Modelling seasonal influenza: The role of weather and punctuated antigenic drift

Abstract

Seasonal influenza appears as annual oscillations in temperate regions of the world, yet little is known as towhat drives these annual outbreaks andwhat factors are responsible for their inter-annual variability. Recent studies suggest that weather variables, such as absolute humidity, are the key drivers of annual influenza outbreaks. The rapid, punctuated, antigenic evolution of the influenza virus is another major factor. We present a newframework for modelling seasonal influenza based on a discrete-time, age-of-infection, epidemicmodel, which allows the calculation of themodel's likelihood function in closed form. This framework may be used to perform model inference and parameter estimation rigorously. Themodelling approach allows us to fit 11 years of Israeli influenza data, with the best models fitting the data with unusually high correlations in which r > 0.9.We show that using actual weather to modulate influenza transmission rate gives better results than using the inter-annual means of the weather variables, providing strong support for the role of weather in shaping the dynamics of influenza. This conclusion remains valid even when incorporating amore realistic depiction of the decay of immunity at the population level, which allows for discrete changes in immunity from year to year. © 2013 The Author(s) Published by the Royal Society. All rights reserved.