Respiratory Syncytial Virus-associated hospitalization in premature infants who did not receive palivizumab prophylaxis in Italy: a retrospective analysis from the Osservatorio Study

Background: Due to different social and epidemiological factors, the eligibility criteria to receive palivizumab prophylaxis may be different between countries, especially in "otherwise healthy" late preterm infants. Methods: We analyzed an Italian database of young children referred to emergency departments for acute lower respiratory tract infection (ALRI) during the RSV season over a four year period, when the use of palivizumab as prophylaxis for RSV disease was not widespread in premature infants. The demographic and environmental characteristics and the RSV positivity (RSV+) in hospitalized and not-hospitalized patients were compared. In the data analysis we divided children according to their chronologic age (age) and their week gestational age (wGA). Results: Out of the 100 children evaluated, 68 were infants (<= 12 month-age): 7.5 and 20.6 % were in the <29 and 29- < 32 wGA groups respectively, and 72.0 % in the 32- < 35 wGA group. Positive hospitalized-to-not-hospitalized ratios were found in all three wGA groups, progressively decreasing (from 4.0 to 1.2), with increasing wGA (p = 0.35). The percentage of hospitalized infants that were also RSV+ was also progressively decreasing (from 40.0 to 28.6 % and 18.4 %) with increasing wGA (p = 0.43). In the >12 month-age group (N = 32), there was positive hospitalized-to-not-hospitalized ratio only in the <29 wGA group with a low RSV+ frequency (< 29 %) in all wGA groups. In the <= 12 month-age group, 41 infants were evaluated with a <= 6 month-age and 27 with a >6-12 month-age. A positive hospitalized-to-not-hospitalized ratios was found in all wGA groups in <= 6 month-age infants, despite a low RSV+ frequency in the 29- < 32 and 32- < 35 wGA group. In the >6-12 month-age group, all infants with a <29 and 29- < 32 wGA were hospitalized with a relatively high RSV+ frequency whilst the 32- < 35 wGA group showed a negative hospitalized-to-not-hospitalized ratio with a lower RSV+ frequency. Conclusions: The hospitalized-to-not-hospitalized ratios and RSV+ frequency in the first 12 months of age in infants born prematurely confirm the vulnerability of these children for clinically important RSV infection, most notably in the <32 wGA category.