Sex- and age-specific incidence and fraction of hospital contacts for infectious mononucleosis cases in Denmark in 1977–2011

Purpose Infectious mononucleosis (IM) is a common presentation of primary Epstein-Barr virus (EBV) infection in adolescence and later, but rare in childhood. Occasionally, IM leads to a hospital contact with an IM diagnosis (hospIM). Knowing the sex-, age- and period- specific probability of IM leading to hospIM allows estimating the overall sex-, age- and period-specific incidence of IM from inverse probability weighting of hospIM occurrences.Methods The probability of hospIM upon IM (hospfrac) was modelled in the Danish Blood Donor Study (DBDS) among persons with self-reported IM at age 0-29 years in 1977 to 2011. The probability of hospIM upon IM was modelled by sex-stratified generalized linear regression modelling of simple functions of age and calendar period with the aim of obtaining a simple, explicit, and transportable model for inverse probability weighting of hospIM occurrences.Results Among 10,619 eligible IM cases, 937 (9%) had hospIM. Hospfrac peaked at around age 5 years with a nadir at age 12 years. No cohort trends in IM incidence were found.Conclusion Absent IM cohort trends implied that childhood EBV sero-conversion patterns could be assumed stable from the 1960'ies. Previous detailed modelling of sex- and age-specific IM incidence for Denmark was extended and improved. Assumed incomplete recall of IM made the exact shape of hospfrac and IM incidence in childhood uncertain. IM and hospIM varied substantially by calendar period, likely reflecting both changing clinical practice and limited recall of events occurring more than 15 years earlier.