Respiratory syncytial virus is the number one virus causing severe lower respiratory disease in preterm infants, while those of younger age and those exposed to young children are at greatest risk, Eric A. F. Simões, MD, of the University of Colorado at Denver, Aurora, and his coauthors reported in the Nov. 29 edition of PLOS ONE.

“These data demonstrate that higher risk for 32 to 35 wGA [weeks gestational age] infants can be easily identified by age or birth month and significant exposure to other young children,” they wrote. “These infants would benefit from targeted efforts to prevent severe RSV disease.”

The prospective RSV Respiratory Events Among Preterm Infants Outcomes and Risk Tracking (REPORT) study in 38 states followed 1,642 preterm infants born at 32-35 weeks’ gestational age who had medically attended acute respiratory illness.

The overall rates of lower respiratory infections per 100 infant-seasons – a season being 5 months of observation from November 1 to March 31 in 2009-2010 or 2010-2011 – were 13.7 for respiratory syncytial virus (RSV), 2.9 for adenovirus, 1.7 for parainfluenza virus type 2, 1.3 for human metapneumovirus, and 0.3 for parainfluenza virus type 2 (PLoS One. 2016 Nov 29. doi: 10.1371/journal.pone.0166226 ).

Infants who had been exposed to young children, either through attending day care or living with non–multiple birth preschool-age siblings, had a twofold higher risk of RSV and human metapneumovirus, and a 3.3-fold greater risk of adenovirus.

The youngest infants showed the highest rate of hospitalizations with RSV: the incidence ranged from 8.2 per 100 infant season in those aged less than 1 month to 2.3 per 100 infant seasons in those aged 10 months of age. Similarly, the incidence of admission to ICU was significantly higher among younger infants.

Infants born in May, before the RSV season, had a much lower incidence of hospitalization, compared with those born in the height of RSV season in February. ICU admission rates also were higher among those born in February, compared with those born in May.

The highest overall rates of hospitalization with RSV – 19 per 100 infant-seasons – were among those born in February, and also those who were exposed to other young children.

“The current results are unique in that they provide continuous age-based risk models for outpatient and inpatient disease for infants with and without young child exposure,” wrote Dr. Simões and his coauthors.

They argued that their findings refute earlier suggestions that the rate of RSV infection in preterm infants is similar to the rate in term infants, and suggested that the limitations of their study may have even underestimated the incidence in preterm babies.

The study was supported by AstraZeneca, parent company of MedImmune. Two authors declared grant support and research funding from AstraZeneca, one author was a former employee of AstraZeneca, one author was a former employee of MedImmune and now contractor to AstraZeneca. One author was a current employee of AstraZeneca and holds stock options. Two authors also declared funding and consultancies with AbbVie.