Thursday, April 29, 2021

India's Second Wave: Role of Mutation & Lax Masking

    Was India's second covid wave have been predicted or was it a complete surprise. Were new mutations like the UK variant, the South Asian variant or the double mutant B1.617 responsible for the second wave? There is not enough information to answer these questions precisely.  What this note does is to look at the available State wise data on the second wave so that we some basis for judging the various hypothesis that are floating around in the media.

I use the weekly growth of active cases to derive a basic picture. at the all India level and the 20 States with the largest number of cases. We look at three sets of dates for each state. The date on which the lowest growth rate was reached and what the rate was, and the dates on which the growth rate exceeded 1% and then 2%. 

Table 1 shows that the active case growth trough was reached on 17 the february 2021 and the growth rate rose gradually over 11 days to over 1% on 12th March, indicating that the phase of declining growth rates was over. During the next 14 days the spread of active cases exploded, reaching 2% by 26th March, confirming that the new wave was underway.

Three States (Uttarakhand, Maharashtra, Punjab) crossed the 1% threshold before the country as a whole, and three States (Madhya Pradesh, Gujarat, Haryana) crossed it soon afterwards (table 1). The stage of explosive growth(above 2%) took much longer to reach in the former set of States, than the latter set. These dates should be noted carefully for the purpose of testing any hypothesis that a specific event such as farmers rallies(august 9, 2020 to February) , or Kumbh mela (12 -27 April) or an influx of NRIs/PIOs from UK, triggered the second wave.Note that that the 2nd wave in Uttarakhand clearly preceded the "Kumbh Mela". Also note that despite the farmers agitation the growth rate of Covid comtinued to decline in Punjab till 10th february. However, we cannot say the same for the farmers agitations which continued after mid-February.

Specifically the issue of the effect of State polls and polling as triggers of the 2nd wave have been raised (hypothesized). Table 2, shows the same data for the poll bound States. It may be recalled that the poll dates for these States were, West Bengal (27th March to 29 April), Tamil Nadu (6th April), Kerala(6th April) and Assam (27th to 29th April). On average election campaigning starts a month before the election, but has varied for different States. The Election Commission of India (ECI) an independent body under the constitution, had issued certain rules and regulations for election rallies during the COVID epidemic. News reports (eg Hindustan Times, 15/4/21), suggest that these were breached more often than they were followed. Table 2 that the growth rate of active cases continued to decline in Kerala after March 6, but started rising during March in Tamil Nadu. There is a similar contrast between Assam and West Bengal, with the growth rate of active cases falling in Assam till 18 March and remaining below 1% for the next 30 days. The role of masking and distancing less significant outdoors, in well ventilated space, and in indoor halls & rooms in which re-circulated air is purged of virus(filters, UV lights).  However prolonged exposure, to unmasked crowds, in still air, would undoubtedly help increase the spread. Their contribution to the explosion of Covid cases during the second wave in India, remains to be quantified, with more detailed analysis.

UK research had earlier indicated that the UK variant of the Corona virus (SARS 2) spread much more rapidly than the original Wuhan virus, though it was not much deadlier and perhaps even less deadly. Based on S curve analysis of the accumulated corona virus cases, we estimate (figure 1) that the actual(orange) corona virus cases crossed the projected 11 million cases(green) on or about march 8, 2021. This suggests that a new corona virus mutation arrived or was generated in India in the second half of February.

Mutation Model (preliminary)

We model the spread of the mutations by assuming that the rate of growth of the original virus had flattened at the minimum growth rate achieved on February 11, 2021. The difference between the projected rate on February 11 onward and the actual cases is taken as due to the new mutations(orange line in figure 2). We then fit the S curve model to estimate the future trajectory of the new mutation (green line in figure 2). Based on this projection, and the one in figure 1 we estimate that the growth rate of cases will go below the  one prevailing on February 11, 2021, by first week of June, after reaching about 21.8 million total cases.

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