Giuseppe
Emeritus
PLoS Medicine - The Effects of Influenza Vaccination of Health Care Workers in Nursing Homes: Insights from a Mathematical Model
The Effects of Influenza Vaccination of Health Care Workers in Nursing Homes: Insights from a Mathematical Model
Carline van den Dool1*, Marc J. M. Bonten1,2, Eelko Hak1, Janneke C. M. Heijne3, Jacco Wallinga1,3
1 Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands,
2 Department of Medical Microbiology, University Medical Center Utrecht, Utrecht, The Netherlands,
3 Center for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands
Background
Annual 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 Findings
We 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.
Conclusions
The 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.
Funding:
The study was funded by a grant from the Netherlands Health Care Organization (ZonMw, number 6120.0015). MJMB is supported by the Netherlands Organization for Scientific Research (VICI NWO Grant 918.76.611). The funders had no role in the study design, data collection and analysis, nor in the decision to publish or in the preparation of the manuscript.
Competing Interests:
The authors have declared that no competing interests exist.
Academic Editor:
Steven Riley, Hong Kong University, Hong Kong
Citation:
van den Dool C, Bonten MJM, Hak E, Heijne JCM, Wallinga J (2008) The Effects of Influenza Vaccination of Health Care Workers in Nursing Homes: Insights from a Mathematical Model. PLoS Med 5(10): e200 doi:10.1371/journal.pmed.0050200Received: March 10, 2008; Accepted: August 28, 2008; Published: October 28, 2008
Copyright: ? 2008 van den Dool et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Abbreviations:
HCW, health care worker; ILI, influenza-like illness
* To whom correspondence should be addressed. E-mail: c.vandendool@umcutrecht.nl
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<cite cite="http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0050200">PLoS Medicine - The Effects of Influenza Vaccination of Health Care Workers in Nursing Homes: Insights from a Mathematical Model</cite>Carline van den Dool1*, Marc J. M. Bonten1,2, Eelko Hak1, Janneke C. M. Heijne3, Jacco Wallinga1,3
1 Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, Utrecht, The Netherlands,
2 Department of Medical Microbiology, University Medical Center Utrecht, Utrecht, The Netherlands,
3 Center for Infectious Disease Control, National Institute for Public Health and the Environment, Bilthoven, The Netherlands
Background
Annual 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 Findings
We 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.
Conclusions
The 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.
Funding:
The study was funded by a grant from the Netherlands Health Care Organization (ZonMw, number 6120.0015). MJMB is supported by the Netherlands Organization for Scientific Research (VICI NWO Grant 918.76.611). The funders had no role in the study design, data collection and analysis, nor in the decision to publish or in the preparation of the manuscript.
Competing Interests:
The authors have declared that no competing interests exist.
Academic Editor:
Steven Riley, Hong Kong University, Hong Kong
Citation:
van den Dool C, Bonten MJM, Hak E, Heijne JCM, Wallinga J (2008) The Effects of Influenza Vaccination of Health Care Workers in Nursing Homes: Insights from a Mathematical Model. PLoS Med 5(10): e200 doi:10.1371/journal.pmed.0050200Received: March 10, 2008; Accepted: August 28, 2008; Published: October 28, 2008
Copyright: ? 2008 van den Dool et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Abbreviations:
HCW, health care worker; ILI, influenza-like illness
* To whom correspondence should be addressed. E-mail: c.vandendool@umcutrecht.nl
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