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.”;
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 “.
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.
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.
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.
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.”
Can we build natural immunity?
The science of immunity to Covid-19 is still developing.
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.”
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.
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.