More than half of Mumbai’s slum residents might have had Covid-19. Here’s why herd immunity could still be a long way off – CNN

While some were alarmed by the results of the study conducted by the Mumbai authorities and the Tata Institute of Fundamental Research, others were optimistic. Mumbai's slums, where social distancing is nearly impossible, might now have some of the highest levels of immunity in the world -- only 23.5% of samples taken by India's National Center for Disease Control tested positive for antibodies in Delhi and 14% tested positive in New York, in a study sponsored by the New York State Department of Health.

Scientists believe it's likely that recovering from coronavirus leaves a person with some immunity, but it's not clear how strong it is or how long it lasts. Herd immunity is the idea that a disease will stop spreading once enough of a population becomes immune -- and is appealing because, in theory, it might provide some protection for those who haven't been ill.

If more than half of people in Mumbai's slums had contracted coronavirus, could they be approaching herd immunity -- without a vaccine?

One expert thought so.

"Mumbai's slums may have reached herd immunity," Jayaprakash Muliyil, chairman of the Scientific Advisory Committee of India's National Institute of Epidemiology, said, according to a Bloomberg report. "If people in Mumbai want a safe place to avoid infection, they should probably go there."

But others have been more cautious. David Dowdy, an associate professor in epidemiology at Johns Hopkins Bloomberg School of Public Health, said it was possible that the researchers had used a test that created false positives.

And Om Shrivastav, an infectious diseases expert in Mumbai, cautioned that, less than eight months into the virus' existence in society, it was too early to make any "decisive, conclusive statements."

The risk of a high death toll is exactly why India's health authorities say the country is not aiming for herd immunity. "Herd immunity can be achieved through immunization -- but that is in future," health official Rajesh Bhushan told reporters last month.

What is herd immunity?

Herd immunity works like this: Assume that each infected person infects three more people. If two of those three people are immune, then the virus is only able to make one person sick. This mean that fewer people are infected by the illness -- and over time, even people who aren't immune end up being protected as they are less likely to be exposed to the virus.

The level of immunity needed in a population depends on the disease. Scientists don't yet know what proportion of a population needs to be immune to achieve herd immunity for the novel coronavirus.

Building up the level of immunity in a population can happen in two ways. People can become immune by being vaccinated, or they catch the virus and develop natural immunity by recovering from it.

And that's where things get controversial.

Most other countries -- including India -- have taken a different approach. "Herd immunity in a country with the size of population of India cannot be a strategic choice, it can only be an outcome and that, too, at a very high cost," said the health official, Bhushan.

As Dowdy puts it: "We could very rapidly develop a population immunity to the coronavirus simply by exposing every single person in the population to the disease ... it's just that millions and millions of people are going to die in the process."

Can we build natural immunity?

The science around immunity to Covid-19 is still developing.

The fact that antibody levels decline over time doesn't necessarily mean that immunity doesn't last, Dowdy says. In other viruses, antibody levels decline over time, too, but the immune response is still able to ramp up again if a person is re-exposed to the virus.

According to Dowdy, our immunity to other coronaviruses tends to last a few years, rather than being life-long. "If those are a guide, then that's what we might expect from this new coronavirus," he said. "But it's hard to say. We don't have any data on this particular virus."

But for now, Tanoto says we don't know how much -- if at all -- these T cells are helping fight off Covid.

In reality, once there is herd immunity -- whether naturally or through vaccines -- it probably won't be the impenetrable shield some people might imagine.

Tanoto's co-author Nina Le Bert, a senior research fellow at Duke-NUS Medical School in Singapore, points out that it's rare to have complete immunity from infection. Instead, immunity often means that a person's body is able to respond fast enough to the virus so that it doesn't gain a foothold -- and doesn't develop enough to infect other people.

"That will be good enough, but that doesn't mean you don't get infected," Le Bert said.

Even if certain areas do achieve herd immunity, it might not last.

The virus could mutate, meaning people who previously had immunity are no longer immune to the new version of the virus, or a person's immunity to the virus might not last long, according to Kleczkowski, from the University of Strathclyde.

"Even if we reach herd immunity at some point in time, we might lose it again," he said. "I don't think it's a silver bullet."

Dowdy says that herd immunity "isn't a magic number" to solve coronavirus.

"It doesn't mean that the disease is going to go away. It means that if you gave it 1,000 years, it would go away."

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More than half of Mumbai's slum residents might have had Covid-19. Here's why herd immunity could still be a long way off - CNN