Absolute humidity modulates influenza survival, transmission, and seasonality

TLDR

Influenza A incidence peaks in winter in temperate regions, yet the basis for this seasonality and the mechanisms of transmission remain poorly understood, though prior work shows relative humidity influences virus transmission and survival, and both outdoor and indoor absolute humidity follow a strong seasonal cycle that drops in winter. The study reanalyzes existing data to explore how absolute humidity affects influenza virus transmission and survival, and proposes that this hypothesis can be further tested by future laboratory, epidemiological, and modeling studies. The authors reanalyze existing datasets to assess the impact of absolute humidity on influenza virus transmission and survival. Absolute humidity explains 50 % of transmission variability and 90 % of survival variability, far exceeding the explanatory power of relative humidity, and its seasonal decline in winter aligns with increased virus survival and transmission, offering a coherent, physically sound explanation for influenza seasonality in temperate regions.

Abstract

Influenza A incidence peaks during winter in temperate regions. The basis for this pronounced seasonality is not understood, nor is it well documented how influenza A transmission principally occurs. Previous studies indicate that relative humidity (RH) affects both influenza virus transmission (IVT) and influenza virus survival (IVS). Here, we reanalyze these data to explore the effects of absolute humidity on IVT and IVS. We find that absolute humidity (AH) constrains both transmission efficiency and IVS much more significantly than RH. In the studies presented, 50% of IVT variability and 90% of IVS variability are explained by AH, whereas, respectively, only 12% and 36% are explained by RH. In temperate regions, both outdoor and indoor AH possess a strong seasonal cycle that minimizes in winter. This seasonal cycle is consistent with a wintertime increase in IVS and IVT and may explain the seasonality of influenza. Thus, differences in AH provide a single, coherent, more physically sound explanation for the observed variability of IVS, IVT and influenza seasonality in temperate regions. This hypothesis can be further tested through future, additional laboratory, epidemiological and modeling studies.