The R-value is a tool epidemiologists and policy makers use to check if cases are growing faster or shrinking quickly. The reproduction factor in India is 1.01. “This means one person is infecting more than one person,” explained Vellore-based senior virologist Dr T Jacob John. In March, as cases went up rapidly the R value was around 1.4 but by May, when overall cases began to fall, it dropped to around 0.7. While rising R0 is a cause for worry, epidemiologists say they don’t red zone a state or district merely because the R value is going up.
“A combination of epidemiological factors including infection growth rate, increasing number of deaths and hospital occupancy rate explain the risk,” said National Institute of Epidemiology director Dr Manoj Murhekar.
At least 10 states have an R-value higher than the national average of 1.01, and Delhi and Maharashtra (both at 1.01) have inched closer to match the national average. Among states, Madhya Pradesh (1.31) has the highest R value in the country followed by Himachal Pradesh (1.30) and Nagaland (1.09). Across India, by August 5, of the eight states that reported over a thousand cases, the R number was higher than one in five states. Kerala, which is reporting more than 20,000 cases a day, has a RO of 1.06. The number is above 1 for Karnataka, Andhra Pradesh and Maharashtra as well.
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Yet, the risk factor is not proportional to the R values in each of these states. For instance, Madhya Pradesh, which has the highest R values, is also reporting fewer than 30 cases a day. “The R value is high because of the erratic daily numbers, but that does not indicate risk because the percentage of people who test positive over the total number of people tested (test positivity rate) is still low,” Murhekar said.
However, some states have increased growth rate and are reporting a high number of cases. For instance, in Tamil Nadu, on June 26, the state saw the steepest fall in the average number of fresh cases. The weekly average of cases was 7.8% less that day compared to the previous week.
On August 1, the weekly average was 1,957 compared to 1,844 seven days ago on July 26. The R value for the state, which was hovering between 0.7 and 0.6 in the first week of June, went above 1 in the last week of July. “But numbers are going up and down. Until we see a steady increase, we won’t be able to declare it as the third wave,” Dr John said.
The public health authorities, however, must monitor infection rate in every district and ensure appropriate response including interruption of transmission, said Bangalore-based epidemiologist Dr Giridhara R Babu, professor of epidemiology of the Public Health Foundation of India (PHFI).
The analysis of R0 for Covid cases across the states points to a few broad patterns. The data shows that the second wave, which was still strong in the country’s northeast, is in a waning phase. Among the northeast states only Nagaland has a greater than one value of this indicator. Among states with over a thousand daily cases the value is less than one for Mizoram, Assam and Odisha. But even in these states, the chances of infection and deaths remain high. And with more states unlocking restrictions, the cases are likely to go up.
Among the six states that reported between 100 to 1,000 cases on August 3 the R0 value is higher for Himachal Pradesh and Nagaland and it is exactly 1 for Jammu & Kashmir, Chhattisgarh and West Bengal. Public health experts say they will have to wait for a few more days before they can declare the spurt in cases of a new wave or dismiss it as a blip on the graph.