Home / Health / More than half of the slum dwellers in Mumbai may have had Covid-19. Here’s why herd immunity could still be a long way off

More than half of the slum dwellers in Mumbai may have had Covid-19. Here’s why herd immunity could still be a long way off

Scientists believe recovery from coronavirus is likely to leave a person with some immunity, but it’s not clear how strong it is or how long it takes. Herd immunity is the idea that once enough of a population becomes immune, once enough of a population becomes immune, a disease will stop spreading – and it’s appealing because in theory it could offer some protection to those who weren’t sick.

If more than half of the people in Mumbai’s slums had coronavirus, could they approach herd immunity – without a vaccine?

An expert believed it.

“Mumbai’s slums may have reached herd immunity,” said Jayaprakash Muliyil, chairman of the scientific advisory board of India̵

7;s National Institute of Epidemiology, according to a Bloomberg report. “If the people of Mumbai are looking for a safe place to avoid infection, they should probably go there.”

India now has more than 2 million confirmed coronavirus cases

But others were more careful. David Dowdy, associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health, said it was possible the researchers used a test that produced false positive results.

And Om Shrivastav, an infectious disease expert in Mumbai, warned that less than eight months after the virus entered society, it was too early to make “decisive, conclusive statements”.

If Mumbai’s slums are on the verge of herd immunity, it comes at a price. Of the more than 2 million coronavirus cases in India, around 5% of these infections have been reported in Mumbai, the country’s commercial capital. According to the city’s health authorities, more than 6,940 people had died in the city on Monday.

The risk of a high death toll is exactly why the Indian health authorities say this Land does not seek herd immunity. “Herd immunity can be achieved through immunization – but that is in the future” Health official Rajesh Bhushan told reporters last month.

What is Herd Immunity?

Herd immunity works like this: suppose each infected person infects three other people. If two of these three people are immune, the virus can only make one person sick. This means fewer people are infected with the disease – and over time, people who are not immune will also be protected as they are less likely to be exposed to the disease Virus.

Migrant workers gather outside the Dharavi slums to board a bus during a nationwide lockdown on May 12, 2020.

The level of immunity required in a population depends on it on the disease. Scientists do not yet know what proportion of a population must be immune to achieve herd immunity against the novel coronavirus.

Currently, scientists believe that any person infected with coronavirus will infect between 2 and 2.5 people, according to the World Health Organization. However, this number can be influenced by other measures – for example, a lockdown could reduce the number of people that each person becomes infected with coronavirus.
It’s hard to know what your herd immunity threshold is. Some estimates initially assume 70% to 90%. Adam Kleczkowski, Professor of Mathematics and Statistics at the University of Strathclyde, Glasgow, calculates it to be 50% to 75% based on what scientists know about the transmission of the virus.
For measles, for example, researchers in the early 20th century found that infections decreased when 68% of children had immunity. However, Dowdy points out that measles outbreaks in areas where people choose not to get vaccinated against measles show how immunity can be lost.

Building immunity levels in a population can be done in two ways. People can get immune from vaccination or catch them the virus and develop natural immunity by recovering from it.

And this is where it becomes controversial.

Medical volunteers wearing PPE equipment take a woman's temperature as they conduct a door-to-door medical screening in Dharavi slums in Mumbai on July 9, 2020.
The UK initially said it would allow the coronavirus to spread within the country in an effort to boost herd immunity. This approach has come under fire and critics warned against it it would come at a high price: overburdened health systems and unnecessarily high fatalities. The UK – which has backtracked on its herd immunity strategy – has now confirmed more coronavirus deaths than most other countries. Of the 20 countries hardest hit by the coronavirus, the UK has the highest death rate with 69.63 deaths per 100,000, compared with 52.28 in the US. This is based on data from Johns Hopkins University.

Most other countries – including India – have taken a different approach. “Herd immunity in a country the size of India’s population cannot be a strategic decision, it can only be an outcome, and at a very high cost,” said Health Official Bhushan.

As Dowdy puts it, “We could develop population immunity to the coronavirus very quickly by simply exposing every single person in the population to the disease … it’s just that millions upon millions of people will die in the process.”

A medical worker takes a blood sample from a man in India.

Can We Build Natural Immunity?

The science around immunity to Covid-19 is evolving.

A paper published last month that has not yet been published in a peer-reviewed medical journal found that antibody responses may decrease 20 to 30 days after symptoms of Covid-19 appear.

The fact that antibody levels decrease over time doesn’t necessarily mean immunity isn’t lasting, Dowdy says. For other viruses, antibody levels also decrease over time, but the immune response can still rise again if a person is exposed to the virus again.

According to Dowdy, our immunity to other coronaviruses lasts for a few years rather than life. “If this is a guide, that’s what we can 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 part of the body’s immune system – there are also T cells that protect the body from infection and B cells that produce the antibodies.
“It’s a well-coordinated 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 an article published in July in the science journal Nature, Tanoto and co-authors found evidence of T cells from people who had recovered from SARS – a coronavirus that spread in 2003 – suggesting that the Cells can have at least one lifetime 17 years.
The common cold is a coronavirus, and scientists believe almost everyone has been infected with a coronavirus in their lifetime. That could mean that many people have T cells that could respond to Covid-19.

But right now, Tanoto says we don’t know how much – if anything – these T cells help fight off Covid.

In reality, herd immunity – whether natural or through vaccines – is unlikely to 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 is rare to have complete immunity to infection. Instead, immunity often means that a person’s body can react to the virus quickly enough 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 won’t get infected,” said Le Bert.

What does this mean for herd immunity?

Even if certain areas achieve herd immunity, it cannot last.

The virus could mutate, meaning that people who previously had immunity are no longer immune to the new version of the virus, or a person’s immunity to the virus doesn’t last long, according to Kleczkowski of Strathclyde University.

“Even if we get herd immunity at some point, we could lose it again,” he said. “I don’t think it’s a silver bullet.”

Dowdy says herd immunity is “not a magic number” for solving the coronavirus.

“It doesn’t mean the disease will go away. It means that if you gave it for 1000 years, it would go away.”

And he notes that the duration of herd immunity – whether in a slum or across a country – depends in part on the amount of movement in and out of that population. If people come to the area without immunity, it will lower the general immunity level of the population. If enough people come in, it could mean there are enough people without immunity for the virus to spread again.

For example, in a slum in Mumbai, people are likely to come and go, which can affect how long herd immunity, if any, lasts. Utture Shankar, president of the Maharashtra Medical Council, said people outside slum areas are dependent on people who live in slums for services like gardening, cleaning and driving, and therefore will be exposed outside of their community.

In 10 years, Kleczkowski expects that some places in the world will still have coronavirus. Even with herd immunity in some areas, virus renewal issues can still arise, especially when people refuse to be vaccinated.
He points out that while humans have had vaccines for 200 years, thanks to a global immunization program led by the World Health Organization, we have only successfully eradicated one disease – smallpox. But it took a long time. A vaccine was discovered in the late 18th century, but smallpox wasn’t officially declared eradicated until 1979.

When it comes to coronavirus, vaccines are key to herd immunity – and to fighting the virus. Says Dowdy.

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

CNN’s Esha Mitra from New Delhi contributed to this story.

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