While we were distracted by COVID-19, a strange thing happened: the flu disappeared. Just take a look at the striking chart below from the World Health Organization (WHO) showing global flu cases plummeting from normal levels in February 2020 to almost nothing by May 2020, and staying there for the rest of the year.
Chart 1: Number of global influenza detections by subtype between November 2019 and December 2020. Source: WHO.
Flu season has been an unpleasant part of winter for as long as we have had winter. So, why did it quietly fade away in 2020? As we documented in a recent article, COVID-19 is definitely not the flu. Will the flu be back?
It is likely no coincidence that flu infection rates decreased sharply with the community mitigation and hygiene guidelines promoted or imposed by governments around the world in response to the COVID pandemic. The deliberate interruption in typical social activity also interrupted influenza transmission.
A similar pattern was observed in Hong Kong during the 2003 SARS (SARS-CoV) outbreak. The first confirmed SARS case was documented there on March 11, 2003, and the last one, June 2, 2003. This period of aggressive social curtailment and hygiene campaigns overlapped the typical flu season in Hong Kong (March to September). As Chart 2 shows, the incidence of respiratory virus infections while SARS was active was considerably lower than the average of the corresponding months in the reference period (1998-2002).
Chart 2: Change in proportion of positive specimens in Hong Kong in 2003 for various viruses with reference to period 1998–2002.
In addition to the relatively obvious effects of intervention, however, the WHO report mentions a phenomenon called “viral interference” that implies viruses attacking viruses. The theory goes that viral pathogens not only interact with the host cells, but interact with each other within the host. Therefore, virus-to-virus interactions can affect the success of one while inhibiting the other (in the case of two viruses fighting it out). As SARS-CoV-2 (COVID-19) and types of influenza are all respiratory viruses, they are in competition for the same cells. Consequently, COVID-19 can inhibit the flu virus. But the opposite can also happen; people who test positive for flu are less likely to test positive for COVID.
An associated effect called “cross-protection” involves a previous or incumbent virus “priming” the immune system to be more effective against a different but related virus. This means that people who have recently had flu or another type of respiratory virus and are then exposed to COVID potentially have greater natural protection. This effect has been observed in flu/common cold competition, and between flu and other respiratory viruses such as Respiratory Syncytial Virus (RSV).
On the microscopic level, there is an epic struggle going on between and amongst viruses, and our immune cells. It is interesting to imagine that perhaps an orchestrated alliance between the immune system and the more benign of the viruses might increase our defences against a more sinister one like COVID-19; a partnership with the lesser of two evils, so to speak. Maybe trying to prevent flu infections increases our vulnerability to more severe viruses.
We know as a society that severe social restrictions must be a temporary measure, so while those are shown to be effective, they are unsustainable. Vaccine development is an important component of pandemic preparedness, but effective vaccines lag the onset of a new virus. Strengthening our immune systems with improved overall health and, yes, exposure to viruses are natural and vital mechanisms of protection.
Will the flu come back? Yes, most likely, as is the case in Chart 2 after the SARS 2003 outbreak was over and people started to mingle again, sharing germs. In the interest of our long-term health, I kind of hope so.
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