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Influence of observable and unobservable exposure on the patient's risk of acquiring influenza-like illness at hospital

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During outbreaks of hospital-acquired influenza-like illness (HA-ILI) healthcare workers, patients, relatives, and visitors are each source of infection for the others. Quantifying the contribution of various exposures will help improve prevention and control of HA-ILI outbreaks.


The objective was to study the influence of observable exposure to contagious patients and HCWs and of unobservable exposure to other sources on the patient's risk of acquiring influenza-like illness at hospital.


On the basis of data from three influenza outbreaks at hospital, we used a statistical model and Bayesian inference to estimate the attributability of HA-ILI to each of: 1) exposure to recorded vs. unrecorded sources; 2) exposure to contagious patient vs. contagious healthcare workers; 3) exposure during observable vs. unobservable contagious period of the recorded sources; and, 4) the moment of exposure.


Among recorded sources, 59% (95% credible interval: 34-83%) of HA-ILIs of patients were associated with exposure to contagious patients and 41% (17-66%) with exposure to contagious healthcare workers. Exposure during the unobservable contagiousness period of source patients and healthcare workers accounted for 49% (19-75%) and 82% (51-99%) of HA-ILIs, respectively. About 80% of HA-ILIs were associated with exposure one day earlier.


Secondary cases of HA-ILI might appear as soon as the day after the detection of a primary case highlighting the explosive nature of HA-ILI spread. Unobservable transmission was the main cause of HA-ILI outbreaks suggesting that symptom-based control measures alone might not reduce transmission. The results support vaccination against influenza of patients and healthcare workers and rapid interventions to control transmission.

Disclosure of interest

None declared.

Author information

Correspondence to N Voirin.

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  • Infectious Disease
  • Influenza
  • Statistical Model
  • Control Measure
  • Bayesian Inference