RSV and HMPV seroprevalence in Tuscany (Italy) and North-Rhine Westfalia (Germany) in the winter season 2009/2010 Maria G. Cusi, a Chiara Terrosi, a Michael Kleines, b Oliver Schildgen c a Department of Biotechnologies, Microbiology Section, University of Siena, Siena, Italy. b Division of Virology, Medical University of Innsbruck, Innsbruck, Austria. c Institute for Pathology, Kliniken der Stadt Ko ¨ln gGmbH, Klinikum der Privaten Universita ¨t Witten / Herdecke. Correspondence: Priv.-Doz. Dr. rer. nat. Oliver Schildgen, Dipl.-Biologe, Kliniken der Stadt Ko ¨ln gGmbH, Krankenhaus Merheim, Klinikum der Privaten Universita ¨t Witten / Herdecke, Institut fu ¨r Pathologie, Ostmerheimer Str. 200, D-51109 Ko ¨ln, Cologne, Germany. E-mail: schildgeno@kliniken-koeln.de Accepted 7 March 2011. Published Online 15 April 2011. Keywords Human metapneumovirus, HMPV, human respiratory syncytial virus, RSV, seroprevalence. To the editor: RSV and HMPV are the major contributors to mild to severe respiratory infections in all age groups. Most infec- tions are recognized in children and frequent re-infections are observed for both viruses [reviewed in 1,2 ]. The seropre- valence for both viruses was described to be age – depen- dent, and the rate of seropositive individuals was significantly higher in subjects older than 10 years. Surpris- ingly, in a recent large study concerning the HMPV sero- prevalence in a German cohort of Bonn, 3 the neutralizing immune response was shown not to correspond to the overall seroprevalence described in previous studies. 4,5 Tak- ing these data into account, we hypothesised that this dis- crepancy could be due to geographical differences, consequently, we prospectively analysed the seroprevalence in children against both viruses during the winter / spring season 2009 / 2010 in two different regions of Europe, Northrhine-Westfalia, Germany (85 serum samples), and Tuscany, Italy (86 serum samples). Unlike the study of Matsuzaki et al., 4 who used a neutralization assay, and Liu et al., 5 who used an in-house recombinant ELISA, the serum samples in the present study have been investigated for the presence of antibodies by indirect immunofluores- cence, as previously described. 6 It appears that only mar- ginal differences were observed in the seropositivity rates of the corresponding groups aged 1-5 years (mean age 2,5 for Italian children and 1,5 for German children) and 6– 10 years (mean age 7,5 for all the children), respectively, of the different geographic provenances (Table 1). It is worthy to note that the sera were drawn from infants older than 6 months, in order to exclude the possibility of an overesti- mation of seropositives due to the presence of specific maternal antibodies. In the youngest group, the HMPV seroprevalence in the two countries was indistinguishable (48Æ8%), whereas the RSV seropositivity rate was slightly higher in the German group, but without a significant dif- ference (P >0Æ05). In the oldest group, the seropositivity rate was similar for both the viruses (P >0Æ05). The data confirm that RSV infection occurs earlier in life, and show that HMPV seropositivity is significantly lower than the RSV seropositivity rate in the cohort of 1– 5 year old children (P <0Æ05). This difference is no more evident in the oldest group of children, indicating that HMPV infection may occur at a later time and most of the young population can be infected by hMPV after the 5th year of life. 7,8 However, the difference in the youngest group of subjects is somehow surprising as there is no sig- nificant epidemiological difference of both viruses in the areas considered. 9,10 Therefore, it appears that the circula- tion of these viruses in these two countries of Europe, which have different climates, is similar and that RSV spreads very efficiently among infants. Moreover, these data confirm that the patterns of RSV and hMPV infection in Italy are similar to those reported for other countries in the northern hemisphere (P <0Æ05). Finally, the lower se- roprevalence to hMPV observed in the youngest German group seems discordant with the analysis performed by Lu ¨sebrink et al. 3 In the previous study, the hMPV seropre- valence, tested by XTT-based neutralization assay, was 93Æ1% in the group of children aged 0–2 years and 100% in the group of children aged 3–9 years. However, this result could be explained by the fact that in this study, we analysed the seroprevalence in young children by IFA, while, in the previous study, it was evaluated by the neutralization test. In this assay, the neutralizing activity against hMPV was evaluated in the presence of DOI:10.1111/j.1750-2659.2011.00252.x www.influenzajournal.com Letter to the Editor 380 ª 2011 Blackwell Publishing Ltd, Influenza and Other Respiratory Viruses, 5, 380–381