Influenza vaccine effectiveness in the tropics: moderate protection in a case test-negative analysis of a hospital-based surveillance population in Bangkok between August 2009 and January 2013.

Influenza in the tropics occurs year round with peaks that correspond variably to temperate regions. However, data on influenza vaccine effectiveness (VE) in the tropics is sparse. We report on the effectiveness of influenza vaccine to prevent medically attended laboratory confirmed influenza from sentinel surveillance conducted at a Thai military medical facility in Bangkok, Thailand from August 2009 to January 2013. Patients ≥6 months old presenting with influenza-like illness underwent combined nasal/throat swabs which were tested by influenza RT-PCR. A case test-negative study design was used to evaluate VE. Of 2999 samples available for analysis,1059 (35.3%) were PCR-positive (cases) and 1940 (64.6%) were PCR-negative (test-negative controls). Five hundred and seven (16.9%) of these patients reported being vaccinated within the previous 12 months. Periods of high and low influenza activity were defined based on publicly available Thai Ministry of Public Health data. Overall VE adjusted for age and epiweek was found to be 50.1% (95%CI: 35.0, 61.9%). The May to April adjusted VE for year 2010, 2011 and 2012 was 57.7% (95%CI: 33.7, 73.8%), 57.1% (95% CI: 35.2, 68.3%) and 37.6% (95% CI: 3.5, 62.9%).During high influenza activity in years with the same vaccine formulation, the adjusted VE was 54.9% (95%CI: 38.9, 66.9%). VE appeared to be much higher during high versus low influenza activity periods. The adjusted point estimate for VE was highest in the 18-49 year age group (76.6%) followed by 6-23 months (58.1%) and 2-17 years (52.5%). Adjusted estimates were not done for those ≥50 years of age due to small numbers. VE in patients with underlying disease was 75.5% compared to 48.0% in those without. Our findings demonstrate moderate protection by influenza vaccination and support the utility of influenza vaccination in the tropics including in very young children and those with underlying disease

Estimates of Influenza vaccine effectiveness for subject in PMK surveillance between August 2009 and January 2013.

<p>* Adjusted for age using recursive spline and epiweek</p><p>** Model produced infinite or undefined confidence intervals</p><p>^‡ Model did not converge</p><p>Estimates of Influenza vaccine effectiveness for subject in PMK surveillance between August 2009 and January 2013.</p

Univariate characteristics of PMK influenza surveillance subjects enrolled between August 2009 and January 2013.

<p>* P-value from Chi-square test does not include missing category</p><p>Univariate characteristics of PMK influenza surveillance subjects enrolled between August 2009 and January 2013.</p

Circulating influenza virus strains relative to trivalent inactivated vaccine strain in Thailand including the study period.

<p>V = Influenza B Victoria linage; Y = Influenza B Yamagata lineage. Table adapted from Influenza viruses in Thailand: 7 years of sentinel surveillance data, 2004–2010, Chittaganpitch et al. Influenza and Other Respiratory Viruses DOI:<a href="http://dx.doi.org/10.1111/j.1750-2659.2011.00302.x" target="_blank">10.1111/j.1750-2659.2011.00302.x</a>.(years 2009–2010) and updated with data from <a href="http://www.thainihnic.org/influenza/main.php?option=newsletter" target="_blank">http://www.thainihnic.org/influenza/main.php?option=newsletter</a> (years 2011 to 2013). Note: Northern Hemisphere vaccine strains did not differ from southern vaccine strains after the inclusion of A/California/7/2009(H1N1) in April 2010.</p><p>Circulating influenza virus strains relative to trivalent inactivated vaccine strain in Thailand including the study period.</p