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Nosocomial Influenza: What Have We Missed?

Many concurrent interventions are needed to control nosocomial influenza (NI). The determinants facilitating its spread and constraint have been reported.1,2 However, certain issues in NI “natural history” were raised recently at the ASM 2016 Microbe Conference (June 16-20, 2016: in Boston, MA). We have summarized some of them here, based on our experience. Exposure to the influenza virus can occur before and during hospitalization. However, the determinants of community exposure differ from those of hospital ex­posure. In addition, some patients might be exposed outside shortly before hospitalization and diagnosed with influenza shortly after hospital admission. It is im­portant to identify sources of exposure because control measures and transmission investigations will diverge. Unfortunately, criteria defining NI have not yet been standardized.
The time interval between hospital admission and on­set ranges from 24 to 72 hours, but can be less than 24 hours in some situations. The 48-hour period between admission and diagnosis, often cited as an epidemiolog­ical definition of bacterial nosocomial infections, cannot be applied to NI.
A standardized definition is needed because attack rates may be impacted by it, sources of exposure may vary in relation to this time interval, and patient char­acteristics may change by time periods adopted for diagnosis. In addition, hospital-attributable risk can be questioned, in terms of legalities, depending on the definition accepted.
A recent survey, conducted in 55 of the 218 centers of the Society for Healthcare Epidemiology of America Research Network, determined that 76% of the centers adopted a standardized definition of NI based on clini­cal (24%), virological (31%) and clinical-virological (45%) features.3 The mean time threshold between admission and onset considered in the definition of NI was 57.1 hours (median 48.0 hours, range 24-96 hours).
The NI definition may be adapted according to objec­tives targeted: early hospital warning, surveillance net­works, collaborative studies, intervention assessment, site comparisons, and preventive measures (clinical tri­als), including vaccination. However, the NI definition is based on clinical features, and is therefore not suitable for asymptomatic cases, which might contribute to the spread of influenza.
Asymptomatic fraction is the proportion of asymp­tomatic influenza virus infections. This disease stage is important to estimate influenza incidence, taking symptomatic and asymptomatic cases into account. Knowledge of infectiousness—to optimize control strategies—may be driven by the presence or absence of symptomatic cases. The asymptomatic fraction is es­timated to include 20% to 50% of patients.4 In outbreak investigations, point estimates of the asymptomatic fraction have ranged from 4% to 28%, with a pooled mean of 16% (95% CI, 13%-19%). In surveillance situ­ations, point estimates of the asymptomatic fraction ranged from 65% to 85%. Estimation of the asymptom­atic fraction is affected by study design (outbreak inves­tigation or planned surveillance system).

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