Effectiveness of the 2010 and 2011 Southern Hemisphere trivalent inactivated influenza vaccines against hospitalization with influenza‐associated acute respiratory infection among Thai adults aged ≥50 years

Abstract

BackgroundInactivated influenza vaccine (IIV) effectiveness has been evaluated among older adults in high-income countries, but data on IIV effectiveness in low- and middle-income countries remain sparse. We conducted a test-negative case–control analysis to estimate 2010 and 2011 trivalent IIV effectiveness against hospitalization with influenza-associated acute respiratory infection (ARI) among persons aged ≥50 years in rural Thailand.MethodsDuring 2010–2011, active surveillance for ARI hospitalization was conducted in two provinces; patients were tested for influenza viruses by real-time RT-PCR. Vaccination status was obtained from vaccine registries. Case and control patients were patients with nasopharyngeal swabs positive and negative for influenza viruses, respectively. Vaccine effectiveness (VE) was estimated for the 6 months after vaccination began. Logistic regression was used to evaluate the association between case status and vaccination while adjusting for age, province, medical conditions, and time.ResultsDuring 2010–2011, there were 1545 patients with ARI, of whom 279 (18%) were influenza-positive case patients and 1266 (82%) were influenza-negative control patients. Of the 279 case patients, 247 (89%) had influenza A and 32 (11%) had influenza B. Fourteen of 279 (5%) case patients and 108 of 1266 (9%) control patients were vaccinated against influenza. The unadjusted IIV effectiveness against hospitalization with influenza-associated ARI was 43% (95% CI: 0–68%); adjusted VE was 47% (95% CI: 5–71%).ConclusionThe 2010 and 2011 IIVs were moderately effective against hospitalization with influenza-associated ARI among Thais aged ≥50 years, but IIV coverage was low. Additional efforts are warranted in Thailand to improve IIV uptake in this target group.

Potential of the test-negative design for measuring influenza vaccine effectiveness: a systematic review

“…The majority of studies made VE estimates for patients of all ages (n=48; one excluded children <10y [85]), with the remainder targeted at adults (≥15y; n=12; 2 restricted to adults ≥50y [27, 87]), children (n=9 [19, 26, 29, 31, 47, 59, 62, 81, 94]), elderly (n=4 [51, 53, 72, 95]), children and adults aged 50+ (n=1 [84]), pregnant women (n=1 [89]), military (n=2 [28, 40]), healthcare workers (n=1 [42]) or people at high-risk or in a target group for vaccination (n=7 [14, 18, 20, 25, 43, 44, 52]).…”

“…Thus, many studies limited the recruitment of patients to those presenting soon after illness onset to reduce the likelihood of a false negative test result (reduced sensitivity). Swabs were taken from patients presenting within 29 days of symptom onset (n=3 [38, 63, 107]), 10 days (n=2 [108, 88]), 9 days (n=2 [18, 25]), 8 days (n=1 [89]), 7 days (n=29), 5 days (n=1 [109]) 4 days (n=10), 3 days (n=7 [27, 28, 40, 44, 46, 47, 72]). Twenty-six studies did not restrict or did not specify how they restricted patients.…”

“…This variable was described as month (n=21), week (n=24) or some other “period” relative to the start [87] or peak of the season (n=3 [27, 31, 91]) or variously defined (n=10 [16, 20–22, 31, 39, 40, 48, 58, 89]). It was often unclear whether the variable was specified as a categorical or linear term.…”

Assessment of potential public health impact of a quadrivalent inactivated influenza vaccine inThailand

“…In particular, we found few estimates of vaccine effectiveness for countries in the Southern Hemisphere in general, and Thailand in particular. There were only two published articles and one abstract that reported the effectiveness of Southern Hemisphere IIV3 used in Thailand, and these reports were limited to key risk groups . Population‐based estimates of influenza incidence in Asia are also limited …”

“…The vaccine effectiveness of the Southern Hemisphere IIV3 was estimated from published data from countries that used Southern Hemisphere vaccines . The estimated vaccine effectiveness by influenza season was calculated by averaging all vaccine effectiveness data from a given season.…”