From my daily briefing, New York Times:

India's discovery.

By mid-March, though, the virus was beginning to reassert itself. A major factor appears to be that many people who previously had mild or asymptomatic cases of Covid remained vulnerable to it. (A recent academic study, done in China, suggests that mild cases confer only limited immunity.) The emergence of contagious new variants is playing a role, too. This combination — less immunity than many people thought, new variants and a resumption of activities — seems to have led to multiple superspreader events, Dr. Jennifer Lighter of New York University told me.