it will be a long-lasting immunity, but I cannot give you an exact number.”

Most scientists have said that herd immunity would only be reached when 60-70% of the community are protected against the virus, although recent studies have noted that the infection rarely spreads beyond about 20% of a local population.

However, Delhi’s government has said the doubt about the duration of the immune response means that “containment measures need to continue with the same rigour”. Research conducted in the UK found that patients who have recovered from Covid-19 may “lose their antibodies” to the disease within three months.

Scientists at King’s College London analysed more than 90 patients and healthcare workers at Guy’s and St Thomas’ Hospital, London and found antibodies that can defeat the virus peaked around three weeks after symptoms first appeared, before suddenly declining.

Other researchers have focused on T-cell immunity to Covid-19, which is thought to offer longer-lasting protection.