Epidemiology and etiology of influenza-like-illness in households in Vietnam; it's not all about the kids!

Abstract

Background: Household studies provide opportunities to understand influenza-like-illness (ILI) transmission, but data from (sub)tropical developing countries are scarce.

Objective: To determine the viral etiology and epidemiology of ILI in households.

Study design: ILI was detected by active case finding amongst a cohort of 263 northern Vietnam households between 2008 and 2013. Health workers collected nose and throat swabs for virus detection by multiplex real-time RT-PCR.

Results: ILI was detected at least once in 219 (23.7%) of 945 household members. 271 (62.3%) of 435 nose/throat swabs were positive for at least one of the 15 viruses tested. Six viruses predominated amongst positive swabs: Rhinovirus (28%), Influenza virus (17%), Coronavirus (8%), Enterovirus (5%), Respiratory syncytial virus (3%), Metapneumovirus virus (2.5%) and Parainfluenza virus 3 (1.8%). There was no clear seasonality, but 78% of episodes occurred in Winter/Spring for Influenza compared to 32% for Rhinovirus. Participants, on average, suffered 0.49 ILI, and 0.29 virus-positive ILI episodes, with no significant effects of gender, age, or household size. In contrast to US and Australian community studies, the frequency of ILI decreased as the number of household members aged below 5 years increased (p=0.006).

Conclusion: The findings indicate the need for tailored ILI control strategies, and for better understanding of how local childcare practices and seasonality may influence transmission and the role of children.

Keywords: Active case finding; Cohort; Household transmission; Influenza-like-illness; Respiratory viruses; Vietnam.

Fig. 1

Frequency of ILI and virus-positive ILI according to characteristics of households and individuals. Results are presented as average episodes/participant in each x-axis category. Upper and lower dashed horizontal lines indicate averages for all participants combined for ILI and virus-positive ILI, respectively. The numbers of participants are shown below each category on the x-axis in parentheses.

Fig. 2

Respiratory viruses detected in swabs from ILI cases. Panel A shows the frequency of detection for each virus, whether detected as a single virus or in combination with others. Numbers above columns indicate the total number of episodes if there were participants who had multiple episodes. Panel B shows the composition of each swab. Numbers detected are indicated next to each virus or combination that was detected more than once.

Fig. 3

Viruses detected in swabs by age category. The proportion of swabs collected from each age group (see legend within the figure) that were positive for the most commonly detected viruses are shown. Dashed horizontal lines indicate the proportions of all swabs that were positive for each virus. Numerators are shown below each bar and denominators are shown in the legend. The inset shows the proportions that were positive for each influenza subtype.

Fig. 4

Distribution of ILI episodes by year and season. The y-axis represents values for lines in both panels whereas the opposite axis in panel A represents values for the stacked bars indicating influenza subtypes. The break in the x-axis indicates the H1N1 pandemic period that was excluded from this analysis.