This year, the number of influenza cases is significantly lower compared to previous seasons, and notably, has the lowest hospitalization rate for flu since the CDC started tracking this statistic in 2005. When thinking about this drastic decrease, one wonders why this might be happening? It could be related to public health protocols, including social distancing, attention to hygiene/hand washing, limited public gatherings, and mask-wearing that have been strictly enforced since the rise of coronavirus cases in the Spring of 2020. Since influenza is a respiratory virus spread by aerosolized particles, it also makes sense that influenza cases have decreased in nursing homes due to limiting access from outside visitors, increased use of personal protective equipment (PPE), and enhanced infection control policies. While all these precautions play a role in reducing this season’s flu cases, the reduction might also be due to a phenomenon called “viral interference.”1

In September 2009, the H1N1 swine flu suddenly appeared in the UK, Portugal, and Spain, leading to a neighboring country, France, to brace itself for impact of infections. This was unusual because these countries typically did not begin to see influenza activity this early in the calendar year1. Following this, healthcare professionals throughout France noticed an increase in the spread of seasonal respiratory-related illnesses. Strangely, there were very few positive H1N1 tests being reported. It was not until a few months later that France saw positive cases of H1N1- so why was this? Some scientists believed that the rapid swine flu spread was averted by rhinovirus, one of the common cold viruses.1 As common cold cases started to dwindle, the pandemic flu surged. Subsequent discussions among scientists have suggested that this “viral interference” likely reduces becoming ill from another virus concurrently.2

Years later, scientists are still investigating the causes and repercussions of viral interference. Dr. Ellen Foxman and colleagues at Yale School of Medicine published a paper this past year suggesting that once the body activates the innate immune system on exposure to a pathogen, it can then repel another viral invader. Foxman modeled the study around the 2009 swine flu (H1N1) pandemic by infecting the apical surface of cultured human epithelial airway tissues; one with rhinovirus and another with H1N1.3 Foxman then exposed the infected rhinovirus tissue to H1N1 three days later, using reverse-transcription quantitative PCR testing.3 The researchers were surprised to observe H1N1 “fizzle out.” To formally test if the previous exposure to rhinovirus inhibited H1N1 from manifesting, the researchers then treated the tissue with BX795, a laboratory reagent, to block the naturally occurring interferons which are produced in response to viral replication. Just like that, the flu virus grew perfectly.3 The investigators concluded that the interferons produced by the body to fight rhinovirus were essentially also blocking the flu from manifesting within the patient’s immune system.3

Today, the COVID-19 pandemic mirrors some of the characteristics of viral interference in the H1N1 pandemic. The novel coronavirus emerged late in the 2019-2020 flu season, which could explain why we did not see a drastic decrease in the number of flu cases reported. On the contrary, coronavirus has consistently dominated viral activity, both leading up to and throughout the current flu season. Has this led to an evasion of influenza activity like that which occurred in 2009 with swine flu and rhinovirus?

There is no confirmation that viral interference is the reason behind this uneventful flu season, however, it does provoke an interesting hypothesis that could lead us to a better understanding of viral transmission and future treatment/prophylaxis.