A significant number of asymptomatic people shed the influenza virus, thereby exposing other individuals in their vicinity, according to research presented at the 29th European Congress of Clinical Microbiology & Infectious Diseases, held April 13 to 16, 2019, in Amsterdam, The Netherlands. Further, temporal and special proximity is insufficient to define a number of newly uncovered transmission clusters that had been undetected in routine surveillance.

Nosocomial influenza acquisition from asymptomatic individuals is a major infection control concern in hospitals. This prospective study was designed to dissect the trajectories of influenza transmission dynamics in a tertiary care hospital by following medical ward patients and acute care healthcare workers in the same wards across 2 consecutive flu seasons. Midturbinate nasal swabs from patients and healthcare workers were used for multiplex real-time polymerase chain reaction and whole-genome sequencing. Daily illness diaries were recorded, and contacts made between study participants were tracked.

Of the 543 inpatients and 152 healthcare workers recruited during the 2015/2016 and 2016/2017 influenza seasons, 3.5% (n=19) of inpatients and 10.5% (n=16) of healthcare workers were diagnosed with influenza. Of the 1241 swabs collected from these 35 participants, 108 returned positive for the influenza virus; the number of per-individual positive swabs ranged from 1 to 13. The majority of participants (91.9% of inpatients and 83.1% of healthcare workers) had symptoms of influenza when they tested positive, and these always included respiratory symptoms.

However, 8.1% (3 of 37) of the influenza-positive swabs from patients and 16.9% (12 of 71) of the influenza-positive swabs from healthcare workers were collected on asymptomatic days. Among the individuals with symptoms, none of the 17 patients had a positive test before influenza symptoms developed, and 14.3% (2 of 14) of healthcare workers had a positive test before influenza symptoms developed. Further, 10.5% (2 of 19) of patients and 12.5% (2 of 16) of healthcare workers remained asymptomatic. Preliminary analyses based on local and temporal proximity of patients and healthcare workers revealed at least 7 clusters of potential transmission events among inpatients, among healthcare workers, or between healthcare workers and patients, with 1 cluster revealing a possible transmission from an asymptomatic healthcare workers to a patient.

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The study investigators concluded, “Influenza infection in acute care is common, and a significant proportion of individuals shed influenza virus without harboring any symptoms, thereby potentially exposing their vicinity. Importantly, our study highlights that spatial and temporal proximity alone is insufficient to define transmission clusters as evidenced by a suspected case of asymptomatic spread that could not be confirmed by phylogenetic analysis.”

Reference

Tamò R, Böni J, Kouyos R, et al. The TransFLUas influenza transmission study in acute healthcare: attack rates, symptoms and transmission clusters. Presented at: 29th European Congress of Clinical Microbiology & Infectious Diseases; April 13-16, 2019; Amsterdam, The Netherlands. Abstract 2292.