Sara van den Berg

58 Chapter 3 stratified by influenza strains (H1N1, H3N2, Influenza B) and age groups (young and old), which led to multiple records per outcome variable for some studies ( Supplementary Figures 1-4 ). Figure 1. Investigated influenza antibody outcomes. The influenza antibody vaccine response is investigated in the context of CMV infection in this review in three ways, based on the variables as indicated by A, B and C . Outcomes are a) the geometric mean titer pre-/post-vaccination ratio (GMR) per CMV-serostatus group, b) the percentage of subjects with a response per CMV-serostatus group and c) the association between the post-vac- cination influenza antibody titers and CMV antibody titers. * 1 study reporting correlations (outcome c) did not correlate the post-vaccination titer, but the fold-increase. HAU = hemagglutination unit. 40 HAU = correlate of protection. Characteristics of the retrieved studies are summarized in Table 1 . Most studies were cohort studies, but some were primarily set up as a vaccine trial with a subgroup analysis for CMV-serostatus. For our research question, all studies could be considered as observational studies. The sample size differed between studies with a range from 37 – 731 participants, leading to a total of 2249 participants. The age groups differed between studies with a range of 19 – 97 years of age and in some studies both young and older adults were included [25, 26, 39-42]. The definition of older adults differed between studies. In this review, either >60 years or >65 years of age was used, as indicated per subgroup or outcome variable. Quality assessment of studies The risk of bias assessment led to an overall quality score per study. Twelve studies were rated as having a high quality, one an intermediate quality and four a low quality ( Table 2 ). Most studies scored high on the selection process (1) with good representativeness of the