“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.

What does this mean for herd immunity?

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.”

And he notes that how long herd immunity lasts — whether it’s in a slum or a whole country — partly depends on how much movement there is in and out of that population. If people without immunity come into the area, that lowers the population’s overall level of immunity. If enough people come in, that could mean that there are enough people without immunity for the virus to spread again.

In a Mumbai slum, for instance, people are likely to be coming and going, which could impact how long herd immunity — if there is any — lasts. Utture Shankar, the president of the Maharashtra Medical Council, said people outside slum areas were dependent on those living in slums for services such as gardening, cleaning and driving, so will be exposed beyond their residential community.

When it comes to coronavirus, vaccines are the key to herd immunity — and controlling the virus, Dowdy says.

“I think this is a disease that’s going to be with us for a while,” he said. “But I don’t think it’s going to be a disease that causes the same level of deaths and suffering as it is right now.”