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