LAUSR

We were pleased to find that the results provided by Navalpotro Rodríguez et al.1 coincide with our data which were published recently in the EIMC journal.2 In their study, more than two-thirds of IgM-negative mumps cases—which obtained positive results with a Real-Time Polymerase Chain Reaction (RT-PCR)—showed elevated specific IgG levels (understood to be above the measurement limit of the chemiluminescence technique employed).1 Unlike what happens with the other components of the MMR vaccine, in those that have international units of IgG (mIU/ml for measles and IU/ml for rubella), which enable the comparison of serological results from different studies,3 in the case of mumps there is no standard serum that can be referred to in international units.4 Moreover, the quantitation of IgG is expressed in terms of titres or arbitrary units relating to the techniques used.1,2 Furthermore, the difficulties regarding the standardisation of quantitation methods for IgG in mumps4 may hinder the comparison of data provided by different laboratories.5 The fact that approximately a quarter of the cases which are negative with RT-PCR will also present a high degree of positivity, can perhaps be partly explained by the trend in the results obtained by laboratory tests for the diagnosis of mumps. RT-PCR techniques prove more sensitive in the early phases, following the onset of symptoms6,7 but may come back negative as the infection advances. Thus, a negative RT-PCR result (in the late stages) does not definitively rule out infection. IgM detection improves from the second week, but lacks sensitivity in the vaccinated population.6,7 Identifying elevated levels of specific IgG may increase this sensitivity. However, raised IgG levels may of course not prove too specific. The current Spanish vaccination schedule involves administering two doses of the MMR vaccine at 12 months and 3–4 years of age. Between 2007 and 2016, vaccination coverage in older children was sustained at 95% with the first dose and 90% with the second dose.8 In our field, the levels of seroprevalence against mumps in young adults are approaching 90%.9 However, despite this, mumps continues to appear in a cyclic presentation in Spain.10 The emergence of periodic epidemic waves may lead to a “booster” effect in vaccinated individuals which