Why has the flu disappeared since the beginning of the coronavirus pandemic?
In 2020, after the global coronavirus pandemic began, influenza viruses mysteriously disappeared from global circulation (see WHO FluNet chart above). Some skeptics suspected that influenza was simply ‘rebranded’ as covid, while many journalists and ‘fact checkers’ 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 epidemics and pandemics, despite face masks, school closures, and other measures.
For instance, Japan, despite its widespread use of face masks, experienced a strong influenza epidemic in early 2019, just one year before the coronavirus pandemic began. Indeed, numerous studies have shown that face masks 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: the 1918 flu virus was displaced by the 1957 flu virus, which in turn was displaced by the 1968 flu virus (see chart).
The 2009 swine flu virus temporarily displaced previous flu viruses, but eventually couldn’t assert itself (see chart). And even during the current coronavirus pandemic, more transmissible virus strains have repeatedly displaced previous coronavirus strains, often within weeks, despite lockdowns.
The last coronavirus pandemic is thought to have occurred in the 1890s (“Russian flu”), which is why a coronavirus displacing influenza viruses was not seen for more than a century. It is well known, however, that influenza vaccinations do not reduce the overall incidence of influenza-like illnesses, as influenza viruses simply get displaced by other respiratory viruses, including coronaviruses.
But why do countries with little or no covid – most of them are islands – also have no influenza? Because they closed their borders early: if the coronavirus doesn’t get in, influenza viruses – which normally oscillate between the northern and southern hemispheres – won’t get in, either.
This viral interference and displacement effect is well-known, but still poorly understood. Another interesting and open question is whether the novel coronavirus might permanently suppress some of the existing influenza virus strains. This might, at last, be a positive development.
Update: The Return of the Flu (November 2021)
Sweden vs. Japan
The flu disappeared both in Sweden (no lockdown, almost no masks) and in Japan (no lockdown, but very high mask compliance). Moreover, in Japan and elsewhere, the flu disappeared several weeks prior to lockdowns and mask mandates. Chart: IanMsc
1) Competition between various respiratory viruses
Temporal patterns of seasonal respiratory viral infections in Glasgow (UK). Red: rhinoviruses, orange and yellow: influenza viruses; light green: coronaviruses.