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Tuesday, March 02, 2021

Explained: The future of SARS-CoV-2 and what endemicity means for India

Will SARS-CoV-2 become endemic or can it be eradicated? A recent modeling study published in Science magazine suggests that in a few years, SARS-CoV-2 may be no more virulent than the common cold.

Written by Mehr Gill , Edited by Explained Desk | New Delhi |
Updated: February 19, 2021 2:12:45 pm
coronavirus, coronavirus variants, virus variant US, coronavirus new variant, coronavirus us variant, what is us coronavirus variant, sars cov 2 virus mutation, virus mutation US, sars-cov-2 virus, sars-cov-2 virus, sars-cov-2 new variantA 3D scan of SARS-CoV-2 virus particles created by design lab Nanographics is seen in this handout released in Vienna, Austria, January 20, 2021. ( via Reuters)

A recent modeling study published in Science magazine suggests that in a few years, SARS-CoV-2 may be no more virulent than the common cold, much like other benign human coronaviruses that are currently circulating in the population and do not cause severe illness.

So what does the future hold for SARS-CoV-2, is it likely to become endemic? If so, what could endemicity mean for India?

What happened to other coronaviruses?

Out of the seven coronaviruses known to infect humans, the ones that have emerged since the last two decades including SARS (fatality rate of 10 percent), MERS (fatality rate between 35-36 percent) and now SARS-CoV-2 are the ones that are a cause for worry since they are capable of causing severe illness and even death.

Out of these three, while humans are still dealing with SARS-CoV-2 and are likely to continue doing so in the coming few years, SARS (emerged in China) and MERS (emerged in Saudi Arabia) were locally contained. The last case of SARS was detected in 2003, however, MERS is still circulating. Even so, while attempts have been made to develop a vaccine for these two coronaviruses, none of them have been approved by the US Food and Drug Administration (FDA) as of now.

In the case of SARS-CoV-2, it was not easily contained and spread rapidly around the world. One reason for this is the spike protein of the virus, which makes it easier for the virus to enter human cells and infect the individual.

While other coronaviruses also have a spike protein, this protein is slightly different in the COVID-19 causing virus compared to its close relatives. One analysis published in Nature nearly a year ago suggested that the spike protein in SARS-CoV-2 has a site on it that is activated by an enzyme in host-cells called furin. Furin is found in lots of human tissues including the lungs, liver and small intestines.

Vaccine development for SARS-CoV-2 has also been faster because of “unprecedented global collaboration and stress” Giridhara Babu, Professor and head, Lifecourse Epidemiology at the Indian Institute of Public Health PHFI, said.

This leaves four other coronaviruses known to infect humans which include 229E, OC43, NL63 and HKU1. The first of these was identified in the mid-1960s. But since these four human coronaviruses were mostly associated with the common cold, a need for developing vaccines for them was not felt. These four have now become endemic, which means that they keep circulating in the human population, some of them, seasonally.

How does a virus evolve?

The goal of the virus is to reach a stage where it can cohabitate with humans because viruses need a host to survive. This means that as the SARS-CoV-2 virus evolves it is looking to become less severe. “The virus evolves to survive among human beings and when it tries to do that it tries to become less and less virulent because if it is fatal people die and it will be a dead-end for the virus as well,” Babu said.

The evolution of the virus depends on a combination of factors, which include the virus, the body of the host and the environment. “Whenever there is a change in the host immune mechanism and the environment around it, the virus is compelled to make modifications,” Babu added.

Further, SARS-CoV-2 is evolving fast because of the scale at which it has infected people around the world. High levels of circulation mean it is easier for the virus to change because it is replicating faster. “Therefore, it is very important to contain and vaccinate because this way even if the virus changes it will be slower and will be less a cause for worry,” Babu said.

So, what then, is the future of SARS-CoV-2?

Even with the vaccine rollout across the world, it will likely take a few more years for the entire population of the world to either attain natural immunity through the disease itself or to be immune by means of a vaccine. Till then, the virus will keep circulating as it will find new hosts that are vulnerable. This means that it is likely that the disease will become endemic in the years to come.

When in the coming few years, a majority of the world gets vaccinated or has acquired natural-immunity to it, then it is likely that the virus will become endemic in most parts of the world. Endemic means that the virus will keep circulating in the population freshly infecting young children and babies.

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But this will take a few years or decades. In a modelling study published in Science magazine, the authors note that while at the beginning of an outbreak, the age distribution of cases mirrors the age of the population, once the demographics of infections reach a steady-state, the model predicts that cases will occur almost entirely in babies and young children, who have a low case fatality ratio and infection fatality ratio.

It is not in the interest of the virus to cause severe disease in this particular demographic because severe disease could kill the host (young children and babies) and the virus with it.

The idea here is that once most of the world has been exposed to the virus, one of the only demographics of the population that will not be familiar with it will be children. In such a case, like some other endemic human coronaviruses, SARS-CoV-2 will cause illness in this particular demographic, but it will most likely be mild.

In their lifetime, this demographic might be reinfected with the virus but because of the immunity they attained in their childhood, they are likely to recover from the illness. This does not mean that the virus will be entirely harmless, it may still have the potential to cause death and severe illness in some people. For instance, while there is a vaccine for seasonal influenza, millions are infected each year and tens of thousands die from flu-related causes each year.

Alternatively, in the case that vaccine-induced immunity from the virus lasts forever then the virus is likely to be eradicated. Measles is one such disease for which the vaccine offers immunity for a lifetime but this is because measles is a “relatively stable organism”. In the case of SARS-CoV-2, the virus is still evolving and it is important that scientists continue to monitor how it is changing so that vaccines can be tweaked or new ones be developed.

What could endemicity mean for India?

Professor Babu explained that depending on the disease, a certain number of cases still occurring may be called endemic. For instance, in the context of India, SARS-CoV-2 may be called endemic when 100-200 cases of the disease are being detected per month from a state. “But this number is arbitrary and will depend on the population of a state, since some states are larger. Generally, this number is defined depending on the population and geographical area and the duration of time is also taken into account,” he said. This means that once the virus is circulating at low levels for a long period of time (at least a few years), the number for endemicity can be defined based on certain factors.

Further, Babu said that while there are over 280 mutations identified in the virus circulating in India, none of them are known to be more infectious and there is no evidence so far to suggest that any new strains that might have emerged in India are more virulent or fatal. “I don’t think any of these mutations have been significant to change the protein configuration of the virus. So the vaccine should be effective and we don’t have to worry about mutations in India,” he said.

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