Vaccination clinics serve a major role in mass vaccination campaigns, as in the COVID-19 pandemic. The requirement of vaccinating as many people as possible in the shortest possible time calls for lowering the time spent by patients in the clinic. The throughput time is then a major Quality-of-Service (QoS) metric. So far, just the average value has been considered as an overall metric. We show that average values may mask significant differences in the quality received by different patients and advocate the introduction of QoS metrics for different segments of the population of patients, in particular by examining the tail distribution of throughput times.