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A hundred years ago, doctors practiced experience-based medicine. That is, they relied on the wisdom of past practices and what they observed with their own patients. There weren’t many scientifically proven remedies for common ailments like colds or flu.

These days, doctors have embraced evidence-based medicine. They look for randomized controlled trials of the medicines they prescribe, and they seek rational evaluations of treatments.

The trouble is that even today, the evidence doesn’t always line up with the belief that something works.

Consider vaccinations against influenza, for example. Public health experts urge almost everyone to get a flu shot now, since the current flu season began early. After all, thousands of people die from influenza every year. Many of these deaths occur among older people at high risk for complications such as pneumonia.

It is disappointing so little data demonstrate that influenza vaccines protect elderly people from the flu. An analysis of 75 studies conducted by the Cochrane Collaboration, an independent and objective nonprofit organization, concluded that: “The available evidence is of poor quality and provides no guidance regarding the safety, efficacy or effectiveness of influenza vaccines for people aged 65 years or older” (Cochrane Database of Systematic Reviews, Feb. 17, 2010).

We spoke with Tom Jefferson, M.D., the lead author of two key flu analyses by the Cochrane Collaboration. He noted that vaccination can reduce symptomatic illness but cautioned, “That does not mean that there is evidence that [flu shots] prevent deaths or person-to-person spread or pneumonia … but the evidence is of extremely poor quality.”

Most people assume that if they get a flu shot, they won’t get sick. That is the implied promise from public health officials. Jefferson suggests that may be overly optimistic. He points out that in healthy young adults for whom the shots work best, “you have to vaccinate between 33 and 99 healthy adults to avoid one case of influenza. … In healthy adults, the vaccines shorten duration of illness by half a working day.”

Based on the available evidence, influenza vaccines are not as helpful as most people imagine. If you have to vaccinate scores of people to protect just one from fever, muscle aches and pains, and all the other symptoms of flu-like illness, those are not great odds.

What about anti-viral drugs? A recent review by the independent Medical Letter on Drugs and Therapeutics (Dec. 10, 2012) concludes that oseltamivir (Tamiflu) or zanamivir (Relenza) can be useful for treating people who come down with influenza and need to be hospitalized. The authors suggest pregnant women and those at high risk for complications could benefit from such drugs.

Staying away from infected people is the best way to avoid catching the flu. That is easier said than done. Frequent hand-washing is essential. Keep hands and fingers away from the face. Take at least 2,000 units of vitamin D-3 every day during winter months because this hormone is essential for proper immune function. Probiotics also may be beneficial in reducing the number of respiratory-tract infections people experience (Cochrane Database of Systematic Reviews, Sept. 7, 2011).