The Epidemiology of Interpandemic and Pandemic Influenza in Vietnam, 2007–2010

Abstract

Prospective community-based studies have provided fundamental insights into the epidemiology of influenza in temperate regions, but few comparable studies have been undertaken in the tropics. The authors conducted prospective influenza surveillance and intermittent seroprevalence surveys in a household-based cohort in Vietnam between December 2007 and April 2010, resulting in 1,793 person-seasons of influenza surveillance. Age- and sex-standardized estimates of the risk of acquiring any influenza infection per season in persons 5 years of age or older were 21.1% (95% confidence interval: 17.4, 24.7) in season 1, 26.4% (95% confidence interval: 22.6, 30.2) in season 2, and 17.0% (95% confidence interval: 13.6, 20.4) in season 3. Some individuals experienced multiple episodes of infection with different influenza types/subtypes in the same season (n = 27) or reinfection with the same subtype in different seasons (n = 22). The highest risk of influenza infection was in persons 5–9 years old, in whom the risk of influenza infection per season was 41.8%. Although the highest infection risk was in school-aged children, there were important heterogeneities in the age of infection by subtype and season. These heterogeneities could influence the impact of school closure and childhood vaccination on influenza transmission in tropical areas, such as Vietnam.

Estimating age‐specific cumulative incidence for the 2009 influenza pandemic: a meta‐analysis of A(H1N1)pdm09 serological studies from 19 countries

“…This may be because our study includes a number of additional middle and low countries who conducted serologic studies because this analysis was published and because we excluded studies which focused on specialized populations17). Additionally, the age‐specific trends we found in our cumulative incidence results are consistent with studies which measured cumulative incidence as a fourfold increase in titers among paired sera36, 42, 57 and similar to studies which measured age‐specific secondary attack rates using RT‐PCR 58, 59…”

Trivalent and quadrivalent influenza vaccination effectiveness in Australia and South Africa: results from a modelling study

“…However, these studies cover at most 5 years and we have chosen to apply average values from these studies to all 11 years considered in our study. Horby et al 9. estimated an average annual infection rate of 21·5% and a symptomatic attack rate of 3·17% over 3 years (2007, 2008 and 2010); this study gives an estimate in a setting (Vietnam) where no seasonal influenza vaccination was used.…”

“…The best estimates available6, 9 are based on surveys that correlate serological RNA RT‐PCR testing and prospective community surveillance. However, these studies cover at most 5 years and we have chosen to apply average values from these studies to all 11 years considered in our study.…”

“…The specific viral strain and lineage for each subtype are updated on an bi‐annual cycle (for the Northern and Southern Hemispheres) based on currently circulating strains 8. Since 2001, two distinct influenza B lineages (Yamagata and Victoria) have cocirculated, with one or both lineages causing a significant proportion of influenza infection in each year 9, 10, 11, 12, 13, 14, 15. In years when the predominant B lineage is incorrectly predicted, or where it is correctly predicted but there is a significant proportion of the other lineage, the TIV for that year leaves vaccinated individuals vulnerable to influenza infection from the ‘missing’ B lineage, since cross‐protection between B lineages is limited 16.…”

Influenza A Virus Shedding and Infectivity in Households

“…We considered age, being an index case patient, and oseltamivir treatment as 3 factors that might influence infectivity and age, receipt of influenza vaccination (trivalent inactivated influenza vaccine, which included seasonal A[H1N1], A[H3N2], and B strains) as 2 factors that might influence susceptibility [2,3,18,[22][23][24] and estimated their effects. We also explored whether the presence of specific symptoms were correlated with individual infectivity and whether sex (using age-sex categories to allow for interactions), household interventions (face mask or hand hygiene), smoking, and the presence of chronic conditions were associated with the susceptibility of household contacts to infection.…”