Predictors of seasonal influenza vaccination among older adults in Thailand

<div><p>Background</p><p>In advance of a large influenza vaccine effectiveness (VE) cohort study among older adults in Thailand, we conducted a population-based, cross-sectional survey to measure vaccine coverage and identify factors associated with influenza vaccination among older Thai adults that could bias measures of vaccine effectiveness.</p><p>Method</p><p>We selected adults ≥65 years using a two-stage, stratified, cluster sampling design. Functional status was assessed using the 10-point Vulnerable Elders Survey (VES); scores ≥3 indicated vulnerability. Questions about attitudes towards vaccination were based on the Health Belief Model. The distance between participants’ households and the nearest vaccination clinic was calculated. Vaccination status was determined using national influenza vaccination registry. Prevalence ratios (PR) and 95% confidence intervals (CIs) were calculated using log-binomial multivariable models accounting for the sampling design.</p><p>Result</p><p>We enrolled 581 participants, of whom 60% were female, median age was 72 years, 41% had at least one chronic underlying illness, 24% met the criteria for vulnerable, and 23% did not leave the house on a daily basis. Influenza vaccination rate was 34%. In multivariable models, no variable related to functional status was associated with vaccination. The strongest predictors of vaccination were distance to the nearest vaccination center (PR 3.0, 95% CI 1.7–5.1 for participants in the closest quartile compared to the furthest), and high levels of a perception of benefits of influenza vaccination (PR 2.8, 95% CI 1.4–5.6) and cues to action (PR 2.7, 95% CI 1.5–5.1).</p><p>Conclusion</p><p>Distance to vaccination clinics should be considered in analyses of influenza VE studies in Thailand. Strategies that emphasize benefits of vaccination and encourage physicians to recommend annual influenza vaccination could improve influenza vaccine uptake among older Thai adults. Outreach to more distant and less mobile older adults may also be required to improve influenza vaccination coverage.</p></div

Forest plot of mean standardized scores* and 95% confidence intervals of the Health Belief Model constructs by vaccination status, Thailand (n = 581).

<p>*Standardized scores (Z scores) were calculated to account for the different number of items comprising each HBM constructs (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188422#pone.0188422.s001" target="_blank">S1 Table</a>), such that all constructs of HBM had a mean of 0 and standard deviation of 1.</p

Descriptive characteristics of survey participants by 2014 influenza vaccination status, Thailand^*.

<p>Descriptive characteristics of survey participants by 2014 influenza vaccination status, Thailand^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0188422#t001fn001" target="_blank">*</a>}.</p

Factors associated with vaccination among survey participants in 2014 (n = 581).

<p>Factors associated with vaccination among survey participants in 2014 (n = 581).</p

Comparison of incidence and cost of influenza between healthy and high-risk children <60 months old in Thailand, 2011-2015

<div><p>Introduction</p><p>Thailand recommends influenza vaccination for children aged 6 months to <36 months, but investment in vaccine purchase is limited. To inform policy decision with respect to influenza disease burden and associated cost in young children and to support the continued inclusion of children as the recommended group for influenza vaccination, we conducted a prospective cohort study of children in Bangkok hospital to estimate and compare influenza incidence and cost between healthy and high-risk children.</p><p>Methods</p><p>Caregivers of healthy children and children with medical conditions (‘high-risk’) aged <36 months were called weekly for two years to identify acute respiratory illness (ARI) episodes and collect illness-associated costs. Children with ARI were tested for influenza viruses by polymerase chain reaction. Illnesses were categorized as mild or severe depending on whether children were hospitalized. Population-averaged Poisson models were used to compare influenza incidence by risk group. Quantile regression was used to examine differences in the median illness expenses.</p><p>Results</p><p>During August 2011-September 2015, 659 healthy and 490 high-risk children were enrolled; median age was 10 months. Incidence of mild influenza-associated ARI was higher among healthy than high-risk children (incidence rate ratio [IRR]: 1.67; 95% confidence interval [CI]: 1.13–2.48). Incidence of severe influenza-associated ARI did not differ (IRR: 0.40; 95% CI: 0.11–1.38). The median cost per mild influenza-associated ARI episode was $22 among healthy and$25 among high-risk children (3–4% of monthly household income; difference in medians: -$1; 95$9 to $6). The median cost per severe influenza-associated ARI episode was$232 among healthy and $318 among high-risk children (26–40$352 to $571).</p><p>Conclusions</p><p>Compared to high-risk children, healthy children had higher incidence of mild influenza-associated ARI but not severe influenza-associated ARI. Costs of severe influenza-associated ARI were substantial. These findings support the benefit of annual influenza vaccination in reducing the burden of influenza and associated cost in young children.</p></div

Number of acute respiratory illnesses (ARI) and influenza-associated ARI among children enrolled in a pediatric respiratory infection cohort in Thailand.

<p>ARI: acute respiratory illness. *partial season.</p

Median cost per acute respiratory illness episode adjusted for distance from residence to the study site in children enrolled in a pediatric respiratory infection cohort in Thailand.

<p>Median cost per acute respiratory illness episode adjusted for distance from residence to the study site in children enrolled in a pediatric respiratory infection cohort in Thailand.</p

Incidence per 1,000 person-years of mild and severe influenza-associated acute respiratory illness in children enrolled in a pediatric respiratory infection cohort in Thailand^γ.

<p>Incidence per 1,000 person-years of mild and severe influenza-associated acute respiratory illness in children enrolled in a pediatric respiratory infection cohort in Thailand<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197207#t002fn002" target="_blank">^γ</a>.</p

One-way sensitivity analyses.

<p>Incremental cost-effectiveness ratios are reported in US dollars per quality adjusted life year gained. Ranges of values used are as follows: Cost of vaccine, 0.5–4.0 times current vaccine cost; Utility weight—well children, 0.73–0.95; Utility weight—children with influenza treated in outpatient department, 0.52–0.66; Utility weight—children with influenza treated in inpatient department, 0.05–0.58; % of second influenza vaccine doses, 0.29–0.45; Proportion of high-risk children, 0–1; % of influenza cases among unvaccinated high-risk children, 4.2–6.1; % of influenza cases among unvaccinated healthy children, 7.3–8.9; % of influenza cases among vaccinated high-risk children, 2.8–6.4; and % of influenza cases among healthy children, 4.6–5.0. ICER: incremental cost-effectiveness ratio; OPD, outpatient department; IPD, inpatient department.</p

Cost-effectiveness of inactivated seasonal influenza vaccination in a cohort of Thai children ≤60 months of age

<div><p>Background</p><p>Vaccination is the best measure to prevent influenza. We conducted a cost-effectiveness evaluation of trivalent inactivated seasonal influenza vaccination, compared to no vaccination, in children ≤60 months of age participating in a prospective cohort study in Bangkok, Thailand.</p><p>Methods</p><p>A static decision tree model was constructed to simulate the population of children in the cohort. Proportions of children with laboratory-confirmed influenza were derived from children followed weekly. The societal perspective and one-year analytic horizon were used for each influenza season; the model was repeated for three influenza seasons (2012–2014). Direct and indirect costs associated with influenza illness were collected and summed. Cost of the trivalent inactivated seasonal influenza vaccine (IIV3) including promotion, administration, and supervision cost was added for children who were vaccinated. Quality-adjusted life years (QALY), derived from literature, were used to quantify health outcomes. The incremental cost-effectiveness ratio (ICER) was calculated as the difference in the expected total costs between the vaccinated and unvaccinated groups divided by the difference in QALYs for both groups.</p><p>Results</p><p>Compared to no vaccination, IIV3 vaccination among children ≤60 months in our cohort was not cost-effective in the introductory year (2012 season; 24,450 USD/QALY gained), highly cost-effective in the 2013 season (554 USD/QALY gained), and cost-effective in the 2014 season (16,200 USD/QALY gained).</p><p>Conclusion</p><p>The cost-effectiveness of IIV3 vaccination among children participating in the cohort study varied by influenza season, with vaccine cost and proportion of high-risk children demonstrating the greatest influence in sensitivity analyses. Vaccinating children against influenza can be economically favorable depending on the maturity of the program, influenza vaccine performance, and target population.</p></div