There are a few limitations in this study when interpreting the study findings. First, we could only infer the type of vaccine (Northern or Southern) that enrolled children received because it was not documented on the vaccination cards or medical records. Compositions of the 2013–2014 Northern Hemisphere and 2013 Southern Hemisphere vaccines were the same for the A(H1N1) and A(H3N2) virus components, but different for influenza B, while those of 2014–2015 Northern Hemisphere and 2014 Southern Hemisphere vaccines were identical. The vaccination card should accommodate recording of information regarding vaccine formulation (Northern vs. Southern Hemisphere) and future studies should collect this information to improve the accuracy of VE estimates. Second, despite conducting the study at the largest pediatric hospital in Thailand, this study did not have adequate statistical power to assess VE by age or influenza type because of low virus activity and vaccination coverage. Future evaluations of influenza VE in Thailand should consider using a multisite network approach to optimize the chances of enrolling a sample size large enough to evaluate VE by age and virus type/subtype. Lastly, we only reported influenza type, subtype and lineage of influenza virus detected in this study, but antigenic characterization of influenza viruses detected was not performed.
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