Scientists believe recovery from coronavirus is likely to leave a person with some immunity, but it’s not clear how strong it is or how long it takes. Herd immunity is the idea that once enough of a population becomes immune, once enough of a population becomes immune, a disease will stop spreading – and it’s appealing because in theory it could offer some protection to those who weren’t sick.
If more than half of the people in Mumbai’s slums had coronavirus, could they approach herd immunity – without a vaccine?
An expert believed it.
“Mumbai’s slums may have reached herd immunity,” said Jayaprakash Muliyil, chairman of the scientific advisory board of India̵
But others were 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 produced false positive results.
And Om Shrivastav, an infectious disease expert in Mumbai, warned that less than eight months after the virus entered society, it was too early to make “decisive, conclusive statements”.
The risk of a high death toll is exactly why the Indian health authorities say this Land does not seek 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: 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 fewer people are infected with the disease – and over time, people who are not immune will also be protected as they are less likely to be exposed to the disease Virus.
The level of immunity required in a population depends on it on the disease. Scientists do not yet know what proportion of a population must be immune to achieve herd immunity against the novel coronavirus.
Building immunity levels in a population can be done in two ways. People can get immune from vaccination or catch them the virus and develop natural immunity by recovering from it.
And this is where it becomes controversial.
Most other countries – including India – have taken a different approach. “Herd immunity in a country the size of India’s population cannot be a strategic decision, it can only be an outcome, and at a very high cost,” said Health Official Bhushan.
As Dowdy puts it, “We could develop population immunity to the coronavirus very quickly by simply 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 around immunity to Covid-19 is evolving.
The fact that antibody levels decrease over time doesn’t necessarily mean immunity isn’t lasting, Dowdy says. For other viruses, antibody levels also decrease over time, but the immune response can still rise again if a person is exposed to the virus again.
According to Dowdy, our immunity to other coronaviruses lasts for a few years rather than life. “If this is a guide, 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 right now, Tanoto says we don’t know how much – if anything – these T cells help fight off Covid.
In reality, herd immunity – whether natural or through vaccines – is unlikely to be the impenetrable shield some people might imagine.
Tanoto’s 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 to infection. Instead, immunity often means that a person’s body can react to the virus quickly enough that it doesn’t gain a foothold – and doesn’t develop enough to infect other people.
“That will be good enough, but that doesn’t mean you won’t get infected,” said Le Bert.
What does this mean for herd immunity?
Even if certain areas achieve herd immunity, it cannot last.
The virus could mutate, 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 doesn’t last long, according to Kleczkowski of Strathclyde University.
“Even if we get herd immunity at some point, we could lose it again,” he said. “I don’t think it’s a silver bullet.”
Dowdy says herd immunity is “not a magic number” for solving the coronavirus.
“It doesn’t mean the disease will go away. It means that if you gave it for 1000 years, it would go away.”
And he notes that the duration of herd immunity – whether in a slum or across a country – depends in part on the amount of movement in and out of that population. If people come to the area without immunity, it will lower the general immunity level of the population. If enough people come in, it could mean there are enough people without immunity for the virus to spread again.
For example, in a slum in Mumbai, people are likely to come and go, which can affect how long herd immunity, if any, lasts. Utture Shankar, president of the Maharashtra Medical Council, said people outside slum areas are dependent on people who live in slums for services like gardening, cleaning and driving, and therefore will be exposed outside of their community.
When it comes to coronavirus, vaccines are key to herd immunity – and to fighting the virus. Says Dowdy.
“I think this is a disease that will be with us for a while,” he said. “But I don’t think it will be a disease that causes the same level of death and suffering as it does now.”
CNN’s Esha Mitra from New Delhi contributed to this story.