Epidemiological and virological characteristics of influenza in the Western Pacific Region of the World Health Organization, 2006-2010

Abstract

Background: Influenza causes yearly seasonal epidemics and periodic pandemics. Global systems have been established to monitor the evolution and impact of influenza viruses, yet regional analysis of surveillance findings has been limited. This study describes epidemiological and virological characteristics of influenza during 2006-2010 in the World Health Organization's Western Pacific Region.

Methodology/principal findings: Influenza-like illness (ILI) and influenza virus data were obtained from the 14 countries with National Influenza Centres. Data were obtained directly from countries and from FluNet, the web-based tool of the Global Influenza Surveillance and Response System. National influenza surveillance and participation in the global system increased over the five years. Peaks in ILI reporting appeared to be coincident with the proportion of influenza positive specimens. Temporal patterns of ILI activity and the proportion of influenza positive specimens were clearly observed in temperate countries: Mongolia, Japan and the Republic of Korea in the northern hemisphere, and Australia, New Zealand, Fiji and New Caledonia (France) in the southern hemisphere. Two annual peaks in activity were observed in China from 2006 through the first quarter of 2009. A temporal pattern was less evident in tropical countries, where influenza activity was observed year-round. Influenza A viruses accounted for the majority of viruses reported between 2006 and 2009, but an equal proportion of influenza A and influenza B viruses was detected in 2010.

Conclusions/significance: Despite differences in surveillance methods and intensity, commonalities in ILI and influenza virus circulation patterns were identified. Patterns suggest that influenza circulation may be dependent on a multitude of factors including seasonality and population movement. Dominant strains in Southeast Asian countries were later detected in other countries. Thus, timely reporting and regional sharing of information about influenza may serve as an early warning, and may assist countries to anticipate the potential severity and burden associated with incoming strains.

Figure 1. WHO's Western Pacific Region with National Influenza Centres and WHO Collaborating Centres on influenza, and years of ILI data contributed by each country/area.

Figure 2

Proportion of influenza specimens reported by Western Pacific Region countries to GISN, 2006–2010 ^∧ . ^∧ The proportion of contribution of viruses reported to FluNet from NICs in the Western Pacific Region ranged from 25–43% during 2006–2010. * Other: Viet Nam, Mongolia, New Zealand, Cambodia, Malaysia, New Caledonia (France), Fiji and Lao PDR.

Figure 3. Proportion of specimens positive for influenza and proportion of consultations meeting ILI case definition in Western Pacific Region countries, 2006–2010.*

^∧ *Panel A: Mongolia and Republic of Korea; Panel B: China; Panel C: Cambodia, Lao PDR, Malaysia, Philippines, Singapore, and Viet Nam; and Panel D: Australia, Fiji, New Caledonia (France), and New Zealand. ^∧ Panel D includes proportion of specimens positive for influenza for all four countries, but ILI ratios only for Australia.

Figure 4. Number of influenza viruses by type/subtype and proportion of specimens positive for influenza in Western Pacific Region countries, 2006–2010.*

^∧ * Panel A: Mongolia and Republic of Korea; Panel B: China; Panel C: Cambodia, Lao PDR, Malaysia, Philippines, Singapore, Viet Nam; and Panel D: Australia, Fiji, New Caledonia (France), New Zealand. ^∧ The number of reported A(H1N1)pdm09 viruses was very high compared to other subtypes/lineages. Thus, to better illustrate the patterns for other subtypes/lineages, the A(H1N1)pdm09 numbers were divided by five.