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Transmission Dynamics of Seasonal Influenza in a Remote Island Population

2022; RELX Group (Netherlands); Linguagem: Inglês

10.2139/ssrn.4244743

1556-5068

Su Myat Han, Alexis Robert, Shingo Masuda, Takahiro Yasaka, Satoshi Kanda, Kazuhiro Komori, Nobuo Saito, Motoi Suzuki, Akira Endo, Marc Baguelin, Koya Ariyoshi,

COVID-19 epidemiological studies

Background: Seasonal influenza outbreaks remain an important public health concern, causing large numbers of hospitalizations and deaths among high-risk groups. Understanding the dynamics of individual transmission is crucial to design effective control measures and ultimately reduce the burden caused by influenza outbreaks. In this study, we analyzed surveillance data from Kamigoto Island, Japan, a semi-isolated island population, to identify the drivers of influenza transmission during outbreaks. Methods: We used rapid influenza diagnostic test (RDT)-confirmed influenza surveillance data from Kamigoto island, Japan and estimated age-specific influenza relative illness ratios over eight epidemic seasons (2010/11 to 2017/18). We reconstructed the probabilistic transmission trees (i.e., a network of who-infected-whom) using Bayesian inference with Markov-chain Monte Carlo method and then we performed a negative binomial regression on the inferred transmission trees to identify the factors associated with onwards transmission risk.Findings: Pre-school and school-aged children were most at risk of getting infected with influenza, with RIRs values consistently above one. The maximal RIR values were 5.99 (95% CI: 5.23, 6.78) in the 7-12 aged-group and 5.68 (95%CI: 4.59,6.99) in the 4-6 aged-group in 2011/12. The transmission tree reconstruction suggested that the number of imported cases were consistently higher in the most populated and busy districts (Tainoura-go and Arikawa-go) ranged from 10-20 to 30-36 number of imported cases across the seasons. The number of secondary cases generated by each case were also higher in the same districts as imported cases, with the highest individual reproduction number (Reff :1.2-1.7) across the seasons. Across all inferred transmission trees, the regression analysis showed that cases reported in districts with lower local vaccination coverage (incidence rate ratio IRR =1.45 (95% CI: 1.02, 2.05)) or higher number of inhabitants (IRR= 2.00 (95% CI: 1.89, 2.12)) caused more secondary transmissions. Being younger than 18 years old (IRR = 1.37 (95%CI: 1.21, 1.57) among 4-6 years old and 1.45 (95% CI: 1.33, 1.59) 7-12 years old) and infection with influenza type A (type B IRR= 0.83 (95% CI: 0.77, 0.90)) were also associated with higher numbers of onwards transmissions. However, conditional on being infected, we did not find any association between individual vaccination status and onwards transmissibility.Interpretation: Our study showed the importance of focusing public health efforts on achieving high vaccine coverage throughout the island, especially in more populated districts. The strong association between vaccine coverage in the region (including neighboring regions), and the risk of transmission may indicate the importance of achieving homogeneously high vaccine coverage. The individual vaccine status may not prevent onwards transmission, though it may reduce the severity or to infection.Funding: The study was funded WISE Program (Doctoral Program for World-leading Innovative & Smart Education) of Ministry of Education, Culture, Sports, Science and Technology. AR was supported by the National Institute for Health Research (NIHR200908).Declaration of Interest: The authors declare no conflicting interests.Ethical Approval: The research was approved by the institutional review boards of Kamigoto Hospital, Nagasaki University Research Ethics Committee (reference number 200619236) the London School of Hygiene and Tropical Medicine Research Ethics Committee (reference number 26706).