The government has maintained that due to the lockdown imposed on March 24, the doubling time of coronavirus cases has increased. In the week before the lockdown, the doubling time across India was 3.4 days. By the week ending April 27, it had improved to 10.77 days, an analysis of daily data shows.
Why this matters
Doubling time is a concept borrowed from the world of finance, where it is routinely used to compute the time taken for an investment to double. In a pandemic such as this, a single-day doubling time would mean that, if there were 100 cases to start with on Day 0, there would be 200 cases on Day 1, 400 on Day 2, 800 on Day 3 and so on. However, if the doubling rate was three days, 100 cases on Day 0 would increase to 200 cases on Day 3. Doubling rate is not known in advance. It has to be calculated, based on the emergence of new cases, and as such, it changes every day.
Sometimes, a large surge or dip in the number of cases can give an erroneous impression of the spread of the disease. Therefore, doubling rate is often calculated using the five-day, seven-day or ten-day moving average of cases to capture trends over longer periods. The government had highlighted seven-day moving averages. Here, we have calculated the doubling rate for five-day moving averages.
Using five-day averages, and data till April 27, we show that the doubling rate at the national level increased from 3.21 days on March 23, just ahead of the lockdown, to 7.82 days in the five-day period ending April 17 (when the first phase of lockdown ended), to 10.77 days in the period ending April 27.
The national figures mask the wide variations in the states. Among the 12 states with the highest number of cases on April 27, the disease was growing at the fastest rate in West Bengal, doubling at just 7.13 days, followed by Maharashtra and Gujarat, at 7.9 and 8.3 days respectively. The slowest doubling was in Telangana, at 58 days (see chart).
West Bengal saw an increase of nearly 93% between April 20 and April 27, which corresponds to a cumulative daily growth rate (CDGR) of more than 10%. Cases in Maharashtra and Gujarat grew by 85% and 93%, respectively, in the same period.
In Kerala, which had registered the first three cases of COVID-19 in India in end-January, the doubling time has improved from about 3 days before the lockdown to 37.17 days now. The state has earned global praise for its containment strategy. While Telangana’s doubling rate is even slower, Kerala’s is significant given that it has a lower number of cases.
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Doubling times have increased in all the top 12 states, underlining the effect of the lockdown so far. Except in West Bengal, the CDGR for the last five days of all other states in the top 12 have dropped below 10%. This has happened for the first time for Maharashtra, whose count grew by an average 8.74% per day as compared to over 17% in the first week of April.
The current Indian Council of Medical Research guidelines allow for testing only people with symptoms, those with travel history, or contacts of infected people. However, 80% of patients are asymptomatic, giving rise to the possibility of many cases going undetected.
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“One would expect that as more people are tested, more cases would come to light each day and the doubling interval will also decrease. Since that is not the case here, it suggests that there is a slowdown in the propagation of the disease. These data need to be read in conjunction with the hospitalisation numbers for serious acute pulmonary infection and the population surveillance data on influenza like illness, to give a three dimensional view,” said Dr K Srinath Reddy, president of the Public Health Foundation of India.