Epidemiology and timing of seasonal influenza epidemics in the Asia-Pacific region, 2010-2017: implications for influenza vaccination programs

Abstract

Background: Description of the epidemiology of influenza is needed to inform influenza vaccination policy. Here we examined influenza virus circulation in countries in the Asia-Pacific region and compared the timing of seasonal epidemics with the timing of influenza vaccination.

Methods: Data were obtained from the World Health Organization (WHO) FluNet database for 2010-2017 for countries in the WHO Asia-Pacific region. Data from countries covering ≥5 consecutive seasons and ≥ 100 influenza positive cases per year were included. Median proportions of cases for each influenza virus type were calculated by country and season. The timing and amplitude of the epidemic peaks were determined by Fourier decomposition. Vaccination timing was considered appropriate for each country if it was recommended ≤4 months before the primary peak of influenza circulation.

Results: Seven hundred eleven thousand seven hundred thirty-four influenza cases were included from 19 countries. Peak circulation coincided with the winter seasons in most countries, although patterns were less clear in some countries in the inter-tropical area due to substantial secondary peaks. Influenza A/H3N2 dominated overall, but proportions of A and B strains varied by year and by country. Influenza B represented 31.4% of all cases. The WHO-recommended timing for influenza vaccination was appropriate in 12 countries. Vaccination timing recommendations were considered inappropriate in Laos, Cambodia, and Thailand, and were inconclusive for India, Sri Lanka, Singapore, and Vietnam due to unclear seasonality of influenza virus circulation.

Conclusions: Influenza virus circulation varied considerably across the Asia-Pacific region with an unusually high burden of influenza B. The recommended timing for vaccination was appropriate in most countries, except for several countries with unclear seasonality, mainly located in the inter-tropical area.

Keywords: Asia-Pacific; Epidemiology; FluNet; Influenza; Vaccination.

Fig. 1

Selection of countries included in the analysis. Influenza surveillance data from week 1 of 2010 to week 52 of 2017 were extracted from FluNet for each of the 53 countries included in the World Health Organization Eastern Asia, Southern Asia, South-East Asia, and Oceania-Melanesia-Polynesia influenza transmission zones. Countries were excluded if they did not have data for at least five consecutive years, with each year having ≥100 laboratory-confirmed influenza cases

Fig. 2

Influenza transmission zones included in the analysis. Countries shaded in grey were not included in the analysis. Abbreviations: AUS = Australia, BGD = Bangladesh, CHN=China, IDN=Indonesia, IND = India, IRN=Iran, JPN = Japan, KHM = Cambodia, KOR = South Korea, LAO = Laos, LKA = Sri Lanka, MNG = Mongolia, NPL = Nepal, NZL = New Zealand, PAK=Pakistan, PHL = Philippines, SGP=Singapore, THA = Thailand, VNM = Vietnam. Map generated using mapchart [16] software

Fig. 3

Circulation of influenza strains by country and overall

Fig. 4

Circulation of influenza strains by year

Fig. 5

Relationship between the timing and amplitude of the primary peak (left panel) and the secondary peak (right panel) of influenza in each country between 2010 and 2017 and the latitude of the country centroid. The annual, semi-annual, and quarterly harmonics obtained from Fourier decomposition were summed to generate a periodic annual function describing the peak of activity and intensity of the seasonality. Latitude of each country centroid were from the US Central Intelligence Agency World Fact Book [15]. The size of each spot is in proportion to the amplitude for the country. Abbreviations: AUS = Australia, BGD = Bangladesh, CHN=China, IDN=Indonesia, IND = India, IRN=Iran, JPN = Japan, KHM = Cambodia, KOR = South Korea, LAO = Laos, LKA = Sri Lanka, MNG = Mongolia, NPL = Nepal, NZL = New Zealand, PAK=Pakistan, PHL = Philippines, SGP=Singapore, THA = Thailand, VNM = Vietnam