['Human bocavirus (HBoV) is a widespread human parvovirus causing acute respiratory illness in young children. The HBoV primary infections are viremic and can be diagnosed serologically.', 'To set up HBoV-IgG-avidity enzyme immuno assays (EIAs) using as antigen recombinant VP2 virus-like particles (VLPs), for diagnosis and timing of primary infections and their distinction from secondary infections or immunoactivations by this recently found virus.', 'The VLPs were utilized in setting up HBoV-IgG-avidity-EIAs of two different types. Paired sera were available from 36 wheezing children with acute primary HBoV infection, single sera from 108 nonsymptomatic university students, and 84 single or follow-up sera from 38 adults with pre-existing HBoV immunity.', 'HBoV-IgG avidity for the VP2-VLPs was measured successfully by protein-denaturing EIAs of two types, employing low concentrations of urea (4.7M and 2.5M). The diagnostic specificities were 99.1% and 90.7%, and diagnostic sensitivities, 94.4% and 91.7%, respectively. Interestingly, of the adults followed up 44% (4/9) exhibited significant titre increases of past-immunity HBoV-IgG.'] Are dating of human bocavirus infection with protein-denaturing IgG-avidity assays-Secondary immune activations ubiquitous in immunocompetent adults?