BackgroundMicroRNAs (miRNAs) are a class of small regulatory RNAs around 21-25 nucleotides in length which govern many aspects of immunity including the host innate and adaptive responses to infection. RT-qPCR studies of select microRNAs show that vaccination alters the expression circulating microRNAs but the effect of vaccination on the global microRNA population (i.e. micronome) has never been studied.AimTo describe vaccine associated changes in the expression of microRNAs 21 days after vaccination in children receiving a pandemic influenza (H1N1) vaccination.MethodSerum samples were obtained from children aged 6 months to 12 years enrolled in an open label randomised control trial of two pandemic influenza (H1N1) vaccines, in which participants received either ASO3B adjuvanted split virion or a whole virion non-adjuvanted vaccine. MicroRNA expression was profiled in a discovery cohort of participants prior to, and 21 days after vaccination using an Agilent microarray platform. Findings were followed up by RT-qPCR in the original discovery cohort and then in a validation cohort of participants taken from the same study.Results44 samples from 22 children were assayed in a discovery cohort. The microarray results revealed 19 microRNAs were differentially expressed after vaccination after adjustment for multiple testing. The microarray detected ubiquitous expression of several microRNAs which could not be validated by RT-qPCR, many of which have little evidence of existence in publicly available RNA sequencing data. Real time PCR (RT-qPCR) confirmed downregulation of miR-142-3p in the discovery cohort. These findings were not replicated in the subsequent validation cohort (n = 22).ConclusionThis study is the first study to profile microRNA expression after vaccination. An important feature of this study is many of the differentially expressed microRNAs could not be detected and validated by RT-qPCR. This study highlights the care that should be taken when interpreting omics biomarker discovery, highlighting the need for supplementary methods to validate microRNA microarray findings, and emphasises the importance of validation cohorts. Data from similar studies which do not meet these requirements should be interpreted with caution.