[nickalaso]

The top level comments are arguing that the cause for this is due to the effectiveness of various health measures. The problem with this argument is we would expect for flu cases to continue to be reported in countries and US states where lockdown measures were never properly enacted.

This doesn't appear to be happening. Instead, "the drop-off in flu numbers was both swift and universal." (article quote)

This universality suggests a data problem to me rather than an environmental change.