The flu has returned to some countries with high rates of natural immunity against SARS-CoV-2. In contrast, countries with high vaccination rates but low natural immunity rates are seeing record new covid waves, although at lower hospitalization and death rates than previously.
In March 2020, shortly after the beginning of the coronavirus pandemic, influenza viruses and some other respiratory viruses mysteriously disappeared from global circulation. Some skeptics suspected that influenza was simply ‘rebranded’ as covid, while many health authorities and journalists claimed influenza was suppressed by face masks and lockdowns.
But influenza has not been rebranded as covid, and influenza viruses have disappeared even in countries without face masks and lockdowns (e.g. Sweden), while they did not disappear during previous flu pandemics, despite face masks, school closures, and other measures. In fact, face masks and most other measures simply aren’t effective against respiratory virus transmission.
Instead, influenza viruses have been displaced by the more infectious novel coronavirus. This “displacement effect” is well known from previous influenza pandemics (e.g. 1918, 1957 and 1968) and is at play every winter, when various respiratory viruses displace each other. Even during the ongoing coronavirus pandemic, new variants have repeatedly displaced previous variants. (1)
Thus, it was only a matter of time until influenza viruses would re-emerge somewhere, most likely at a place that had achieved a high rate of natural immunity against the novel coronavirus.
This moment came in July 2021, when the first major influenza wave since early 2020 hit India, shortly after its large Delta coronavirus wave had ended in June (see WHO flu chart below). At the time, the natural immunity rate in India was about 80% at the national level and about 97% in the capital city, Delhi, while the covid vaccination rate was only about 5%. (2)