Signals from the southern hemisphere suggest we’re about to see a silver lining in the current pandemic: a sharp reduction in the number of seasonal influenza cases as a result of public health restrictions.
We’ve modelled how these restrictions may impact selected high-income countries in the northern hemisphere (France, Germany, Italy, Japan, Spain, United Kingdom and the United States), bearing in mind that the influenza season in these countries straddles the end of one year and the beginning of another.
Assuming we have a population-wide vaccination program completed before the 2021/2022 influenza season, we should see comparable drops in total cases, albeit spread throughout the 2020 and 2021 calendar years given the split impact between these two seasons.
Influenza-like-illness cases expected to drop amid public health restrictions
Countries assessed: France, Germany, Italy, Japan, Spain, United Kingdom and the United States
Source: Decision Resources Group, 2020
Just as the kind of public health restrictions put in place to control spread of SARS-CoV-2 – the virus that causes COVID-19 – can reduce the risk of transmission of that virus (measured as the infamous R number), so it will also reduce the transmission of any other virus that is transmitted in a similar fashion. Specifically, strains of seasonal influenza.
Moreover, this reduction may not be in proportion. Milder public health restrictions could merely reduce the rate of increase of SARS-CoV-2, for example, reducing R from 3 to 1.5. But the same restrictions, if they also halved the transmission risk of influenza, which typically has a smaller R than SARS-CoV-2, could push it from a figure greater than 1 to a figure less than 1. This could make all the difference in an epidemic: R>1 indicates an exponentially increasing number of infections, whereas R<1 indicates exponentially decreasing numbers.
With the southern hemisphere’s seasonal influenza peak occurring while the northern hemisphere basks in the summer warmth, we have evidence of this hypothesis: cases of seasonal influenza in the southern hemisphere showed huge drops in 2020, typically in excess of 90% compared to 2019 numbers.