Data

Data | Who gets tested for coronavirus determines infection rates better than how many are

A health worker testing using the antibody-based rapid test kits at a centre in Chennai on April 19, 2020.

A health worker testing using the antibody-based rapid test kits at a centre in Chennai on April 19, 2020.   | Photo Credit: B. Jothi Ramalingam

Generally, the number of positive COVID-19 cases increases when the testing rate increases. However, this is not always the case, as data from Gujarat show.

Who gets tested determines the number of positive cases more than how many get tested. In Tamil Nadu, the number of positive cases went up when more people from the Nizamuddin cluster were tested and went down when more people outside the cluster were tested.

Cases, tests in India

The chart plots the daily samples tested (blue line, left axis) and daily cases recorded (yellow line, right axis) in India since April 3.

image/svg+xml After Apr. 9, the two lines mimic each other. When tests go up, cases go up & vice-versa 45000 35000 25000 15000 5000 1500 1100 700 300 April 5 April 12 April 9 Apr. 8 April 19 Until Apr. 9, cases and tests did not mimic each other. In fact on Apr. 8, even when tests went up, cases went down
 

(If chart doesn't appear in full, click here)

As seen above, after April 9, a somewhat linear relationship emerges between samples tested daily and the number of new cases recorded.

Cases, tests in Gujarat

However, the above is not always the case, as the following example from Gujarat shows. The chart plots the daily samples tested (blue line, left axis) and daily cases recorded (yellow line, right axis) in Gujarat since April 3.

image/svg+xml On Apr. 11, Gujarat conducted 3,025 tests (the second highest), but recorded only 90 cases On Apr. 19, the State conducted 452 tests (among the lowest) but recorded 367 cases (the highest yet) 3500 2500 1500 500 400 300 200 100 0 April 5 April 12 April 19

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Testing clusters

The graph shows daily cases recorded in Tamil Nadu (bar, right axis), daily samples tested of those in the Nizamuddin cluster (red line, left axis) and daily samples tested of those not in the cluster (blue line, left axis).

image/svg+xml 1500 1000 500 0 100 80 60 40 April 5 April 7 April 9 April 11 and samples from outside the cluster were the highest ever In the period analysed, the lowest no. of daily cases was recorded on Apr. 8. On that day, samples tested from people inside the cluster were the lowest ever
 

Tests vs positives

The graph shows the % of positive cases among the samples tested of those in the Nizamuddin cluster (red line) and those outside the cluster in Tamil Nadu (blue line).

image/svg+xml Apr. 5 Apr. 9 60 40 20 0 The % of positive cases inside the cluster were very high before April 9 and sharply dropped thereafter
 

As the probability of positives inside clusters is high, on days when a State tests more people who belong to clusters, the case count is higher. And the days on which a State tests more people from outside clusters, where the probability of positives is much lower, the case count is also lower.

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Printable version | Aug 15, 2020 7:26:37 PM | https://www.thehindu.com/data/data-coronavirus-who-is-getting-tested-is-more-important-than-how-many-are/article31392265.ece

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