Transmission of Influenza Virus From Asymptomatic Healthcare Workers and Inpatients in the Acute Care Hospital Setting

The epidemiology and transmission dynamics of influenza in hospitals are only poorly understood, particularly with respect to subjects without symptoms of influenza infection (e.g. without fever, cough, sore throat, nasal congestion, weakness, headache, loss of appetite, or myalgia). Knowledge about whether asymptomatic subjects are able to transmit influenza is of major importance. If they do transmit influenza, vaccination of patients and healthcare workers (HCW) before start of the influenza season, the permanent use of masks by HCW during influenza season, and quarantine for previously exposed inpatients may be the only available measures to reduce the number of influenza transmission events from asymptomatic subjects in acute care hospitals. Closure of this knowledge gap would be of major benefit to infection prevention and control recommendations, and may in turn reduce morbidity and mortality associated with influenza in hospitals through improved patient management.

The epidemiology and transmission dynamics of influenza in hospitals are only poorly understood, particularly with respect to subjects without symptoms of influenza infection (e.g. without fever, cough, sore throat, nasal congestion, weakness, headache, loss of appetite, or myalgia). Knowledge about whether asymptomatic subjects are able to transmit influenza is of major importance. If they do transmit influenza, vaccination of patients and healthcare workers (HCW) before start of the influenza season, the permanent use of masks by HCW during influenza season, and quarantine for previously exposed inpatients may be the only available measures to reduce the number of influenza transmission events from asymptomatic subjects in acute care hospitals.

The investigators' key aim is therefore to define whether exposure to asymptomatic subjects with influenza infection constitutes a risk for influenza transmission in an acute care hospital setting through active, prospective surveillance.

The investigators' secondary aims are to describe the prevalence of community-acquired symptomatic and asymptomatic influenza upon hospital admission and the incidence of asymptomatic and symptomatic nosocomial influenza among inpatients; to assess transmission dynamics of symptomatic influenza infection in acute care; and to study the incidence of asymptomatic and symptomatic influenza, absenteeism (i.e. being absent from work due to influenza), presenteeism (i.e. being present at work despite influenza infection) associated with influenza, and compliance with infection control recommendations to prevent spread of influenza in acute care HCW.

The investigators plan to enroll 1,260 inpatients and 180 HCW from medical wards at the University Hospital Zurich in a prospective study over two consecutive influenza seasons in order to detect at least one transmission event from an asymptomatic individual shedding influenza virus. Flocked mid-turbinate nasal swabs will be collected daily from consenting inpatients starting from day of admission until two days after discharge and from HCW over the influenza (winter) season and analyzed for influenza A and B using polymerase chain reaction. Simultaneously, signs and symptoms of influenza infection (including cough, sore throat, fever \>37.8°C, nasal congestion, weakness, headache, loss of appetite or myalgia) as well as contact patterns between inpatients and HCW will be recorded. Reconstruction of influenza transmission chains will be based on phylogenetic analyses derived from next-generation sequence data and epidemiological contact tracing.

Eligibility Criteria:
Inclusion Criteria:

* 18 or more years of age;
* Available for follow-up during the study period;
* If a HCW: employed full- or part-time (≥50% full-time equivalent);
* Understand the study, agree to its provisions, and give written informed consent (as documented by signature).

Exclusion Criteria:

* If a HCW: planning to spend more than two consecutive weeks outside of Switzerland during the winter study period (November 1st to April 31st);
* If a HCW: planning to take leave from work for more than two consecutive weeks during the winter study period (e.g. maternity or medical leave);
* Inability to follow the procedures of the study, e.g. due to language problems, psychological disorders or dementia of the subject;
* Known or suspected non-compliance.