The science around immunity to Covid-19 is still developing.
A paper released 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 Covid-19 symptoms emerge.
The fact that antibody levels decline over time doesn’t necessarily mean that immunity doesn’t last, Dowdy says. In other viruses, antibody levels decline over time, too, but the immune response is still able to ramp up again if a person is re-exposed to the virus.
According to Dowdy, our immunity to other coronaviruses tends to last a few years, rather than being life-long. “If those are a guide, then that’s what we might expect from this new coronavirus,” he said. “But it’s hard to say. We don’t have any data on this particular virus.”
Antibodies are only 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 the antibodies.
“It is a well co-ordinated orchestra,” said Anthony Tanoto, a senior research fellow at Duke-NUS Medical School in Singapore, who worked on research into T cells in Covid-19 patients
In a paper published in science journal Nature in July, Tanoto and his co-authors found evidence of T cells from people who had recovered from SARS –
– a coronavirus which spread in 2003 — indicating that the cells can last for at least
The common cold is a coronavirus and scientists believe that almost everyone
has been infected with a coronavirus in their lifetime. That could mean that many people have T cells that might respond to Covid-19.
But for now, Tanoto says we don’t know how much — if at all — these T cells are helping fight off Covid.
In reality, once there is herd immunity — whether naturally or through vaccines — it probably won’t 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’s rare to have complete immunity from infection. Instead, immunity often means that a person’s body is able to respond fast enough to the virus so 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 don’t get infected,” Le Bert said.
What does this mean for herd immunity?
Even if certain areas do achieve 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, from the University of Strathclyde.
“Even if we reach herd immunity at some point in time, we might lose it again,” he said. “I don’t think it’s a silver bullet.”
Dowdy says that herd immunity “isn’t a magic number” to solve coronavirus.
“It doesn’t mean that the disease is going to go away. It means that if you gave it 1,000 years, it would go away.”
And he notes that how long herd immunity lasts — whether it’s in a slum or a whole country — partly depends on how much movement there is in and out of that population. If people without immunity come into the area, that lowers the population’s overall level of immunity. If enough people come in, that could mean that there are enough people without immunity for the virus to spread again.
In a Mumbai slum, for instance, people are likely to be coming and going, which could impact how long herd immunity — if there is any — lasts. Utture Shankar, the president of the Maharashtra Medical Council, said people outside slum areas were dependent on those living in slums for services such as gardening, cleaning and driving, so will be exposed beyond their residential community.
In 10 years’ time, Kleczkowski expects that some places in the world will still have coronavirus. Even if there is herd immunity in some places, there may still be an issue with the virus resurfacing, especially if people refuse to get vaccinated
He points out that, although humans have had vaccines for 200 years, we have only successfully eradicated one disease affecting humans — small pox, 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 smallpox was only officially declared eradicated in 1979.
When it comes to coronavirus, vaccines are the key to herd immunity — and controlling the virus, Dowdy says.
“I think this is a disease that’s going to be with us for a while,” he said. “But I don’t think it’s going to be a disease that causes the same level of deaths and suffering as it is right now.”