Sara van den Berg

59 3 CMV on influenza vaccination: a systematic review exposed cohort and selection of the non-exposed cohort from the same population. Many different ways of reporting the influenza vaccine outcome were used and the quality of the reported outcome varied between the studies. All cohort studies measured the outcome of interest also before vaccination (pre-vaccination titers), which granted them an additional star compared to the three cross-sectional studies. In five studies, influenza titers were assessed using ELISA, which is generally regarded a less reliable method than the hemagglutination inhibition (HI) assay [43]. All studies had an adequate follow-up time, which was considered to be more than two weeks for humoral immunity outcomes [44, 45]. Figure 2. PRISMA-flow diagram of identification and selection of studies .