Research is lacking on whether seasonal influenza epidemiology differs across geographically and climatically similar regions and whether the COVID-19 pandemic has caused changes to influenza dynamics that transcend these differences. We adopted two complementary databases covering ten consecutive years of seasonal influenza epidemics across the four nations of the United Kingdom (UK). The Global Influenza Surveillance and Response System (GISRS) database mainly provides weekly numbers of tested individuals and infected cases, while the UK Health Security Agency (UKHSA) database primarily offers the onset and offset weeks of each influenza season as well as vaccination information. Using these data, we quantified the divergences in peak positivity, total seasonal positivity, and epidemic timing for each influenza season. Before the COVID-19 pandemic, across five influenza seasons in the UK, seasonal influenza peak positivity, total seasonal positivity, and epidemic timing metrics—including onset weeks, offset weeks, and epidemic duration in weeks—varied by nation, season, and/or virus type. Post the COVID-19 pandemic, changes in the aforementioned parameters also varied by season, region, and/or virus strain. The main patterns that can be summarized are as follows: aggregated peak positivity and total seasonal positivity for influenza A(H3N2) and influenza B were reduced across all three nations with available post-pandemic data; influenza A(H1N1)pdm09 and A(H3N2) exhibited a consistent alternating dominance pattern across the four nations before the COVID-19 pandemic, which was disrupted post-pandemic; and in Wales, the onset week advanced, the offset week delayed, and the epidemic duration prolonged significantly in the post-pandemic period, whereas no statistical differences in these timing parameters were observed in England. Pre-COVID-19, influenza seasonality in the UK varied by nation, season, and/or virus strain. The pandemic disrupted prior dominant strain patterns with considerable consistency, yet it did not consistently alter the timing and intensity of seasonal influenza epidemics across nations and/or virus types. Not applicable.
Yuan et al. (Mon,) studied this question.