Do Healthy Adults Really Need a Flu Shot?

A Best Evidence Review

Charles P. Vega, MD


December 17, 2010

In This Article

Best Evidence Reference

Jefferson T, Di Pietrantonj C, Rivetti A, et al. Vaccines for preventing influenza in healthy adults. Cochrane Database Syst Rev. 2010;7:CD001269.


This study was selected from Medscape Best Evidence, which uses the McMaster Online Rating of Evidence System. Of a possible top score of 7, this study was ranked as 6 for newsworthiness and 6 for relevance by clinicians who used this system.

The Background

The Centers for Disease Control and Prevention (CDC) now recommends universal vaccination against influenza among adults and children, but is the vaccine effective for healthy individuals? Moreover, can it prevent complications of influenza, which occur less frequently among adults without chronic illness? The current analysis from the Cochrane Database of Systematic Reviews investigates these issues and finds weak evidence of vaccine efficacy.

Prevention is a profound theme in medicine. Physicians and patients are constantly in search of therapy that can reduce or eliminate the risk for illness and mortality. Vaccinations are one of the best examples of preventive medicine, and the influenza vaccination is widely used around the globe.

The potentially severe consequences of influenza infection, particularly the risk associated with pandemic disease, are well-documented. In the decade prior to 1999, influenza virus was associated with an average of 51,203 deaths per year in the United States.[1] The majority of these deaths were due to respiratory and circulatory illness, and 90% of these fatal cases occurred among adults aged 65 years and older. During a similar period, the annual number of hospitalizations for influenza-related illness exceeded 1 million in the United States.[2] The number of primary influenza and pneumonia hospitalizations among adults aged 85 years and older was nearly double that of individuals between the ages of 5 and 49 years of age.

Younger adults may be at lower risk for the complications of influenza, but the infection has a significant impact on them as well. In 1 study of adults between the ages of 50 and 64 years, the prevalence of influenza-like illness during the 2006-2007 season was 17.1%.[3] Among people who didn't receive a flu vaccine, influenza-like illness accounted for 39% of illness-related lost work days and 49% of all days with reduced productivity due to illness. A review of 28 studies found that the average number of work-days lost per episode of physician-diagnosed influenza was 3.7 to 5.9.[4] Laboratory-confirmed influenza resulted in 1.5-4.9 days lost from work. Overall, the annual average direct medical costs related to influenza in the United States exceed $10 billion, and projected lost earnings due to illness and loss of life total $16.3 billion annually.[5]

Influenza promotes significant loss of productivity even among young adults. In a study of 3249 college students, 36.7% of participants reported at least 1 influenza-like illness in 2002-2003.[6] Colds and influenza accounted for a total of 45,219 days of illness, and nearly one fourth of participants had sought medical care for their infection. Infections led to 27.8% of participants performing poorly on an examination and 46.3% doing poorly on a class assignment. Influenza-like illness was more likely than colds to affect school performance.

The influenza vaccine is frequently cited as a means to reduce morbidity and mortality associated with the infection, and the CDC now calls for universal influenza vaccination for individuals at age 6 months or older.[7] For the first time, these recommendations include healthy adults who are not in contact with individuals at high risk for the complications of influenza. The new recommendation for vaccinating healthy adults is built upon several assumptions:

  • The vaccine will reduce the number of cases of influenza;

  • The vaccine will reduce complications of influenza;

  • The vaccine will reduce the transmission of influenza; and

  • The vaccine will safely accomplish these goals.

But does the influenza vaccine fulfill these criteria, particularly for a low-risk group such as healthy adults? The current meta-analysis examines this issue.


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