On Temporal Patterns and Circulation of Influenza Virus Strains in Taiwan, 2008-2014: Implications of 2009 pH1N1 Pandemic

• Published in: PloS one 2016-05, Vol.11 (5), p.e0154695-e0154695
• Main Authors: Hsieh, Ying-Hen, Huang, Hsiang-Min, Lan, Yu-Ching
• Format: Article
• Language: English
• Subjects: Analysis; Biology and life sciences; Circulation patterns; Disease control; Distribution; Epidemics; Evolution; Growth rate; Historical account; Humans; Immunization; Influenza; Influenza A; Influenza A Virus, H1N1 Subtype - physiology; Influenza B; Influenza virus; Influenza viruses; Influenza, Human - epidemiology; Mathematical models; Medicine and Health Sciences; Models, Statistical; Northern Hemisphere; Orthomyxoviridae; Pandemics; Pandemics - statistics & numerical data; People and Places; Physiological aspects; Prevalence studies (Epidemiology); Strains (organisms); Summer; Surveillance; Taiwan - epidemiology; Time Factors; Vaccines; Viruses; Waves; Winter
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0154695

It has been observed that, historically, strains of pandemic influenza led to succeeding seasonal waves, albeit with decidedly different patterns. Recent studies suggest that the 2009 A(H1N1)pdm09 pandemic has had an impact on the circulation patterns of seasonal influenza strains in the post-pandemic years. In this work we aim to investigate this issue and also to compare the relative transmissibility of these waves of differing strains using Taiwan influenza surveillance data before, during and after the pandemic. We make use of the Taiwan Center for Disease Control and Prevention influenza surveillance data on laboratory-confirmed subtyping of samples and a mathematical model to determine the waves of circulating (and co-circulating) H1, H3 and B virus strains in Taiwan during 2008-2014; or namely, short before, during and after the 2009 pandemic. We further pinpoint the turning points and relative transmissibility of each wave, in order to ascertain whether any temporal pattern exists. For two consecutive years following the 2009 pandemic, A(H1N1)pdm09 circulated in Taiwan (as in most of Northern Hemisphere), sometimes co-circulating with AH3. From the evolution point of view, A(H1N1)pdm09 and AH3 were able to sustain their circulation patterns to the end of 2010. In fact, A(H1N1)pdm09 virus circulated in six separate waves in Taiwan between summer of 2009 and spring of 2014. Since 2009, a wave of A(H1N1)pmd09 occurred every fall/winter influenza season during our study period except 2011-2012 season, when mainly influenza strain B circulated. In comparing transmissibility, while the estimated per capita weekly growth rates for cumulative case numbers (and the reproduction number) seem to be lower for most of the influenza B waves (0.06~0.26; range of 95% CIs: 0.05~0.32) when compared to those of influenza A, the wave of influenza B from week 8 to week 38 of 2010 immediately following the fall/winter wave of 2009 A(H1N1) pdm09 was substantially higher at r = 0.89 (95% CI: 0.49, 1.28), in fact highest among all the waves detected in this study. Moreover, when AH3 or A(H1N1)pdm09 exhibit high incidence, reported cases of subtype B decreases and vice versa. Further modeling analysis indicated that during the study period, Taiwan nearly experienced at least one wave of influenza epidemic of some strain every summer except in 2012. Estimates of R for seasonal influenza are consistent with that of temperate and tropical-subtropical regions, while estimate of