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Home / Health / More than half of Mumbai’s slum dwellers may have ever had Covid-19. This is why herd immunity is still far from established

More than half of Mumbai’s slum dwellers may have ever had Covid-19. This is why herd immunity is still far from established

Scientists believe that recovering from coronavirus gives a person immunity, but it’s unclear how strong it is or how long it lasts. Herd immunity is the idea that a disease stops spreading once there is enough population to become immune – and attractive because, in theory, it could provide some protection for humans. not sick yet.

If more than half of people in the slums of Mumbai have been infected with coronavirus, will they be able to achieve herd immunity – without vaccines?

One expert thought so.

“The Mumbai slums may have already reached herd immunity,” said Jayaprakash Muliyil, Chair of the Scientific Advisory Committee of the National Institute of Epidemiology, India. “If people in Mumbai want a safe place to avoid infection, they should probably go there.”


India currently has more than 2 million confirmed coronavirus infections

But others have been more cautious. It’s possible the researchers used an experiment that produced false positives, says David Dowdy, associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

And Om Shrivastav, an expert on infectious diseases in Mumbai, warns that, less than eight months since this virus exists in society, it is too early to make any “decisive and conclusive statements. Come on “.

If the Mumbai slums were on the verge of herd immunity, it would have come at a cost. Of India’s more than 2 million coronavirus infections, about 5% of them were reported in Mumbai, the country’s commercial capital. As of Monday, more than 6,940 people have died in the city, according to the city’s health authorities.

The high risk of death is precisely why India’s health authorities report The country does not aim for 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: Assume that each infected person infects three more people. If two of the three people are immune, the virus can only make one person sick. This means fewer people are infected – and over time, even those who are not immune are protected as they are less likely to be exposed to the disease. virus.

Migrant workers gathered outside the Dharavi slums to board buses during the nationwide shutdown on May 12, 2020.

The level of immunity required in a population depends about disease. Scientists still do not know the proportion of the population that need immunity to achieve herd immunity to the new coronavirus.

Currently, scientists believe that each person infected with coronavirus infects between 2 and 2.5 people, according to the World Health Organization. But that number could be influenced by other measures – for example, closing off could reduce the number of people each with coronavirus infection.
It is difficult to know what the immunity threshold of the herd is. Initially, some estimates put the figure 70% to 90%. Adam Kleczkowski, professor of mathematics and statistics at Strathclyde University, Glasgow, calculates this ratio between 50% and 75%, based on what scientists know about how the virus is transmitted.
With measles, for example, researchers in the early 20th century found that infection decreased as 68% of children became immune. But Dowdy points out how measles outbreaks in areas where people choose not to get a measles vaccine show how much of their immunity is lost.

Building immunity levels in the population can happen in two ways. People can become immune by getting vaccinated, or they catch virus and develop natural immunity by recovering from it.

And that’s where things get controversial.

Medical volunteers wearing PPE equipment measure a woman's temperature as they conduct a home medical examination inside the Dharavi slums in Mumbai on July 9, 2020.
The UK initially said that it would allow the spread of the coronavirus in the water to build immunity to the herd. That approach has been countered, with critics warning that it comes at a high price: the medical system is overworked and the death fee is unnecessarily high. The UK – which has been hampered in its herd immunization strategy – currently has more confirmed coronavirus deaths than most. Of the 20 countries most affected by coronavirus, the UK has the highest mortality rate, with 69.63 deaths per 100,000 people compared with 52.28 for the United States, according to data from Johns Hopkins University. .

Most other countries – including India – have taken a different approach. “Herd immunity in a country as large as India cannot be a strategic option, it can only be an outcome,” said Bhushan, a health official. high”.

As Dowdy put it: “We can very quickly develop population immunity to coronavirus simply by letting everyone in the population become sick … just millions of millions of people will die in the this progress.”

A medical worker collects a blood sample of a man in India.

Can we build natural immunity?

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

An article released last month – which has yet to be published in a peer-reviewed medical journal – suggests that the antibody response can begin to decrease 20 to 30 days after Covid-19 symptoms. appear.

The fact that antibody levels decline over time doesn’t necessarily mean that immunity doesn’t last long, Dowdy said. In other viruses, antibody levels also decline over time, but the immune response can still increase dramatically if a person is re-exposed to the virus.

According to Dowdy, our immunity to other coronaviruses tends to last a few years, instead of a lifetime. “If that’s a guideline, 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, which help protect the body from infection and the B cells, which produce antibodies.
“It’s a well-coordinated orchestra,” said Anthony Tanoto, a senior research fellow at Duke-NUS School of Medicine in Singapore who has studied T cells in Covid-19 patients.
In an article published in the scientific journal Nature in July, Tanoto and co-authors found evidence of T cells from people who had recovered from SARS – a coronavirus that spreads in 2003 – suggesting that these cells are least likely to exist 17 years.
The common cold is a coronavirus, and scientists believe that almost everyone has been infected with coronavirus in their lifetime. That means many people with T cells can react to Covid-19.

But for now, Tanoto says, we don’t know how many – if any – these T cells are helping fight Covid.

In fact, once the swarm immunity becomes available – whether naturally or through a vaccine – it probably won’t be the invulnerable shield some people might imagine.

Tanoto’s co-author Nina Le Bert, a senior research fellow at Duke-NUS School of Medicine in Singapore, points out that it is rare to have complete immunity from infection. Instead, immunity often means that a person’s body can react quickly enough to the virus so that it has no place – and not growing enough to infect others.

“That should be good enough, but that doesn’t mean you are not infected,” said Le Bert.

What does this mean for the herd’s immunity?

Even if certain areas achieve herd immunity, it may not last long.

According to Kleczkowski, from Strathclyde University, people who already have immunity are no longer immune to the new version of the virus, or a person’s immunity to the virus may not last long.

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

Herd immunity “isn’t a magic number” for coronavirus, Dowdy said.

“It doesn’t mean the disease will go away. It means if you give it 1,000 years, it will go away.”

And he notes how long the herd’s immunity lasts – whether it’s in a slum or an entire country – partly depends on how well the population moves in and out. If people do not have immunity to this area, it will decrease the general immunity level of the population. If enough people come, it means there are enough people who don’t have immunity for the virus to spread again.

In a Mumbai slum, for example, more people are likely to come and go, which could affect the herd’s immunity – if any – prolonged. Utture Shankar, chairman of the Maharashtra Medical Council, said people living outside of the slums depend on those living in the slums for services such as gardening, cleaning and driving, thus exposure outside of their community.

In the next 10 years, Kleczkowski hopes that some parts of the world will still have coronavirus. Even with swarm immunity in some areas, there may still be a problem with viral re-emergence, especially if people refuse to be vaccinated.
He points out that, although humans have had vaccines for 200 years, we have only successfully eliminated one disease that affects humans – small chickenpox, thanks to the global immunization program run by the Foundation. Health World leads. But it took a long time. A vaccine was discovered in the late 18th century, but smallpox was officially declared wiped out in 1979.

When it comes to coronaviruses, vaccines are key to the herd’s immunity – and to control viruses, Dowdy said.

“I think this is a disease that will stay with us for a while,” he said. “But I don’t think it will be a disease that’s causing the current level of death and suffering.”

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

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