European Congress of Clinical Microbiology and Infectious Diseases

European Congress of Clinical Microbiology and Infectious Diseases

April 20, 2019
2 min read

‘Significant’ proportion of health care workers, patients shed flu virus before symptoms appear

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Stefan Kuster, MD, MSc, MBA
Stefan Kuster

Researchers tracking influenza transmission at a tertiary care hospital in Switzerland found that a “significant” proportion of patients and health care workers shed influenza virus before the appearance of symptoms.

The researchers said they found several influenza transmission clusters undetected by routine surveillance, and seven overall clusters of potential transmission among health care workers, including one that appeared to include transmission from an asymptomatic health care worker to an inpatient. The findings were presented at the European Congress of Clinical Microbiology & Infectious Diseases.

Stefan Kuster, MD, MSc, MBA, a senior physician in the division of infectious diseases and hospital epidemiology at the University Hospital Zurich, and colleagues conducted a prospective study of 543 inpatients and 152 acute-care health care workers working on the same wards during the 2015-2016 and 2016-2017 influenza seasons at the University Hospital Zurich.

They collected nasal swabs daily to track influenza infection and tracked contacts between study participants.

Nineteen inpatients (3.5%) and 16 HCWs (10.5%) were diagnosed with influenza. Kuster and colleagues wrote that the majority of those who tested positive (83.1% of HCWs and 91.9% of patients) experienced influenza symptoms, always including respiratory symptoms, at the time of their positive test.

According to the findings, 16.9% of influenza-positive swabs from HCWs and 8.1% influenza-positive swabs from patients were collected on days they did not report symptoms.

The researchers also reported that 12.5% of HCWs and 10.5% of inpatients who tested positive remained asymptomatic. Two (13%) of the HCWs and none of the symptomatic patients had a positive influenza test before symptoms developed.

“Our findings suggest that influenza infection in acute care is common and a significant proportion of individuals appear to shed influenza virus without harboring any symptoms, making the spread of flu very difficult to control, even with self-diagnoses and current infection control practices,” Kuster told Infectious Disease News. “Influenza vaccination is not perfect but remains the best tool we have to protect health care workers and their patients from severe illness.”

Previous study findings showed that more than 50% of health care trainees reported showing up for work with influenza-like illness, despite knowing the risk to others. – by Bruce Thiel


Cowman K, et al. Am J. Infect Control. 2019;doi:10.1016/j.ajic.2018.02.004.

Tamó R, et al. Abstract 2292. Presented at ECCMID; April 13-16, 2019; Amsterdam.

Disclosures: The authors report no relevant financial disclosures.