Review
Effectiveness of seasonal influenza vaccination in healthcare workers: a systematic review

https://doi.org/10.1016/j.jhin.2011.08.004Get rights and content

Summary

Vaccination is considered a key measure to protect vulnerable groups against influenza infection. The objectives of this review are to determine the effect of influenza vaccinations in reducing laboratory-confirmed influenza infections, influenza-like illnesses (ILIs), working days lost among vaccinated HCWs, and associated adverse effects after vaccination. Twenty-two healthcare-related databases and internet resources, as well as reference lists, and the bibliographies of all of the retrieved articles were examined. All randomized controlled trials (RCTs) comparing the effectiveness of any kind of influenza vaccine among all groups of HCWs with a placebo/vaccine other than the influenza vaccine/no intervention were included in the review. Only three RCTs matched the inclusion criteria. There is a limited amount of evidence suggesting that receiving influenza vaccination reduces laboratory-confirmed influenza infections in HCWs. No evidence can be found of influenza vaccinations significantly reducing the incidence of influenza, number of ILI episodes, days with ILI symptoms, or amount of sick leave taken among vaccinated HCWs. There is insufficient data to assess the adverse effects after vaccination. There is no definitive conclusion on the effectiveness of influenza vaccinations in HCWs because of the limited number of related trials. Further research is necessary to evaluate whether annual vaccination is a key measure to protect HCWs against influenza infection and thus increase their confidence in the vaccine. In the mean time, the direction of promoting influenza vaccination to HCWs can be shifted from staff protection to patient protection, with accurate information to address concerns and misconceptions.

Introduction

Influenza is a highly contagious viral infection that leads to the abrupt onset of respiratory symptoms. The attack rates of seasonal influenza generally range from 10% to 20% in healthy adults, but can exceed 80% in hospital outbreaks and long term care facilities.1, 2, 3, 4, 5 The transmission of influenza A (H1N1) causing the pandemic in 2009 was even higher, as the majority of population did not have such immunity to this strain, especially the younger age group.6 A review of the influenza pandemic in 2009 suggested that 15–45% of the world’s population has already become infected by this newly emergent strain.7 Although influenza is mostly a mild and self-limiting illness, it can pose a significant health risk to vulnerable groups.8, 9 Vaccination has been demonstrated to be the most effective way to prevent influenza-related complications among high risk populations such as children, elderly and patients with comorbidity.10, 11, 12 Healthcare workers (HCWs) are also a target for the influenza vaccination because they may serve as the vector for transmitting influenza virus to patients after they become infected from either the community or healthcare setting.2, 13, 14, 15, 16

Despite general consensus and recommendations for influenza vaccination of HCWs, vaccine coverage among HCWs has remained low and is far below the desired level to achieve herd immunity.17, 18, 19, 20 The low vaccination rate among HCWs may be associated with doubts over the effectiveness of the vaccine and fear of its adverse effects.21 Negative information regarding safety can be an influencing factor for HCWs to reject newly developed vaccines, such as the pandemic H1N1 vaccine.22 Although several systematic reviews have already evaluated the effectiveness of influenza vaccine in different groups of high risk persons, no review has yet focused on the outcomes in HCWs. Evidence of such effectiveness is important to increase the confidence of HCWs in influenza vaccines and provide a rational backdrop for the future development of vaccination policies. This systematic review evaluates the effectiveness of influenza vaccines in terms of (i) preventing laboratory-confirmed influenza infections, (ii) preventing influenza-like illness (ILI), and (iii) reducing working days lost among HCWs. This review also aims to determine the adverse effects among HCWs after receiving the influenza vaccine.

Section snippets

Search strategy

The authors used 22 electronic healthcare databases and internet resources to search for eligible studies (Box 1). All databases were searched from the date of their launch up to 14 March 2011. Searches were conducted using a combination of the following keywords with relevant MeSH or abbreviated terms if applicable. The keywords included influenza vaccines (influenza, human/prevention and control; influenza vaccin*; inoculation; immuni*), effectiveness (efficacy), health personnel (medical

Characteristics of the studies

In total, 5090 studies were identified using the predefined search strategy and keywords. Of these, 5070 were excluded after the titles and abstracts of the articles had been reviewed. Of the remaining 20 studies, 17 were eventually excluded after a full review of the report, because of irrelevant outcome measures or study designs (Table I).25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41 Three RCTs meeting the inclusion criteria were finally identified (Table II).15, 42, 43

Discussion

This is the first systematic review evaluating the effectiveness of influenza vaccinations on HCWs by comparing laboratory-confirmed influenza infections, ILI, working days lost, and adverse effects between vaccinated and unvaccinated HCWs. An extensive search revealed only three RCTs conducted in hospital settings that fulfilled the selection criteria. All of these studies used trivalent inactivated parenteral seasonal influenza vaccine, and they involved a total of 967 HCWs. A pooled analysis

Conflict of interest statement

None declared.

Funding sources

None.

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