India, June 24 -- Five years later, the summer of 2021 still evokes a distinct and unsettling memory. There was an eerie feeling, a growing sense that something unprecedented was unfolding, even before its full scale became apparent. This was very different from the first wave. Cases were rising rapidly, but so too was the severity of illness. Hospitals were filling at an alarming pace, oxygen demand was escalating, and mortality was increasing across the country. In terms of deaths occurring over such a short time span, the only comparable event was the tragedy of Partition.

The first indications of the impending crisis emerged through discussions within disaster-response and public-health networks. What initially appeared to be concern...