About SUTRA

• SUTRA (Susceptible, Undetected, Tested (positive), and Removed Approach) first came into public attention when one of its expert members announced in October that India was “past its peak”.
• Unlike many epidemiological models that extrapolated cases based on the existing number of cases, the behaviour of the virus and manner of spread, the SUTRA model chose a “data centric approach”.
• However, the surge in the second wave was several times what any of the modellers had predicted.
• The predictions of the SUTRA model were too variable to guide government policy.

So, what went wrong in the model

• The SUTRA model was problematic as it relied on too many parameters, and recalibrated those parameters whenever its predictions broke down.
• The more parameters you have, the more you are in danger of overfitting.
• One of the main reasons for the model not gauging an impending, exponential rise was that a constant indicating contact between people and populations went wrong.
• Further the model was ‘calibrated’ incorrectly.
• The model relied on a serosurvey conducted by the ICMR in May that said 0.73% of India’s population may have been infected at that time.
• This calibration led our model to the conclusion that more than 50% population was immune by January.
• The SUTRA model’s omission of the importance of the behaviour of the virus; the fact that some people were bigger transmitters; a lack of accounting for social or geographic heterogeneity and not stratifying the population by age as it didn’t account for contacts between different age groups also undermined its validity.

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