Insights into the association of H1N1 seasonality with the COVID-19 pandemic in Brazil: an ecological time series analysis.

During the COVID-19 pandemic, H1N1 seasonality disappeared worldwide. In Brazil, information on how coronavirus impacted this seasonality is scarce. In this study, we aimed to verify whether COVID-19 pandemic was associated with changes in the seasonality of H1N1, modeling the time series of H1N1 between pre-pandemic (2018 and 2019), pandemic (2020 and 2021) and post-pandemic (2022 and 2023) periods. For this purpose, we superimposed on this time series cases of COVID-19 from 2020 to 2023. Our findings highlighted that H1N1 exhibited a consistent seasonal pattern in the pre-pandemic period, with peaks mainly in months with the highest rainfall. However, this seasonality disappeared during the pandemic, with a significant decrease in the number of cases, in contrast with the predicted seasonality of H1N1 for the same period. In addition, the seasonal pattern of H1N1 in the post-pandemic showed a return to that observed in the pre-pandemic period, especially in 2023. We observed that the COVID-19 pandemic was consistently associated with changes in H1N1 seasonality in Brazil, underscoring the relative importance of monitoring patterns of respiratory syndromes to enhance our understanding of how coronavirus is associated with changes in seasonal diseases.