Community-based pathogen-specific incidence of influenza-like illness due to respiratory viruses in South-central Vietnam in 2009–2012, after a pandemic of influenza A viruses

Background. Influenza-like illness (ILI) is one of the most common illnesses caused by various respiratory viruses and brings high expense to household directly or indirectly. However, the pathogen-specific incidence and health-seeking behaviour in communities is not well described. Method. A longitudinal cohort study using a self-recorded health calendar among 1,000 households was performed in South-central Vietnam from October 2009 to September 2012. Endemic respiratory viruses in the community were monitored by random sampling in polyclinics. Monthly incidence of specific pathogens was calculated using Bayesian method. Findings. Among 5,016 household members, 3,687 ILI episodes were reported during study period. Monthly incidence of ILI was 21.7 (95% confidence interval 21.0–22.4) per 1,000 population for all ages and highest in children under 2 years old with 71.6 (64.7–81.8) followed by 2–4 years old with 71.3 (65.8–78.2). Rhinovirus had the highest incidence with 22.5 among the age under 2 years, followed by adenovirus and respiratory syncytial virus (RSV) with 12.5 and 9.9, respectively. The majority of individuals sought treatment from drug stores rather than healthcare facilities except for younger children. RSV outbreaks significantly increase the numbers of health care visit among children under 2 years but not in older age groups. Interpretation. Several surges of ILI were attributed by multiple respiratory viruses. Health care seeking patterns were varied by pathogens. Highly transmissible viruses like rhinovirus and adenovirus have potential risk of next pandemic.