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

"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."

If Mumbai's slums are on the brink of herd immunity, it has come at a cost. Of India's more than 2 million coronavirus cases, about 5% of those infections were reported in Mumbai, the country's commercial capital. As of Monday, more than 6,940 people had died in the city, according to the city's health authorities.

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.

Currently, scientists believe that each person infected with coronavirus infects between 2 to 2.5 people, according to the World Health Organization. But that number can be affected by other measures -- a lockdown, for instance, could bring down the number of people that each person with coronavirus infects.

It's hard to know what the threshold is for herd immunity. Initially, some estimates put the figure at 70% to 90%. Adam Kleczkowski, a professor in mathematics and statistics at the University of Strathclyde, Glasgow, calculates it at 50% to 75%, based on what scientists know about how the virus transmits.

With measles, for instance, researchers at the start of the 20th century noticed that infections declined when 68% of children had immunity. But Dowdy points out that measles outbreaks in areas where people are opting not to get vaccinated against measles shows how immunity can be lost.

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.

The United Kingdom initially said that it would allow the spread of coronavirus within the country to build up to herd immunity. That approach came under fire, with critics warning that it would come at a heavy price: overburdened health systems and unnecessarily high death tolls. The UK -- which backtracked on its herd immunity strategy -- now has more confirmed coronavirus deaths than most countries. Of the 20 countries most affected by coronavirus, the UK has the highest mortality rate, with 69.63 deaths per 100,000 compared to the US' 52.28, according to data from Johns Hopkins University.

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.

A paper released last month -- which has not yet been published in a peer-reviewed medical journal -- found that antibody responses may start to decline 20 to 30 days after Covid-19 symptoms emerge.

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."

Antibodies are only one part of the body's immune system -- there are also T cells, which help protect the body from infection, and B cells, which produce the antibodies.

"It is a well co-ordinated orchestra," said Anthony Tanoto, a senior research fellow at Duke-NUS Medical School in Singapore, who worked on research into T cells in Covid-19 patients.

In a paper published in science journal Nature in July, Tanoto and his co-authors found evidence of T cells from people who had recovered from SARS -- a coronavirus which spread in 2003 -- indicating that the cells can last for at least 17 years.

The common cold is a coronavirus and scientists believe that almost everyone has been infected with a coronavirus in their lifetime. That could mean that many people have T cells that might respond to Covid-19.

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.

In 10 years' time, Kleczkowski expects that some places in the world will still have coronavirus. Even if there is herd immunity in some places, there may still be an issue with the virus resurfacing, especially if people refuse to get vaccinated.

He points out that, although humans have had vaccines for 200 years, we have only successfully eradicated one disease affecting humans -- small pox, thanks to a global vaccination program led by the World Health Organization. But it took a long time. A vaccine was discovered in the late 18th century, but smallpox was only officially declared eradicated in 1979.

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."