Study design
All infants born at the Hôpital Femme Mère Enfant (HFME; part of the university hospitals of Lyon, France) from August 26, 2019 to February 27, 2020 (during the RSV epidemic season in France [4]) with a cord blood sample available were eligible for inclusion in this single-center prospective observational cohort study. Cord blood samples were collected for all included patients. Stillborn children, those living outside the Auvergne-Rhône-Alpes region or those with insufficient sample were excluded.
The administrative registry of all infants born in the university hospitals of Lyon (Hospices Civils de Lyon, HCL) was used to recover the following variables of interest: sex (male/female), month of birth, gestational age (weeks of amenorrhea, WA), maternal parity (primiparity/multiparity), type of pregnancy (simple/multiple gestation), mode of delivery (vaginal birth/caesarean section), and birth weight. Preterms were classified as either “moderate-to-late preterm” [from 32 to 36(+6) WA], “very preterm” [28 to 31(+6) WA], and “extremely preterm” [22 to 27(+6) WA], as defined by the World Health Organization (WHO) [21]. Low birth weight was defined as less than 2500 g [22].
Infants aged 3 months or younger who visited the emergency department of the HFME hospital (either directly or transfer from another hospital) between August 26, 2019 to May 27, 2020 with laboratory-confirmed RSV bronchiolitis recognized by the International Statistical Classification of Diseases and Related Health Problems (ICD-10) codes (J21.0, J12.1 and/or J20.5) were identified in the informatics’ database of the hospital. A 3-month follow-up seemed to be optimal to detect a protection purposed by maternal antibodies as children under 3 months of life experience the greatest risk of hospitalization and mortality [5,23], and as previous studies have shown that maternal antibody concentrations against RSV decreased between 2 and 6 months [1,24], with a half-life of approximately 27 days [25]. According to local protocols, all infants hospitalized with an LRTI diagnosis were tested for RSV on a nasopharyngeal sample. Laboratory-confirmed RSV infection were those diagnosed by real-time reverse transcriptase (RT)-PCR, as previously described [26]. Clinical records were reviewed to further classy these as VS-LRTI, as defined by the WHO: cough or difficulty breathing, associated with fast breathing or peripheral capillary oxygen saturation (SpO2) < 90% or inability to feed or unconscious [13]. The peak of the epidemic was defined as December 9, 2019 based on both pediatric emergency admissions for bronchiolitis and RSV detection data in Lyon, in accordance with data from previous years.