Influenza epidemics have different seasonalities in different regions, likely due to different climate conditions. In temporal regions, epidemics occur predominantly in winter months, which have been associated with low humidity (see why here). In subtropical and tropical climates, however, the transmission dynamics of influenza are highly diverse and remain poorly characterized. In two recent projects, we studied flu transmission dynamics in Uganda, an African country in the deep tropics.
The impact of weather conditions and co-circulating influenza types/subtypes on influenza epidemic dynamics in tropical Africa
The association of influenza with meteorological variables in tropical climates remains controversial. In this study, we investigate the impact of weather conditions on influenza in the tropics and factors that may contribute to this uncertainty. We computed the monthly viral positive rate for each of the 3 circulating influenza (sub)types (ie, A/H1N1, A/H3N2, and B) among patients presenting with influenza-like illness (ILI) or severe acute respiratory infections (SARI) in 2 Ugandan cities (Entebbe and Kampala). Using this measure as a proxy for influenza activity, we applied regression models to examine the impact of temperature, relative humidity, absolute humidity, and precipitation, as well as interactions among the 3 influenza viruses on the epidemic dynamics of each influenza (sub)type. A full analysis including all 4 weather variables was done for Entebbe during 2007-2015, and a partial analysis including only temperature and precipitation was done for both cities during 2008-2014.
For Entebbe, the associations with weather variables differed by influenza (sub)type; with adjustment for viral interactions, the models showed that precipita- tion and temperature were negatively correlated with A/H1N1 activity, but not for A/H3N2 or B. A mutually negative association between A/H3N2 and B activity was identified in both Entebbe and Kampala.
Our findings suggest that key interactions exist among influenza (sub) types at the population level in the tropics and that such interactions can modify the association of influenza activity with weather variables. Studies of the relationship between influenza and weather conditions should therefore determine and account for co-circulating influenza (sub)types.
Yang W, Cummings MJ, Bakamutumaho B, Kayiwa J, Owor N, Namagambo B, Byaruhanga T, Lutwama JJ, O’Donnell MR, Shaman J. Dynamics of influenza in tropical Africa: Temperature, humidity, and co-circulating (sub)types. Influenza Other Respir Viruses. 2018;12(4):446-56.
Transmission dynamics of influenza in two major cities of Uganda
In this study, we analyze the epidemic characteristics of the three co-circulating influenza viruses (i.e., A/H1N1, A/H3N2, and B) in two tropical African cities—Kampala and Entebbe, Uganda—over an eight-year period (2008–2015). Using wavelet methods, we show that influenza epidemics recurred annually during the study period. In most months, two or more influenza viruses co-circulated at the same time. However, the epidemic timing differed by influenza (sub)type. Influenza A/H3N2 caused epidemics approximately every 2 years in both cities and tended to alternate with A/H1N1 or B. Influenza A/H1N1 and B produced smaller but more frequent epidemics and biennial epidemics of these two viruses tended to be synchronous. In addition, epidemics of A/H3N2 were more synchronized in the two cities (located ca.37 km apart) than that of A/H1N1 or influenza B.
Yang W, Cummings MJ, Bakamutumaho B, Kayiwa J, Owor N, Namagambo B, Byaruhanga T, Lutwama JJ, O’Donnell MR, Shaman J. Transmission dynamics of influenza in two major cities of Uganda. Epidemics. 2018;24:43-8.