One reason for the relatively slow increase in the number of novel coronavirus patients in India, as of now, could be the fact that every infected person has been passing on the virus only to another 1.7 people on an average. This is remarkably lower than what has been observed in the worst-affected countries, a study by scientists at the Institute of Mathematical Sciences in Chennai shows.
In Wuhan in China, where the novel coronavirus originated, every person transmitted the virus to 2.14 persons on an average. In Iran, this number, referred to by scientists as R0 (R-naught or reproduction number) was 2.73, while in Italy, it has been 2.34, according to the study by Soumya Easwaran and Sitabhra Sinha.
If the same rate persists, India is likely to have around 200 positive cases in the next five days, Sinha told The Indian Express. “However, at the upper end of this projection, the number can go up to as many as 500,” he said.
R-naught is a frequently used mathematical metric to estimate how contagious an infectious disease can be. It can help in making projections for the number of people likely to be affected by such a disease and is often used to decide on the kind of policy interventions required to halt the epidemic.
A less than one value for R-naught would mean that disease would not take the form of an epidemic. Any value more than one indicates an exponential rise in the number of patients. Significantly, this number is applied only in cases of outbreak of new diseases, in which the entire population is equally susceptible to being infected.
There could be several reasons for a low R-naught. The climatic conditions might not be conducive for a long enough survival of the virus, or patients might have been isolated promptly thereby restricting their interaction with others, or lockdown measures might have been implemented efficiently, reducing the possibility of general public coming in contact with infected people.
But Sinha said that in the absence of any supporting data as of now, all these reasons could only be speculative. “Right now, we do not know the exact reason for low R-naught. May be a week from now on, when the impacts of government measures start getting reflected in the data, we would have some idea,” he said.
R-naught is also a dynamic metric, and is sensitive to the kind of interventions made to contain the spread. “The steps that the government has been taking in the last couple of weeks may force a change in this number in the coming days. The impact of these steps is not yet evident. As we keep adding more data points to our analysis, this number would likely change,” Sinha said.
A higher R-naught is not the only factor that could lead to a rapid growth in the number of positive cases. A wider base of infected people with the same R-naught would also have the same effect.
So, if more people coming from abroad had tested positive for the virus — considering them to be the first point of transmission in the community — the growth in the numbers would have been much higher even at the same current rate of R-naught of 1.7.
“So tomorrow, if we discover that there are some cases of community transmission, we will see the numbers rise rapidly even if they were still passing on the virus to only 1.7 people on an average,” Sinha said.
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Data from other countries show that there was a very rapid rise in number of positive cases once the infected number of people had reached 100.
Countries like Iran and Italy had more than 500 positive cases in less than three days after reaching the figure of 100 while others like Spain, France, Germany, the United States, Switzerland, Sweden and few others took a couple of more days to traverse that distance.
India, on the other hand, touched the 100-figure mark on March 15, and in the next three days the number of infected people had risen to 152.
This relatively slow rise has led many to raise concerns about the possible under-reporting of cases in India, and the health apparatus missing out on infected people in the community because of the rather rigorous criteria for testing suspected cases.
Sinha, however, said at least the low R-naught number for India did not seem very unusual. “If we plot the R-naught numbers of other countries against the latitudes in which they are situated, India more or less fits into the curve,” he said, suggesting that how far the country was located from the equator might have some correlation with how fast the virus was spreading. But he said there was no indication, yet, that this was linked to temperature. “We do not have any evidence yet that temperature might be playing a part. As of now, we are unable to explain this correlation,” he said.
There are other things, Sinha said, that change with the change in latitude, like the duration of sunlight. “We are still exploring the likely impact of these,” he said.
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