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

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Data | Who gets tested for coronavirus determines infection rates better than how many are
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When testing is done for people within a cluster, the number of positive cases seem to be higher, and vice-versa

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+xmlAfter Apr. 9, the two lines mimiceach other. When tests go up,cases go up & vice-versa45000350002500015000500015001100700300April 5April 12April 9Apr. 8April 19Until Apr. 9, cases and tests did not mimic eachother. In fact on Apr. 8, even when tests wentup, 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+xmlOn Apr. 11,Gujarat conducted3,025 tests (thesecond highest),but recorded only90 casesOn Apr. 19, the State conducted 452 tests (amongthe lowest) but recorded 367 cases (the highest yet)3500250015005004003002001000April 5April 12April 19

Also read: 96% migrant workers did not get rations from the government, 90% did not receive wages during lockdown: survey

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+xml150010005000100806040April 5April 7April 9April 11and samples from outside the cluster were the highest everIn the period analysed, the lowest no. of daily caseswas recorded on Apr. 8. On that day, samples testedfrom 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+xmlApr. 5Apr. 96040200The % ofpositive casesinside thecluster werevery highbefore April 9and sharplydroppedthereafter

 

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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