This column has often mentioned the similarities between the spread of the coronavirus disease in the US and India, both geographically large and populous countries. The US is larger than India; and India is more populous than the US. But there is a big difference — one that points to a mystery in the Indian numbers.

Across US states, there is a significant similarity in terms of the number of cases per 100,000 people. The vast difference between the absolute number of cases in the US and India makes comparisons difficult, but it is possible to look for similarities within the countries.

The difference (per 100,000 people; and from here on, wherever this column refers to cases it is per 100,000 of the population) between the state ranked first (Louisiana) and the state ranked 20th is a third of the number of cases in the first. And many of the states in the top 10 (excluding Louisiana) are clustered within 20% of the cases in the state with the second highest number of cases, Florida. The difference between Louisiana and the state ranked 40th is 43% of the number of cases in the former.

In India, Delhi is the city-state with the most number of cases per 100,000 people (875; this analysis has excluded smaller states and Union Territories). It is followed by Andhra Pradesh (813), Maharashtra (640), Tamil Nadu (557), Karnataka (510), and Telangana (330). The difference per 100,000 cases between Delhi and Telangana is around 62% of the number of cases in the former. The numbers drop off sharply after that.

Indeed, some of India’s most populous states figure towards the bottom of the list — West Bengal, with 165 cases per 100,000; Bihar with 113; and Uttar Pradesh (100). At the national level, India has 271 cases per 100,000 of the population. All numbers are from HT’s dashboard as of Sunday night.

Testing explains some of this. For instance, India had tested 31,741 people per million of its population till Sunday night. But Uttar Pradesh, Bihar and West Bengal had tested only 24,404, 25,913, and 19,043 people per million respectively. In a country that is a laggard when it comes to testing, these states are in the lower quartile in terms of tests per million. Among the 10 countries ranked by the most cases in the world, only Mexico tests fewer per million than India. In the 20 countries ranked by the most cases, only four (Mexico, Argentina, Bangladesh and Pakistan) test fewer people per million.

But testing doesn’t explain it all. On Sunday, for instance, Uttar Pradesh carried out the most tests but its positivity rate was among the lowest — 4.42%. Bihar’s was lower still (1.92%) and it carried out the most tests after Uttar Pradesh. Even West Bengal’s, on a much lower base of tests, was around 7%. In contrast, Tamil Nadu, which carried out the third highest number of tests after Uttar Pradesh and Bihar on Sunday, saw a positivity rate of 7.8%. And Maharashtra, which carried out the fourth highest number of tests, saw a positivity rate of 22%. Delhi saw a positivity rate of 9.9%.

Among these states, Delhi has seen its positivity rate plunge (from highs in the early 30s to lows in the 5-7% range) before it started climbing again. Tamil Nadu was an early convert to the merits of aggressive testing, but while the positivity rate has come off its peaks, the state is clearly on a long plateau. As for the rest, their positivity rates aren’t, in most part, seeing the kind of trend that should be seen with more testing — a rise, a long plateau, and a fall.

Some of the disparity between Indian states can be explained by the first wave of infections largely being restricted to the large urban centres. But this newspaper has written about how that is changing with 55.3% of the cases in the third million (India ended Monday with 3.68 million cases) coming from rural districts. The mystery, then, is that large states such as Uttar Pradesh, Bihar, and West Bengal are not seeing the kind of numbers they should.