Influenza Vaccine

Influenza vaccination effectiveness and influenza disease burden


Three real-world effectiveness studies are taking place with partners at the US Centers for Disease Control and Prevention, to evaluate the effectiveness of the influenza vaccine in three higher risk populations: pregnant women, children and health care professionals.

Context and Aims

In order to prevent flu infections, limit influenza epidemics and reduce serious complications of flu infections, an understanding of vaccination uptake and effectiveness, is needed, particularly among high risk groups. Three of these high risk groups are healthcare professionals, pregnant women, and children.

Persistently low rates of vaccine uptake among health care professionals are of particular concern given their close contact with patients and the risk of transmitting the virus to many vulnerable patients.

Pregnant women and children disproportionately suffer from secondary complications of influenza infection, and are also more likely to be hospitalized due to influenza. This high risk for hospitalization and severe influenza among pregnant women and children was observed during the 2009 A(H1N1) pandemic and other prior pandemics.

Three studies are currently being conducted to estimate influenza disease incidence, and to determine the effectiveness of the influenza vaccine among these three high-risk populations.

Research Approach

All three studies will identify the annual incidence of disease using laboratory-confirmed influenza, and will report on vaccination rates among the respective study populations. The study among health care professionals is a prospective cohort study, while the other two studies among pregnant women and children are retrospective studies.

The effectiveness of the influenza vaccines will be tested through adjusted and stratified models, including evaluations for age groups, virus strains, and other strata.

Health care professionals: The study evaluating vaccine effectiveness among health care professionals in two hospitals in the Clalit health system includes active surveillance for acute illness during flu season. This active surveillance is conducted through SMS text messages to capture episodes of acute illness among participants. Specific to health care workers, this study is examining the effectiveness of vaccination in preventing symptomatic and asymptomatic influenza illness and days of missed work due to influenza illness. The study will also evaluate the association between repeated influenza vaccination and immune response to influenza vaccines. Electronic surveys are also being conducted to capture knowledge, attitudes and practices (KAP) among participating healthcare professionals about flu vaccine and flu-associated illnesses.

Pregnant women: The study of flu vaccine effectiveness among pregnant women is a multi-center study, including sites in North America, Israel and Australia. The study examines the effectiveness of maternal vaccination in preventing influenza-associated hospitalizations and ICU admissions. In addition, the study will describe the burden of flu-associated hospitalizations in pregnant women, detail the clinical course of influenza in infected pregnant women, and evaluate flu vaccine effectiveness in preventing adverse birth outcomes.

Children: The study among children is examining vaccine effectiveness among children hospitalized in multiple Clalit hospitals and the influence that vaccination in a previous season has on the vaccine effectiveness of the current season.

Key Findings and/or Potential Impact

These three ongoing studies aim to identify the burden of influenza infection and the effectiveness of influenza vaccine among higher risk populations. Findings from these evaluations can potentially improve prevention efforts, provide insights into factors that contribute to influenza-associated hospitalizations, and inform vaccination strategies for these three high-risk groups.