Discussion

Viral nucleic acid sequencing and phylogenetic analyses of 7 HRV NICU outbreak strains revealed 4 distinct HRV clades. Thus, multiple sources of HRV infections were contemporaneous to a genetically diverse community summer HRV outbreak. The 2 HRV-C clades infected groups of 2 and 3 patients. This points to probable common sources for these clades, possibly an infected family member (in the case of the twins), shared infected healthcare worker (HCW), improperly disinfected shared equipment, or horizontal transmission via contaminated HCW hands. Coincidentally, HCW hand hygiene audits during the outbreak period recorded suboptimal compliance of 68%. With the exception of the twins, each patient had a private room. Notably, single rooms alone did not fully prevent HAI horizontal transmission.Reference Julian, Burnham and Sellenriek ⁸

The HRV genetic analyses of specimens obtained during NICU outbreaks have previously been used to confirm nosocomial transmission and to differentiate prolonged HRV shedding periods from reinfections with a different strain.Reference Marcone, Carballal, Irañeta, Rubies, Vidaurreta and Echavarría ⁴ Marcone et alReference Marcone, Carballal, Irañeta, Rubies, Vidaurreta and Echavarría ⁴ reported an HRV outbreak involving 6 preterm NICU patients whose genotyping identified 4 different and 2 identical strains, pointing to nosocomial transmission of the latter.Reference Marcone, Carballal, Irañeta, Rubies, Vidaurreta and Echavarría ⁴ Reid et alReference Reid, Anderson, Cooley, Williamson and Mcgregor ³ described a neonatal HRV outbreak involving 8 patients. Screening of asymptomatic patients allowed them to detect 4 of these cases, which could otherwise have been unidentified sources of further transmission. These studies show a potential adjunctive role of genotyping in outbreak investigations. Nonetheless, genotyping should not override the use of a case definition combined with the epidemiologic context to establish the presence of an outbreak.

Similarly to previously described HRV outbreaks, 5 of our 7 patients presented mild respiratory symptoms.Reference Reese, Thompson, Price and Young ^{1 ,} Reference Reid, Anderson, Cooley, Williamson and Mcgregor ^{3 ,} Reference Marcone, Carballal, Irañeta, Rubies, Vidaurreta and Echavarría ⁴ However, our case of HRV-associated mortality progressed from no respiratory support to mechanical ventilation, showing the potential severity of HRV outbreaks in vulnerable populations.Reference Zinna, Lakshmanan and Tan ⁹ Among outbreak cases, this patient was the only baby born at term and the only patient without active pulmonary comorbidities. However, the presence of HRV bronchiolitis in the context of underlying symptomatic progressive hypertrophic cardiomyopathy likely contributed to his evolution to respiratory failure and death.

This study has several limitations. Asymptomatic or very mild HRV infections may not have been recognized because no systematic screening was undertaken. Furthermore, we did not test symptomatic family members, visitors, and HCWs for HRV during the outbreak; thus, we could not fully determine the sources of nosocomial HRV transmission. Nonetheless, our epidemiologic and viral sequencing data demonstrate multiple distinct introductions of HRV into our NICU and possible secondary transmission. Given the observed morbidity of HRV infection in NICU patients and potential for severe outcomes, these cases underscore the importance of HCW and visitor hand hygiene and respiratory hygiene, as well as HCW compliance to additional precautions, especially during community HRV outbreaks, which may not occur during periods typically associated with high levels of respiratory virus circulation. The MCH NICU has since continued to reinforce additional measures to prevent outbreaks and to limit their extent, namely by cohorting infected infants and through sensitizing parents and HCWs.