Waning immunity could also explain our effectiveness estimate. Those who were vaccinated more than 10 years earlier were at greater risk of developing mumps than

those vaccinated later, this simple analysis is however limited, since no correction for possible confounding factors is done. Other studies report diverse results on waning immunity. A 2003 Belgian study and a 2006 study in the USA, both in outbreak settings, reported that protection against mumps declined with increasing time since last vaccination [6], [31] and [32]. A specific second sample of students frequently working in bars was compared to the first random sample of students. The main purpose of this design was to evaluate if dense social contacts would KRX-0401 mouse affect attack rates. We felt that the response rate on our survey would suffers from questions such as time spent in student bars and also that the

quality of answers on such questions might be low. We therefore selected a second cohort. This second cohort worked in student bars for 2–3 evenings a week. This was used as a proxy for dense social contacts. Differentiating student bar workers from the other students in the first sample would have also been possible, but see more would have required a much larger first sample, since only a small proportion of students worked in bars. No students were present in both cohorts. It is possible that confounders were present as the second cohort might differ from the general student population on more than working in bars often crowded with a lot of peers. Age, gender and vaccination coverage were however comparable between cohorts. We found a higher attack rate in students working in student bars as compared to the general student population.

Other studies in populations with a high coverage of two doses of mumps-containing vaccine have also reported close and prolonged social contacts as an important risk factor for transmission [9]. Intense social contacts in close environments may contribute to over come vaccine-induced protection. Terminal deoxynucleotidyl transferase Avoiding these whilst infectious will limit the spread of a mumps outbreak. An important limitation of such a control measure is however that persons might be infectious up to 6 days before exhibiting symptoms [33]. The specific contribution of social activities in overcoming vaccine induced protection, certainly if this protection is incomplete due to vaccine effectiveness, incomplete coverage and waning, is a topic for further research. Our study is subject to certain limitations. First, our use of self-reported clinical symptoms de facto consisted in parotitis surveillance. Mumps can be asymptomatic, without parotitis, and on the other hand parotitis can be caused by other pathogens, especially when incidence of other respiratory infections is high.