Seasonality Cycles of Novel Influenza Strains

It is well known that non-pandemic influenza has a seasonal repeating periodicity, especially in temperate climates [1]. The causes of seasonal cycles of influenza infections are not well understood. However, an analysis of seasonality of influenza around the world in a recent PLOS article indicates that cold-dry and humid-rainy conditions are associated with peaks in the frequency of seasonal influenza cases in different regions [2]. The authors in this article suggest that “these two distinct mechanisms account for influenza seasonality in temperate and tropical climates, perhaps due to changes in the dominant mode of transmission.”

In the past decade there have been several outbreaks of novel influenza infections, including (A)H5N1, (A)H7N9, and (A)H10N8. Based on limited data, it does appear that novel influenza infections follow the same seasonal pattern as non-pandemic influenza.

For example, H7N9 was first reported by the Republic of China to the World Health Organization (WHO) in early 2013. Since then more than 440 cases have been reported, all originating in China. The graph below shows the frequency of more than 400 WHO confirmed H7N9 human cases by month from 2013 and 2014 based on symptom onset date. From about 20 months of data, H7N9 shows a seasonal increase between December and May in China.

Case data for H5N1 infections has been accumulating for more than a decade from 15 countries. Based on WHO data, the distribution of symptom onset dates for more than 600 H5N1 cases since 2003 also shows a periodic seasonal increase in cases between December and May as shown in the graph below.

Currently, only three cases of a H10N8 have been reported, all from China (two confirmed). This is an insufficient number of cases to plot on a seasonal basis however, all three these cases were reported in period from November 2013 to February 2014.

The data suggest that even novel influenza infections seem to be constrained by the same environmental factors that control the infection cycle of seasonal non-pandemic influenza.

[1] Influenza Seasonality: Underlying Causes and Modeling Theories

[2] Environmental Predictors of Seasonal Influenza Epidemics across Temperate and Tropical Climates