Possible role of aerosol transmission in a hospital outbreak of influenza

Abstract

Background: We examined the role of aerosol transmission of influenza in an acute ward setting.

Methods: We investigated a seasonal influenza A outbreak that occurred in our general medical ward (with open bay ward layout) in 2008. Clinical and epidemiological information was collected in real time during the outbreak. Spatiotemporal analysis was performed to estimate the infection risk among patients. Airflow measurements were conducted, and concentrations of hypothetical virus-laden aerosols at different ward locations were estimated using computational fluid dynamics modeling.

Results: Nine inpatients were infected with an identical strain of influenza A/H3N2 virus. With reference to the index patient's location, the attack rate was 20.0% and 22.2% in the "same" and "adjacent" bays, respectively, but 0% in the "distant" bay (P = .04). Temporally, the risk of being infected was highest on the day when noninvasive ventilation was used in the index patient; multivariate logistic regression revealed an odds ratio of 14.9 (95% confidence interval, 1.7-131.3; P = .015). A simultaneous, directional indoor airflow blown from the "same" bay toward the "adjacent" bay was found; it was inadvertently created by an unopposed air jet from a separate air purifier placed next to the index patient's bed. Computational fluid dynamics modeling revealed that the dispersal pattern of aerosols originated from the index patient coincided with the bed locations of affected patients.

Conclusions: Our findings suggest a possible role of aerosol transmission of influenza in an acute ward setting. Source and engineering controls, such as avoiding aerosol generation and improving ventilation design, may warrant consideration to prevent nosocomial outbreaks.

Figure 1.

Epidemic curve of the influenza outbreak. Patients were shown according to their symptom onset date (fever or new respiratory symptoms); the order does not necessarily reflect the order in which they acquired infection. The arrow indicates the time when the index patient (patient A) commenced bi-level positive airway pressure (BiPAP) ventilation support. Prior to that, he was receiving supplemental oxygen therapy via nasal cannula. The BiPAP ventilation lasted for >16 h; he was subsequently transferred to the intensive care unit. Patient I started to receive oseltamivir prophylaxis on 4 April (the ward was closed and sick patients were isolated); however, he soon became unwell and developed fever on 10 April, despite receipt of prophylaxis. Staff 1 and 2 had symptoms; however, the results of serological tests for recent influenza infection were negative (Table 1).

Figure 2.

Figure showing layout of the outbreak ward and the locations of affected patients. Patient A (circled) was the index case. Dark-colored blocks represent high-efficiency particulate absorbing (HEPA) filters placed at the wall end of each ward bay. Dates of symptom onset were stated for all infected patients. Patient D had been staying at 2 bed locations (front row then back row).

Table 1.

Clinical and Virological Data for Individuals Affected in the Outbreak

Figure 3.

The measured airflow rates (in the unit of L/s) at different ward locations. High-efficiency particulate absorbing (HEPA) air purifiers were turned to the low setting in bay B and to the medium setting in bay C during time of measurement.

Figure 4.

Airflow pattern at the mid-plane across the 2 high-efficiency particulate absorbing (HEPA) air purifiers in the rear bays B and C.

Figure 5.

The spatial distribution of normalized concentration of hypothetical virus-laden aerosols (modeled as gaseous tracer) in the outbreak ward at a height of 1.1 m. The flow rates used in this model were those described in Figure 3. All high-efficiency particulate absorbing (HEPA) filters were assumed to function with 100% filtration of the modeled droplet nuclei. The 3 HEPA air purifiers are shown as black boxes, the 4 diffusers are shown by a square with an X, and the 4 returns are shown as a small rectangular filled box. Affected patients are represented by white ovals (the index patient is marked as a red oval).