The effects of influenza vaccination of health care workers in nursing homes: insights from a mathematical model

Abstract

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.

Figure 1. Influenza Virus Attack Rates among Patients for Increasing Health Care Worker Vaccination Rates

(A) Increased vaccination of HCWs decreases the expected influenza virus attack rate among patients. Squares indicate the mean attack rates, dashed lines the median attack rates, and light blue boxes the 2.5th to 97.5th percentiles. (B–D) The distribution of the influenza virus attack rates among patients shifts to the left when vaccine uptake among HCWs is increased from 0, to 0.5, and 1 for (B), (C), and (D), respectively. Each distribution is based on 5,000 simulations.

Figure 2. Effects of Increased Health Care Worker Vaccination on Influenza Virus Attack and Introduction Rates

Increased vaccination of health care workers (HCWs) reduces the influenza virus attack rate among HCWs. It also reduces the rate of introduction of influenza virus into the patient population and the attack rate among patients following an introduction. As a consequence it reduces the total attack rate among patients. All relationships appear linear.

Figure 3. Effects of Increased Health Care Worker Vaccination on Patient Attack Rates in Different Scenarios

For all scenarios under study, the influenza virus attack rate among patient decreases in an approximately linear way when the health care worker vaccination rate is increased. (A) The expected attack rates for the open and closed departments, where patients have many or no contacts with individuals from the community, respectively. (B) The attack rates for seasons with high (15% community attack rate) and low (5% community attack rate) influenza virus activity. (C) The attack rates for departments with high and low HCW/patient ratios.