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.