What proportion of India’s population is infected with Sars-CoV-2, the virus that causes the coronavirus disease?

What proportion of tests return a positive result in Delhi?

Why are so many passengers landing in Coimbatore testing positive?

These are important questions — and linked ones.

In Delhi, and in Tamil Nadu and Maharashtra, the proportion of tests returning a positive result remains high. In the seven days to May 21, the average daily positivity (as it is called) rate was 9.4% in Delhi. It was 17.9% in Maharashtra, and it was 5.7% in Tamil Nadu.

And in the seven days till Thursday, May 28, the corresponding numbers were 12.2%, 18%, and 6.1% respectively.

All three states tested more people on average in the seven days ended May 28 compared to the previous seven-day period. Delhi did 5,370 tests a day as compared to 4,950; Maharashtra, 14,244 against 11,366; and Tamil Nadu 14,189 as against 10,838.

These positivity rates would be alarming if not for the fact that they pretty much reflect the trend across most countries, including the US, which has seen the most number of cases and deaths. Positivity rates keep increasing with the number of tests, plateau at a certain level as the tests increase further, and finally start falling as the number of tests increases even more. Given the trend of the disease in other countries, in India, the states of Delhi, Maharashtra, and Tamil Nadu will likely be the first to see the positivity rates declining — but they aren’t there yet. The two-week trend line for Maharashtra and Tamil Nadu would appear to be flattening, but this needs to happen for a longer period before it can be safely assumed that the positivity rates have peaked.

The high positivity rates can also be explained by India’s testing protocol, which still skews the sample towards those likely to be infected; it is difficult to get a test done unless one has severe symptoms, has been exposed to an infected person, or takes a flight to certain states (or certain cities within those states). More on the last anon.

Is there a relationship between the positivity rate and the proportion of a country’s population that has been exposed to the virus (and which is, as a result, immune)?

There should be, but this only applies when a population has been tested widely, not at the current levels of testing in India.

Now to the issue of passengers landing in Coimbatore — flights started earlier this week — testing positive, as reported by both local officials and airlines. That’s probably because everyone landing in Coimbatore is tested — just like everyone landing in Jammu is. Not all states require arriving domestic air passengers to be tested; nor do all cities within a state. Some make do with screening and quarantining. In the case of the passengers who tested positive in Coimbatore, most are reported to be asymptomatic. The interesting thing about them is, because their Aarogya Setu apps showed them to be “safe”, and because they didn’t have symptoms, they would have never been tested if they had not taken the flight to a city that mandated tests for all arrivals. This is another piece of evidence that points to the number of people infected by the coronavirus in India actually being far higher than the tested numbers. This is actually a good thing because it means the case fatality rate in India, already at 2.8%, much lower than the global average of 6.1%, is actually even lower and because it means we are slowly inching our way up towards achieving herd immunity. To be sure, we are a long way from that, but till a vaccine is discovered, that remains our best bet.

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