The transmission of coronavirus is exponential, and not linear. Hence, merely looking at the number of cases and deaths per day is most likely to reveal an incomplete picture of the extent of the transmission.
To get an in-depth idea of the extent of the virus spread, epidemiologists have proposed a series of measurement parameters that can help us understand this disease better.
Pronounced “R-naught,” it represents the number of new infections estimated to stem from a single case. In other words, if R0 is 2.5, then one person with the disease is expected to infect, on average, 2.5 others. An R0 below 1 suggests that the number of cases is shrinking, possibly allowing societies to open back up. An R0 above 1 indicates that the number of cases is growing, perhaps necessitating renewed lockdowns or other measures. But R0 is messier than it might look. It is built on hard science, forensic investigation, complex mathematical models — and often a good deal of guesswork. It can vary radically from place to place and day to day, pushed up or down by local conditions and human behavior.
Number of tests per day
If no tests are conducted there is no data and there is no way to find out the extent of spread of the pandemic. Testing is a window onto the pandemic and how it is spreading.
The number of confirmed cases depends on a country’s testing strategy.
A major reason for high Covid-19 cases in India is the ramped-up testing facilities. According to the health ministry, India had conducted 11.31 million tests on July 10.
Number of cases per million population
Due to the difference in populations of various countries, gauging the extent of pandemic sheerly based on the total number of confirmed cases will not provide an apt picture.
India, with 672,644 confirmed cases, had 487 cases per million population as on July 3. In comparison, Qatar, with 99,183 confirmed cases, had 35,324 cases per million population.
Test positivity rate
The bigger the outbreak, the more extensive the tracking should be to properly monitor the spread of the virus.
Test positivity rate (TPR) is a measure if a country is testing sufficiently. Test positivity rate is the share of tests resulting positive results. It is also defined as the number of confirmed cases per 100 tests.
The consolidated all-India positivity rate of all states together, as of July 4, has risen to a worrisome 8.63% during this period as compared to 7.77% in the previous fortnight and 7.4% the fortnight before.
Mexico and Bolivia — with TPR of 62.10% and 52%, respectively — reported the highest TPR on July 4. Australia and South Korea — with TPR of 0.10% and 0.40% — reported the lowest TPR.
Also referred to as the fatality ratio, is a ratio of the number of people who have died against those who have the virus.
Since it is not possible to test the entire population and a large number of cases are asymptomatic, the number of reported positive cases is always less than the number of actual cases. Therefore, the number of cases is several multiples above the number of reported cases. The number of deaths are also under-reported as many patients are not hospitalised and not tested.
Hence, the mortality ratio, which is based on the number of reported cases, is often overestimated.
There is another common glitch often encountered while calculating death rate. A covid-19 death follows after two or three weeks of occupancy. The numerator should be taken as the number of deaths and denominators as the number of cases two weeks ago, not today. That actually reflects your mortality rate/case fatality rate.
The virus affects individuals and populations in a very different way. The fatality rate varies with region, age and in some cases, even race.
Initially, the World Health Organisation had estimated the mortality rate from Covid-19 to be 3.4% as of March 3. In India, the current mortality rate is around 2.6%.
Here is a look at mortality rate of some other countries as per John Hopkins University —
It is the time taken by the number of confirmed cases to double. It is a statistical principle that is also applied to population growth, inflation, resource extraction, consumption of goods, compound interest, the volume of malignant tumours, etc.
In mathematical terms, DT is used to measure components where the growth is ‘exponential’ and not ‘linear’. It gives an indication of the pace at which the virus is transmitted.
To calculate doubling time, 5/7-day moving averages are considered.
The doubling rate in the country on March 23, a day before the lockdown, was 3.21 days.It increased to 7.82 days on April 17, when the first phase of lockdown ended. On April 27, the doubling rate was 10.77.
5/7-day moving average
Since the daily growth in the number of cases is not consistent, hence a concept of moving average is used to undo the effect of rapid fluctuations from skewing the data.
A 5-day moving average is calculated for each day by averaging the values of that day, the two days before, and the two next days.
Similarly, a 7-day moving average is calculated for each day by averaging that day, the three days before, and the three next day.