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Home / Health / More than half of Mumbai’s impoverished residents may have had Covid-19. That’s why herd immunity can still be far away

More than half of Mumbai’s impoverished residents may have had Covid-19. That’s why herd immunity can still be far away

Scientists believe that recovery from coronavirus is likely to leave a person with an immunity, but it is not clear how strong it is or how long it lasts. Herd immunity is the idea that a disease will stop spreading once a population becomes immune – and it is attractive because, in theory, it can provide protection for those who have not been sick.

If more than half the people in Mumbai slums had contracted the coronavirus, could they have approached herd immunity – without a vaccine?

One expert thought so.

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


India now has more than 2 million confirmed cases of coronavirus

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

And Om Shrivastav, an infectious disease expert in Mumbai, warned that, less than eight months into the existence of the virus in society, it was too early to make any “decisive, final statement”.

If Mumbai neighborhoods are on the verge of herd immunity, it has come at a cost. Of India’s more than 2 million cases of coronavirus, about 5% of these 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 city health authorities.

The risk of a high number of deaths is exactly why the health authorities of India say this the country is not targeting herd immunity. “Herd immunity can be achieved through immunization – but that’s the future,” he said. 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 these three people are immune, then the virus is only able to make one person sick. This means that fewer people are infected with the disease – and over time, even people who are not immune end up being protected as they are less likely to be exposed to virus.

Migrant workers gather outside Dharavi neighborhood neighborhoods to board a bus during a nationwide blockade on May 12, 2020.

The level of immunity needed in a population depends on the disease. Scientists still do not know what percentage of a population must be immune to achieve cluster immunity to the new coronavirus.

Currently, scientists believe that each person infected with the coronavirus infects 2 to 2.5 people, according to the World Health Organization. But that number could be affected by other measures – a blockage, for example, could bring down the number of people infecting each person with the coronavirus.
It is difficult to know what the threshold for herd immunity is. Initially, some estimates put the figure at 70% to 90%. Adam Kleczkowski, professor of mathematics and statistics at the University of Strathclyde, Glasgow, estimates it at 50% to 75%, based on what scientists know about how the virus transmits.
For measles, for example, researchers in the early 20th century noted that infections were reduced when 68% of children had immunity. But Dowdy points out that measles outbreaks in areas where people are choosing not to be vaccinated against measles show how immunity can be lost.

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

And that’s where things get controversial.

Medical volunteers wearing PPE clothing take a woman's temperature reading as they perform a door-to-door medical examination within the Dharavi neighborhood in Mumbai on July 9, 2020.
The UK initially said it would allow the coronavirus to spread domestically to boost herd immunity. This approach came under fire, with critics warning that would come at a heavy price: overloaded health systems and unnecessary numbers of unnecessary deaths. The UK – which withdrew from the herd immunity strategy – has now confirmed more coronavirus deaths than countries. Of the 20 countries most affected by the coronavirus, the UK has the highest mortality rate, with 69.63 deaths per 100,000 compared to 52.28 in the US, according to data from Johns Hopkins University.

Most other countries – including India – have taken a different approach. “Flock immunity in a country the size of India’s population cannot be a strategic choice, it can only be a result and that too, at a very high cost,” said health official Bhushan.

As Dowdy puts it: “We could very quickly develop a population immunity to the coronavirus, simply by 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 staff collects a blood sample from a man in India.

Can we build natural immunity?

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

A paper released last month – which has not yet been published in a peer-reviewed medical journal – found that antibody responses may begin to fall 20-30 days after the onset of Covid-19 symptoms.

The fact that antibody levels fall over time does not mean that immunity does not last, Dowdy says. In other viruses, antibody levels also drop over time, but the immune response is still able to recur if a person is exposed to the virus again.

According to Dowdy, our immunity to other coronaviruses tends to last for several years, rather than being long. “If they are a guide, then this is what we can expect from this new coronavirus,” he said. “But it’s hard to say. We have no data on this particular virus.”

Antibodies are just 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 antibodies.
“It’s a well-coordinated orchestra,” said Anthony Tanoto, a senior research researcher at Duke-NUS Medical School in Singapore, who worked on T cell research in Covid-19 patients.
In a paper published in the scientific journal Nature in July, Tanoto and his co-authors found T cell evidence from people who had recovered from SARS – a coronavirus that spread in 2003 – showing that cells could last 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. This could mean that many people have T cells that can respond to Covid-19.

But now, Tanoto says we do not know how much – if at all – these T cells are helping fight Covid.

In reality, having herd immunity – either naturally or through vaccines – will certainly not be the impenetrable shield some people can imagine.

Tanoto 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 from infection. On the contrary, immunity often means that a person’s body is able to respond quickly enough to the virus, so that it does not get a ground – and does not develop enough to infect other people.

“It will be good enough, but it does not mean that you will not become infected,” said Le Bert.

What does this mean for herd immunity?

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

The virus can change, 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 may not last long, according to Kleczkowski, of the University of Strathclyde.

“Even if we achieve herd immunity at some point in time, we can lose it again,” he said. “I do not think it is a silver bullet.”

Dowdy says clutch immunity “is not a magic number” to solve the coronavirus.

“It does not mean that the disease will go away. It means that if you gave it 1000 years, it would go away.”

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

For example, in a humid Mumbai, people are more likely to come and go, which can affect how long the herd’s immunity lasts – if any. Utture Shankar, president of the Maharashtra Medical Council, said people outside the slums depend on those living in slums for services such as gardening, cleaning and running, so they will be exposed beyond their residential community.

At the age of 10, Kleczkowski expects some countries in the world to still have coronavirus. Even if there is herd immunity in some countries, there may still be a problem with the virus recurring, especially if people refuse to be vaccinated.
He points out that, although humans have had vaccines for 200 years, we have only successfully eradicated a disease that affects humans – minor illnesses, 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 the liver was only officially declared extinct in 1979.

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

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

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

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