RSV activity in Bologna presents the pattern described in temperate areas, with onset and peak during winter season. Meteorological conditions in the 4 weeks of peak activity for each season were similar to data presented by other authors [1–3].
These conditions are the mean minimum temperature 2-6°C and mean relative humidity 45-65% and are associated to the greatest activity of the virus [11–13].
In 2009–2010 the onset occurred later than the previous years; a delay in RSV epidemics during the season 2009–2010 has been observed in France by Casalegno et al . These authors observed that RSV epidemic started more gradually with a delayed peak and hypothesized that this delay might be partially caused by pandemic H1N1 Influenza in 2009 through viral interference and increased hygiene measures.
A study performed in Croatia  presented a biennial cycle of RSV outbreaks with one large and one small season; the correlation between RSV epidemic and climatic factors was observed only for the larger season.
In the present study we cannot demonstrate the effects of other respiratory viruses on RSV delayed peak since we included only RSV detection which can be performed in a few minutes at Paediatric Emergency Unit to all infants with respiratory symptoms.
Moreover, a study period including several additional winter seasons would probably be warranted in order to determine the overall patterns of RSV epidemic onset in Bologna and Emilia-Romagna and to confirm whether a biennial cycle of RSV outbreak is detectable in our geographic area.
RSV activity is greater at cold temperature because the virus is more stable in secretions by which it is transmitted. Moreover, cold temperature might drive populations indoor where RSV spreads more readily.
The correlation between mean minimum temperature and RSV epidemic activity persists in the three different time lags. We hypothesize that a decrease in the mean minimum temperature may determine a greater RSV activity in the following weeks associated with more prolonged permanence indoor, sometimes in overcrowded rooms; these conditions may determine an increase in RSV diffusion, which is followed by viral incubation (2–7 days) and lead to an increase in number of RSV infection.
The weak relationship between RSV epidemics and relative humidity can be explained by the small variability and the irregular weekly pattern of this parameter during the whole winter season in Bologna. This result is different from data collected in other countries such as the Netherlands , where relative humidity has greater seasonal fluctuations and the effect of relative humidity is more consistent. We also analyzed the correlation between RSV detection and air pollutants concentration. Many authors [5–7, 9] showed a positive correlation between fine particulate matter (PM10 and PM2,5) and morbidity for respiratory infections and other respiratory conditions such as asthma and chronic obstructive pulmonary disease. This correlation is explained by increased respiratory symptoms, reduced lung function and bronchial reactivity related to air pollution exposure.
Moreover, these effects are increased in paediatric population, especially young infants, because of their higher respiratory rate that increases air pollutants per kilogram of body weight exposure.
The correlation with RSV infection [8, 9] was found both for acute exposure (one week) and for subchronic and chronic exposure (30–60 days). We observed a similar correlation in our study between the weekly number of RSV detections and the mean particulate matter concentration of the week before. We examined only acute air pollution exposure (considering PM10 and PM2,5 concentration in the same week or in the week before) and not subchronic and chronic exposure because we considered the whole population who referred to our Paediatric Emergency Unit without considering differences in their residential address and the distance from the air pollution monitoring station.
Furthermore, the reduction of air pollution in urban areas could lead to an improvement in infants morbidity as it could determine a reduction in respiratory symptoms and individual susceptibility to respiratory infection.