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

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Last month, researchers in one of India’s largest cities made a startling discovery. Of nearly 7,000 blood samples taken from residents of Mumbai’s slums, 57% tested positive for anti-coronavirus antibodies.While some were alarmed by the results of the study conducted by the Bombay authorities and the Tata Institute of Fundamental Research, others were optimistic. The slums of Mumbai, where social distancing is nearly impossible, may now have some of the highest levels of immunity in the world – only 23.5% of samples taken by India’s National Center for Disease Control have been tested positive for antibodies in Delhi and 14% tested positive in New York, in a study funded by the New York State Department of Health.

Scientists believe that healing from the coronavirus is likely to leave a person with some immunity, but it is not known how strong it is or how long it lasts. Herd immunity is the idea that a disease will stop spreading once a sufficient population is immunized – and it’s appealing because, in theory, it could offer some protection to those who haven’t been sick. .If more than half of Mumbai’s slum dwellers had contracted coronavirus, could they come close to herd immunity – without a vaccine?

An expert thought so.

“The slums of Mumbai may have achieved collective immunity,” said Jayaprakash Muliyil, chairman of the scientific advisory board of the National Institute of Epidemiology of India, according to a Bloomberg report. “If the people of Mumbai want a safe place to avoid infection, they probably should go. ”

But others have been 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 created false positives.

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

If Mumbai’s slums are on the verge of collective immunity, it comes at a cost. Of the more than 2 million cases of coronavirus in India, around 5% of these infections have been reported in Mumbai, the commercial capital of the country. 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 Indian health officials say the country is not targeting herd immunity. “Herd immunity can be achieved through vaccination – but that’s in the future,” health official Rajesh Bhushan told reporters last month.

What is collective immunity?

Collective 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 that fewer people are infected with the disease – and over time even people who are not immune end up being protected because they are less likely to be exposed to the virus.

The level of immunity needed in a population depends on the disease. Scientists do not yet know what proportion of a population must be immunized to achieve herd immunity against the new coronavirus.

Currently, scientists estimate that each person infected with a coronavirus infects between 2 and 2.5 people, according to the World Health Organization. But that number may be affected by other measures – a lockdown, for example, could lower the number of people infected by each person with coronavirus.

It is difficult to know what the collective immunity threshold is. Initially, according to some estimates, the figure was between 70 and 90%. Adam Kleczkowski, professor of mathematics and statistics at the University of Strathclyde, Glasgow, calculates it between 50% and 75%, based on what scientists know about the transmission of the virus.

With measles, for example, researchers in the early 20th century noticed that infections decreased when 68% of children were immunized. But Dowdy points out that measles outbreaks in areas where people choose not to get the measles vaccine show how immunity can be lost.

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

And this is where things get controversial.

The UK initially said it would allow the spread of the coronavirus in the country to expand to herd immunity. This approach has been criticized, and critics have warned that it will come at a high price: overburdened health systems and unnecessarily high death tolls. The UK – which has backed down on its herd immunity strategy – now has more confirmed coronavirus deaths than most countries. Of the 20 countries most affected by the coronavirus, the UK has the highest death rate, with 69.63 deaths per 100,000 people compared to 52.28 in the US, according to data from Johns Hopkins University .

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

As Dowdy puts it, “We could build up population immunity to coronavirus very quickly just by exposing every 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.

An article published 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 symptoms of Covid-19 appear .

Just because antibody levels decrease over time doesn’t necessarily mean immunity doesn’t last, Dowdy says. In other viruses, antibody levels also decline over time, but the immune response is still able to speed up if a person is re-exposed to the virus.

According to Dowdy, our immunity to other coronaviruses tends to last for a few years, rather than being lifelong. “If these are a guide then this is what we might expect from this new coronavirus,” he said. “But it’s hard to say. We have no 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 against infection, and B cells, which produce antibodies.

“It’s a well-coordinated orchestra,” said Anthony Tanoto, senior researcher at Duke-NUS Medical School in Singapore, who has worked on T cell research in patients with Covid-19.

In an article published in the scientific journal Nature in July, Tanoto and his co-authors found evidence of T cells from people who had recovered from SARS – a coronavirus that spread in 2003 – indicating that the cells can 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 could respond to Covid-19.

But for now, Tanoto says we don’t know to what extent, if at all, these T cells help fight Covid.

In reality, once there is herd immunity – whether naturally or through vaccines – it probably won’t be the impenetrable shield some might imagine.

Tanoto co-author Nina Le Bert, senior researcher at Duke-NUS Medical School in Singapore, points out that it’s rare to have full immunity to infection. Instead, immunity often means that a person’s body is able to respond quickly enough to the virus that it doesn’t gain a foothold – and doesn’t grow enough to infect other people.

“It will be enough, but it doesn’t mean you won’t get infected,” Le Bert said.

What does this mean for collective immunity? Even if some areas get 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 of the University of Strathclyde.

“Even if we achieve collective immunity at some point, we could lose it again,” he said. “I don’t think it’s a quick fix. ”

Dowdy says herd immunity “isn’t a magic number” for solving the coronavirus.

“That doesn’t mean the disease will go away. This means that if you gave it 1000 years old it would disappear. ”

And he notes that the duration of herd immunity – whether in a slum or across a country – depends in part on the extent of movement within and outside that population. If people without immunity enter the area, it reduces 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.

In a slum in Mumbai, for example, people come and go, which could impact the duration of collective immunity – if there is one. Utture Shankar, chairman of the Maharashtra Medical Council, said people outside the slums depend on those who live in slums for services such as gardening, cleaning and driving, and will therefore be at risk beyond their residential community.

In 10 years, Kleczkowski expects some parts of the world to still have coronavirus. Even though there is herd immunity in some places, there may still be a problem with the reappearance of the virus, especially if people refuse to be vaccinated.

He points out that although humans have had vaccines for 200 years, we have only succeeded in eradicating one disease affecting humans – smallpox, thanks to a global immunization program led by the World Health Organization. . But it took a long time. A vaccine was discovered at the end of the 18th century, but smallpox was not officially declared eradicated until 1979.

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

“I think it’s a disease that’s going to last for a while,” he said. “But I don’t think it’s going to be a disease that causes the same level of death and suffering as it does today. “

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