Significant Decrease in Seasonal Influenza in the COVID-19 Era: Impact of Global Movement Restrictions?

In early June 2020, the first wave of the COVID-19 pandemic ended in many countries. What followed were the strictest historical migration and travel blockades implemented worldwide. At that time, healthcare professionals witnessed the sudden early end of the 2019 2020 seasonal flu season in the northern hemisphere. According to medical and public health experts, the significant decrease in those presenting for influenza testing may have affected the eventual count of those infected with seasonal influenza in 2019 2020. The significant decline in seasonal flu counts in 2019 2020 is also due to the same infection precautions as those recommended to prevent COVID-19 infection. During the COVID-19 epidemic setting in the United States (US), the number of people infected with the influenza virus this season has plummeted by 98% compared to an average year. This is tempered by a 61% reduction in flu sample analysis volumes [1]. The US Centers for Disease Control and Prevention rated the recent flu season “moderate” despite an estimated incidence of 38 million illnesses and the death of 22,000 individuals. Compared to the average year, the number of seasonal influenza infections and deaths is significantly lower [1]. The numbers infected with seasonal influenza, often feared as a simultaneous epidemic with COVID-19, have been reported in Japan. The number of cases reported in the week to December 13, 2020 is 57 nationwide (counted across 5,000 institutions). Compared to historical averages for Japan, the 2020 2021 infection counts continue to be significantly lower. According to a report by the National Institute of Infectious Diseases in Japan, the signs of the epidemic of seasonal influenza in Japan have not yet been recognized. The Japanese government is calling on the Japanese people to be aware of the upcoming seasonal influenza epidemic. In the case of seasonal influenza infection, if the number of patients per week per medical institution exceeds 1, it is judged an epidemic. Fortunately the fixed-point prevalence of infections is 0.01. This same report indicated that the number of people infected with seasonal influenza is more than 20,000 at the same time of any other flu season year. The number of people infected with seasonal influenza in 2020 2021 continues to be significantly lower. After the early end of the season of seasonal influenza in the northern hemisphere, there was no epidemic of 2020 seasonal influenza in the southern hemisphere. In the southern hemisphere from April to July 2020, influenza case counts remained extremely low. In Australia, Chile and South Africa, of more than 83,000 samples for seasonal influenza, only 51 people tested positive [1]. Some countries in South America have inconsistent and/or ineffective public health policies directed against COVID-19 (Note 1). Despite what appears to be a general challenge for seasonal and/or epidemic illnesses, the number of people infected with seasonal influenza is extremely small (Note 1). In other words, the absence of people infected with seasonal influenza is not solely due to wearing masks, washing hands and social distance as a countermeasure against the COVID-19 pandemic.