Global Spatio-temporal Patterns of Influenza in the Post-pandemic Era

Abstract

We study the global spatio-temporal patterns of influenza dynamics. This is achieved by analysing and modelling weekly laboratory confirmed cases of influenza A and B from 138 countries between January 2006 and January 2015. The data were obtained from FluNet, the surveillance network compiled by the the World Health Organization. We report a pattern of skip-and-resurgence behavior between the years 2011 and 2013 for influenza H1N1pdm, the strain responsible for the 2009 pandemic, in Europe and Eastern Asia. In particular, the expected H1N1pdm epidemic outbreak in 2011/12 failed to occur (or "skipped") in many countries across the globe, although an outbreak occurred in the following year. We also report a pattern of well-synchronized wave of H1N1pdm in early 2011 in the Northern Hemisphere countries, and a pattern of replacement of strain H1N1pre by H1N1pdm between the 2009 and 2012 influenza seasons. Using both a statistical and a mechanistic mathematical model, and through fitting the data of 108 countries, we discuss the mechanisms that are likely to generate these events taking into account the role of multi-strain dynamics. A basic understanding of these patterns has important public health implications and scientific significance.

Figure 1. Spatio-temporal patterns of H1N1pdm and H3N2.

(Panels a–h) Square-root of weekly lab-confirmed cases of H1N1pdm (red triangle) and H3N2 (blue circle) in eight geographical regions between January 2009 and January 2015. Black arrows indicate ‘skip’ seasons for H1N1pdm. Northern America (panel a) exhibits annual epidemics without a skip; Central America (panel b) exhibits seemingly biennial epidemic with a skip in both 2010/11 and 2012/13 seasons; South America (panel c) is irregular in pattern; Eastern Asia (panel d) and Europe (panel e–h) exhibit annual epidemics with a skip of H1N1pdm and a substantial epidemic of H3N2 during the 2011/12 season. Panel (i) summarises the global pattern during the 2011/12 season. Periodicity was estimated based on Fourier spectrum. The map is made with the free software R (http://www.r-project.org) and the country borders are from Sandvik B., World Borders Dataset http://thematicmapping.org (2009), date of access: 11/04/2015).

Figure 2. Countries where H1N1pdm skipped the 2011/12 season (ordered in latitude from North to South).

Countries with α₁ < log1/10 are coded in red and countries with log1/10 < α₁ < log1/5 are in blue. The color (grey) scheme is in a square-root scale.

Figure 3. Fitting an SEIR model, with a cubic-spline function in the transmission rate, to influenza A confirmations in 10 countries.

Panel a, a flowchart of the model. Panel (b–k), the results in 10 countries. Each panel shows the simulation (red) versus the observed (black), with the best fitting parameters. The dotted vertical lines indicate the two timings for the reporting ratio changes. The simulations are median values for each week of 1000 simulations and shaded region show the 95% range. The inset panel shows the profile log-likelihood for the duration of immunity.

Figure 4. Distribution of Mean Confirmation Time for three strains, H1N1 (H1N1pre and H1N1pdm), H3N2 and Influenza B, in Northern Hemisphere in eight flu seasons.