Much discussion around
Kerala model these days.
Here is my take and also thinking of scientists who who are doing Sutra modeling etc.
(The below requires some basic mathematics and scientific thinking -- These are *not* predictions like astrologers but rather - how we are interpreting the current data. Please ignore - if you don't trust these scientist, if you want more details please check other sources too - Good mathematical background on Sutra model is at
https://arxiv.org/abs/2101.09158) -
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There are two sides of the debate: using same data of
a) low seropositivity and b)
high TPR --to argue opposite points!
One side says: -
low seropositivity demonstrate success of
containment strategy.
-(and)
high TPR show that
smart testing is being done.
Other side says: -
low seropositivity mean there is m
uch larger susceptible population that other states which means pandemic has still a long way to run there,
and
high TPR indicates that
infection is spreading significantly.
Both are true, but, only partially.
The plot below is the entire timeline for Kerala showing how
the values of reach and contact rate parameters have changed to influence the trajectory.
(The values are reasonably precise thanks to calibration via serosurvey)
Last year, until August-end, reach in Kerala was a mere 3%. This demonstrates success of containment strategy adopted by the state -- the pandemic could not even reach 97% of population! In September, reach jumped to 11% due to onset of festival season. It jumped again to 21% in October-end, but the cases did not rise due to reduction in contact rate. Both the parameters increased slowly over next four months resulting in almost flat trajectory that eventually came down in February.
And then delta-variant struck!
This resulted in sharp increases in both reach and contact rate. Reach is now nearly 94%
thanks to another sharp rise during mid-June to mid-July.
The containment strategy adopted by the state was a success until February. It managed to keep most of the
population away from the pandemic. However, with onset of delta-variant, the strategy has not worked well
resulting in spread of pandemic everywhere.
With seropositivity around 52% at present and pandemic spread over nearly entire population, there is a long way
to go before herd immunity is reached. There are now two options available: keep contact rate low through
lockdowns, or allow it to increase further by opening up.
As posted in earlier post : if contact rate is brought down from its current value of 0.33, the numbers will start reducing, however, herd immunity will not be reached and lockdown will have to be kept until a large fraction is vaccinated. (See graph in earlier post)
If strict lockdown is lifted and basic rules are enforced like wearing masks, avoiding crowding etc, the contact rate should settle down around 0.35. The resulting trajectory was shown in my previous post - it shows a peak of around 25K infections by mid August. By September, seropositivity would be 75% and then even without any precautions cases will not rise.
This is why, some of the scientists are recommending lifting of the strict lock-down in Kerala.