Coronavirus: Herd immunity with just 10% infections?

As Karnataka battles to contain Covid-19, an interesting new study suggests that herd immunity can be achieved if only 10% of the effective population is infected, a marked departure from the common theory that a wide section of the population needs to be infected to develop immunity.

The technical report by a team of European scientists has found that heterogeneity in exposure and susceptibility and transmission of Covid-19 infection reduce the number of people required to be infected before herd immunity kicks in.

Heterogeneity comprises people who cannot transmit the virus, people in contact with infected individuals and a proportion of “the effective population” not susceptible to infection owing to some pre-existing cross-infection.

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The “effective population” is a subset of the total census population and is made up of individuals who are in contact with contagious individuals. The rest of the population is assumed to be shielded or geographically separated from a regional outbreak.

The report titled Effective Immunity and Second Waves has implications for Bengaluru Urban, where the government might consider herd immunity as a strategy, said a member of the Covid Task Force. Epidemiologists with the ICMR, who have examined the report, said that herd immunity is the silver lining.

“The question is how much of the population would need to be infected for us to break the chain of infection,” said leading epidemiologist Jayaprakash Muliyil, former Principal of Christian Medical College, Vellore.

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Dr Muliyil had previously stated that a rate of 60% infection was required. However, the outbreak in Mumbai’s Dharavi slum prompted a rethink.

“At Dharavi, we began to see the number of new cases fall after 40% of the population was infected. We still don’t understand why herd immunity kicked in so quickly,” he told DH.

However, Muliyil urged caution, saying the study’s 10% rate was not apt for India.

“India’s epidemic is veering away from international findings because our way of life amid the outbreak has been mostly unchanged. People are still gathering in crowds and not wearing masks,” he said.

Padarayanapura example

Bengaluru’s Padarayanapura ward also makes a case for the lower herd immunity theory.

According to Bengaluru Bruhat Mahanagara Palike (BBMP) war room and scientists mapping the locality for random-testing measures, Padrayanapura (with a population 35,213) was reeling from the city’s highest Covid-19 caseload from April to late June. However, no new cases have been reported from there in the last 14 days.

Instead, the Palike data shows the outbreak as having migrated to new areas of the city.

“I think it is clear what happened at Padarayanapura,” said Dr Giridhar Babu, who is with the ICMR and State’s Expert Committee.

He added that the outbreak in Padarayanapura hit a peak and went down thereafter.