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Secondary attack rates from asymptomatic and symptomatic influenza virus shedders in hospitals: Results from the TransFLUas influenza transmission study


Tamò, Raphaël; Turk, Teja; Böni, Jürg; Kouyos, Roger D; Schmutz, Stefan; Huber, Michael; Shah, Cyril; Bischoff-Ferrari, Heike A; Distler, Oliver; Battegay, Edouard; Giovanoli, Pietro; Guckenberger, Matthias; Kohler, Malcolm; Müller, Rouven; Petry, Heidi; Ruschitzka, Frank; McGeer, Allison; Sax, Hugo; Weber, Rainer; Trkola, Alexandra; Kuster, Stefan P (2022). Secondary attack rates from asymptomatic and symptomatic influenza virus shedders in hospitals: Results from the TransFLUas influenza transmission study. Infection Control and Hospital Epidemiology, 43(3):312-318.

Abstract

OBJECTIVE: Nosocomial transmission of influenza is a major concern for infection control. We aimed to dissect transmission dynamics of influenza, including asymptomatic transmission events, in acute care.

DESIGN: Prospective surveillance study during 2 influenza seasons.

SETTING: Tertiary-care hospital.

PARTICIPANTS: Volunteer sample of inpatients on medical wards and healthcare workers (HCWs).

METHODS: Participants provided daily illness diaries and nasal swabs for influenza A and B detection and whole-genome sequencing for phylogenetic analyses. Contacts between study participants were tracked. Secondary influenza attack rates were calculated based on spatial and temporal proximity and phylogenetic evidence for transmission.

RESULTS: In total, 152 HCWs and 542 inpatients were included; 16 HCWs (10.5%) and 19 inpatients (3.5%) tested positive for influenza on 109 study days. Study participants had symptoms of disease on most of the days they tested positive for influenza (83.1% and 91.9% for HCWs and inpatients, respectively). Also, 11(15.5%) of 71 influenza-positive swabs among HCWs and 3 (7.9%) of 38 influenza-positive swabs among inpatients were collected on days without symptoms; 2 (12.5%) of 16 HCWs and 2 (10.5%) of 19 inpatients remained fully asymptomatic. The secondary attack rate was low: we recorded 1 transmission event over 159 contact days (0.6%) that originated from a symptomatic case. No transmission event occurred in 61 monitored days of contacts with asymptomatic influenza-positive individuals.

CONCLUSIONS: Influenza in acute care is common, and individuals regularly shed influenza virus without harboring symptoms. Nevertheless, both symptomatic and asymptomatic transmission events proved rare. We suggest that healthcare-associated influenza prevention strategies that are based on preseason vaccination and barrier precautions for symptomatic individuals seem to be effective.

Abstract

OBJECTIVE: Nosocomial transmission of influenza is a major concern for infection control. We aimed to dissect transmission dynamics of influenza, including asymptomatic transmission events, in acute care.

DESIGN: Prospective surveillance study during 2 influenza seasons.

SETTING: Tertiary-care hospital.

PARTICIPANTS: Volunteer sample of inpatients on medical wards and healthcare workers (HCWs).

METHODS: Participants provided daily illness diaries and nasal swabs for influenza A and B detection and whole-genome sequencing for phylogenetic analyses. Contacts between study participants were tracked. Secondary influenza attack rates were calculated based on spatial and temporal proximity and phylogenetic evidence for transmission.

RESULTS: In total, 152 HCWs and 542 inpatients were included; 16 HCWs (10.5%) and 19 inpatients (3.5%) tested positive for influenza on 109 study days. Study participants had symptoms of disease on most of the days they tested positive for influenza (83.1% and 91.9% for HCWs and inpatients, respectively). Also, 11(15.5%) of 71 influenza-positive swabs among HCWs and 3 (7.9%) of 38 influenza-positive swabs among inpatients were collected on days without symptoms; 2 (12.5%) of 16 HCWs and 2 (10.5%) of 19 inpatients remained fully asymptomatic. The secondary attack rate was low: we recorded 1 transmission event over 159 contact days (0.6%) that originated from a symptomatic case. No transmission event occurred in 61 monitored days of contacts with asymptomatic influenza-positive individuals.

CONCLUSIONS: Influenza in acute care is common, and individuals regularly shed influenza virus without harboring symptoms. Nevertheless, both symptomatic and asymptomatic transmission events proved rare. We suggest that healthcare-associated influenza prevention strategies that are based on preseason vaccination and barrier precautions for symptomatic individuals seem to be effective.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > Institute of Medical Virology
04 Faculty of Medicine > University Hospital Zurich > Clinic and Policlinic for Internal Medicine
04 Faculty of Medicine > University Hospital Zurich > Clinic for Radiation Oncology
04 Faculty of Medicine > University Hospital Zurich > Rheumatology Clinic and Institute of Physical Medicine
04 Faculty of Medicine > University Hospital Zurich > Clinic for Cardiology
04 Faculty of Medicine > University Hospital Zurich > Clinic for Infectious Diseases
04 Faculty of Medicine > University Hospital Zurich > Clinic for Reconstructive Surgery
04 Faculty of Medicine > University Hospital Zurich > Department of Aging Medicine
04 Faculty of Medicine > University Hospital Zurich > Clinic for Oncology and Hematology
Dewey Decimal Classification:610 Medicine & health
Scopus Subject Areas:Health Sciences > Epidemiology
Health Sciences > Microbiology (medical)
Health Sciences > Infectious Diseases
Language:English
Date:March 2022
Deposited On:26 May 2021 14:44
Last Modified:24 Feb 2024 02:51
Publisher:Cambridge University Press
ISSN:0899-823X
OA Status:Hybrid
Free access at:PubMed ID. An embargo period may apply.
Publisher DOI:https://doi.org/10.1017/ice.2021.112
PubMed ID:33952361
  • Content: Published Version
  • Language: English
  • Licence: Creative Commons: Attribution 4.0 International (CC BY 4.0)