The Origins of the Second Wave of COVID-19 in India - a Model-based investigation

Background. After months of declining or steady rates of daily COVID-19 cases, in April 2021 India experienced the onset of a massive second wave of the epidemic which overstressed its healthcare facilities and caused lakhs of casualties. During the wave, the reproduction number R in some places was 1·8 or greater. The origins of this wave are at best partially understood. Socioeconomic activities had been running at a relatively high level for several months prior to the wave, and there was no relaxation of non-pharmaceutical interventions (NPI) in the immediately preceding days. A causal connection between the second wave and organized gatherings such as election activities, religious festivals and farmers’ protests is impossible to establish due to absence of spatio-temporal correlation. Analysis of genomic sequencing data suggests that the emergence of the double mutant B1.617 and its sub-lineages cannot be the sole cause of the wave as well. Methods and results. In a companion study [1] we have constructed a mathematical model for COVID-19 transmission which accounts for heterogeneity in people’s interaction patterns. This model yields two phenomena of concern which are outside the purview of classical infectious disease dynamical understanding. These phenomena are • cryptogenic instability in which relatively small-scale social gatherings such as marriage functions and birthday parties play a disproportionately high role in driving spread, and • critical mass effect in which very low case counts result in a temporary and artificial suppression of an existing instability. Principal findings. We believe that the above two phenomena are primarily responsible for the suddenness and severity of the second wave of COVID-19 in India. Complacency prevailing in early 2021 and emergence of mutant strain are not root causes of the wave but contributory factors to disease burden. The origins of the wave are complex and multifactorial, being largely governed by infectious disease dynamics which were unknown at the time of its onset. The lessons learned from the experience must be used to guide future course of NPI, not just in India but in other countries as well until significant fractions of their respective populations have been vaccinated.