Why Covid-19 herd immunity may be closer than we believed so far

Experts say Covid-19 antibodies last at least 6 months, and they are not the only weapons our body has against coronavirus.

Representational image of Covid-19 testing | Photo: Manisha Mondal | ThePrint

New Delhi: The early days of the Covid-19 pandemic in India saw the R0 value, a metric that describes how many people are likely to get infected from one patient, climb to 1.83. This led experts to suggest that as many as 60 per cent of the population will have to gain immunity — by recovery or vaccination — for herd immunity to kick in.

This was an alarming assessment as the 27,67,273 cases recorded in India so far (accounting for approximately 0.2 per cent of the population) have already led to 52,889 deaths. Even the government has warned that, without vaccination, the country can only achieve herd immunity at a high human price.

However, emerging research showing that the body can launch a robust immune response to SARS-CoV-2, the virus that causes Covid-19, even in the absence of previous exposure, along with evidence that antibodies for the disease last in the system for at least six months, points to the fact that achieving herd immunity may be easier than thought. 

Herd immunity refers to a concept where enough members of the population are immune to a disease, making it difficult for the pathogen to find new hosts to infect. It is not necessarily achieved with vaccinations alone, and can also be gained if enough people develop antibodies against the pathogen after first contracting it. 

If a significant proportion of the population becomes immune to a virus, it will run out of fresh people to infect and the chain of transmission will be disrupted.

New knowledge

In the early days of the pandemic, it wasn’t understood whether the antibodies launched by the body against the novel coronavirus were long-lasting. Reports of re-infections in South Korea triggered fears that those who had recovered from the infection could contract it again. 

But a study conducted by the South Korean government confirmed in May that this was not true. 

According to the study, people who tested positive after discharge from hospitals only had leftover viral debris in their bodies, and not live virus — a distinction RT-PCR tests failed to make. 

One set of worries addressed, another arose when a Chinese study suggested in June that antibodies against the novel coronavirus start fading within two to three months. 

However, experts say there is more to the body’s Covid-19 response than just antibodies.

Immunologist Sunil K. Noothi, who was a postdoctoral scholar at the University of Alabama, told ThePrint that antibodies are not the only part of the immune system protecting the body against infections. 

The immune response to any kind of pathogen is in incremental steps or layers. Immune system cells are primarily made up of leukocytes or white blood cells, which circulate through our body and scan for suspicious objects.

As soon as we are infected, the first kind of immune system, the “innate” kind, kicks in. It is non-specific, so it protects against all pathogens the same way.

The innate immune response triggers the adaptive immune response, which is more specific to the kind of pathogen infecting our bodies. The adaptive immune response consists of T cells and B cells. 

B cells are the ones that produce antibodies or immunoglobulins to fight off the infection and help recovery. 

T cells and B cells also produce memory cells that are capable of storing information about antigens. These cells, called memory T cells and memory B cells, take a few days to trigger after the first instance of infection, but invoke a swift and efficient response the next time the pathogen is encountered. 

Fresh research on memory T cells with respect to Covid-19 shows that the goal of herd immunity may be closer than believed so far. 

A study published this month in the journal Science showed memory T cells that recognise common cold coronaviruses also identify matching sites on SARS-CoV-2.

The researchers found that certain protein structures in the SARS-CoV-2 virus are similar to those in common cold coronaviruses. In people who have been previously exposed to common cold coronaviruses, the study said, the T cells can “see” the similarities and launch an attack against the novel coronavirus. The research may explain why some people have milder Covid-19 cases than others.

The study built on previous research by the same group, which was published in the journal on 25 June and showed that 40 to 60 per cent of people never exposed to SARS-CoV-2 had T cells that recognised the virus. 

This finding was reported in people from the Netherlands, Germany, the United Kingdom and Singapore, according to the study authors. 

Another study, conducted by Chinese and German researchers and also released this month, found that certain types of antibodies specific to the coronavirus may last as long as six months. The study, which is yet to be peer-reviewed, involved patients from Wuhan, who were among the first to be affected by the pandemic. 

Even in mild cases of Covid-19, the body launches robust memory T cell responses, even in the absence of detectable virus-specific antibody responses, researchers wrote in the journal Cell on 11 August.  

The findings are encouraging not only because they suggest that natural immunity among those exposed to the coronavirus may be long-lasting, but also because the vaccines — that are designed to trigger such responses in the body — will have a long-lasting effect. 

Already, several sero-surveillance studies in Indian cities have shown that antibodies are present in a significant part of the population. In Delhi, one in four people had antibodies against the SARS-Cov-2, according to the results of a study conducted late June-early July, while 50 per cent of those tested in Pune between July and August had antibodies.

“If already some fraction of the population has recovered from this disease and immunity to reinfection is sufficiently long-lasting, then we need not vaccinate people who can show that they have already had the disease,” Gautam Menon, a professor of physics and biology at Ashoka University, told ThePrint.

“Right now, we are slowly beginning to understand the body’s immune response to Covid-19, but much still needs to be understood, including the question of how ‘long-lasting’ the immune response will be,” he added.