Date Published: October 28, 2008
Publisher: Public Library of Science
Author(s): Carline van den Dool, Marc J. M Bonten, Eelko Hak, Janneke C. M Heijne, Jacco Wallinga
Abstract: BackgroundAnnual influenza vaccination of institutional health care workers (HCWs) is advised in most Western countries, but adherence to this recommendation is generally low. Although protective effects of this intervention for nursing home patients have been demonstrated in some clinical trials, the exact relationship between increased vaccine uptake among HCWs and protection of patients remains unknown owing to variations between study designs, settings, intensity of influenza seasons, and failure to control all effect modifiers. Therefore, we use a mathematical model to estimate the effects of HCW vaccination in different scenarios and to identify a herd immunity threshold in a nursing home department.Methods and FindingsWe use a stochastic individual-based model with discrete time intervals to simulate influenza virus transmission in a 30-bed long-term care nursing home department. We simulate different levels of HCW vaccine uptake and study the effect on influenza virus attack rates among patients for different institutional and seasonal scenarios. Our model reveals a robust linear relationship between the number of HCWs vaccinated and the expected number of influenza virus infections among patients. In a realistic scenario, approximately 60% of influenza virus infections among patients can be prevented when the HCW vaccination rate increases from 0 to 1. A threshold for herd immunity is not detected. Due to stochastic variations, the differences in patient attack rates between departments are high and large outbreaks can occur for every level of HCW vaccine uptake.ConclusionsThe absence of herd immunity in nursing homes implies that vaccination of every additional HCW protects an additional fraction of patients. Because of large stochastic variations, results of small-sized clinical trials on the effects of HCW vaccination should be interpreted with great care. Moreover, the large variations in attack rates should be taken into account when designing future studies.
Partial Text: Annual influenza vaccination of institutional health care workers (HCWs) is advised in most Western countries to reduce transmission of influenza to vulnerable patients . A few clinical trials have indeed demonstrated protective effects of this intervention for patients in nursing homes at relatively low HCW vaccine uptake rates [2–5]. However adherence to the recommendation is generally low [6–9] and it is uncertain what the effect of a further increase of vaccine uptake among HCWs is and whether herd immunity can be attained in health care institutions . Empirical data from previous trials did not reveal a clear association between the number of HCWs vaccinated and the number of prevented influenza virus infections in patients. This absence of a clear association might be due to substantial variation between the studies in the endpoints measured, the departments (of varying size) involved, and vaccine coverage among patients. Furthermore, the effect of HCW vaccination is highly dependent on annual factors such as influenza virus activity and the match between the circulating influenza virus strain and the current vaccine. To control for all uncertainties potentially modifying the effects of HCW vaccination an exceptionally large clinical trial would be needed.
Our model reveals a linear relationship between the number of HCWs vaccinated and the expected number of influenza virus infections among patients in a nursing home department. No threshold for herd immunity can be detected and even when HCW vaccine uptake is maximal, due to stochastic effects that are inherent to the transmission process, the variation in patient attack rates is high and large outbreaks can occur. In fact, the value of small-sized clinical trials in estimating the protective effect of HCW vaccination on patient outcome seems questionable due to the large impact of chance effects within the small environment of nursing home departments.