I had said in an earlier blog article that the SARS Cov-2 virus responsible for the current pandemic is likely to evolve rapidly towards reduced virulence. The reason why I expect this is that on the one hand, almost all countries are implementing strict quarantine measures for all detected positive cases. But on the other hand, we cannot afford to do mass testing, leading to many undetected asymptomatic cases roaming around and spreading the virus. The virus reaches huge populations and also has a high mutation rate, so all possible variants will keep on arising. A virulent strain is most likely to cause severe infection which will invite testing and ultimately quarantine. A mild variant, on the other hand is more likely to lead to asymptomatic or mild symptomatic infections which are more likely to escape screening followed by quarantine and therefore keep on spreading. In several generations of the virus, which is a short time for us, natural selection will favour the mild variants.

While all research on the virus is engaged in developing vaccine, studying pathogenic mechanisms or suggesting treatments, nobody seems to talk about evolution of the virus. This is for two reasons. One is that people in medicine are never trained to think of evolution. The other is that virulence is difficult to quantify. It is easier to sequence the virus, study its proteins, look for antibodies in the host etc. Researchers typically do what is easy to do rather than what is scientifically more relevant. Since one cannot measure a change in virulence easily, nobody will even talk about any hypothesis related to it. This is what I call “evidence bias” in science. If it is difficult to find evidence to either falsify or support a hypothesis, people will avoid talking about the hypothesis because it cannot make a paper. Whether the hypothesis is relevant to public health is not an important issue, whether you can publish a paper is.

But in the epidemiological trend at the global as well as the Indian scene, there are definite signs of reduction in virulence. Although the infection is growing, the death rate is consistently reducing with time. Look at the patterns. From mid-April, although the total number of new cases per day has been increasing, the total reported deaths per day is decreasing.

The same is happening in India too. In fact, the case fatality rate in India was always low and it is decreasing further, although the absolute number of deaths per day has not started decreasing yet.  

Data from: https://ourworldindata.org/mortality-risk-covid

I plotted the time trend in the ratio of daily reported positive cases to daily reported deaths starting from the day the new death count exceeded 50. Although there are expected daily chance fluctuations, there is a clear decreasing trend.

Data from https://www.covid19india.org

Now if we make a simplistic assumption that the linear trend continues, then we can come out with a prediction that in India in about 35 days, Covid 19 will remain only as dangerous as any seasonal flu. The assumption of linearity is of course an oversimplification, the slope may not remain the same throughout. The second caveat is that case fatality rate cannot be exactly equated to mortality rate. In a growing epidemic, case fatality rate is an underestimate or mortality. But that need not affect the trend. The estimate of 35 days might be too optimistic. It may take somewhat longer. But the direction is assuring. Anecdotally I heard from some of my clinician friends that the proportion of patients needing critical care is already low.

The vaccine trial and mass production is going to take many months and may not immediately become available or affordable to the masses. For the huge population of India, acquiring herd immunity is a huge task and will not happen for a year or two. But much before either of the two becomes useful for public health, evolution would have taken care of the deadliness of the virus. We need to continue quarantine and good medical care of symptomatic cases, but not be fussy about the asymptomatic ones. Because they are going to be the saviours. Let us wait for a couple of months to see whether the prophecy turns out to be true qualitatively or quantitatively. If it does, it has a long term lesson for medicine. Virulence management strategies should become an integral part of public health planning. This is not the last time that a new virus arises. This will keep on happening. Understanding of evolutionary dynamics is certainly required to manage public health.