Flu shots can save lives, especially among the elderly who account for most of the 20,000 flu-related deaths in the United States each year. But physicians have debated whether vaccinating healthy, younger adults is worth the time and money.

The answer is yes, according to a new study based on a computer model. The study was led by Patrick Lee, MD, a resident in internal medicine at Stanford Hospital & Clinics.

"There's been a lot of debate about optimal treatment strategies," Lee said. "Our study shows that society as a whole benefits if you vaccinate the entire population and use antiviral medications on those who get sick."

Influenza, a viral disease characterized by nasal congestion, dry cough and fever, affects 10 percent to 20 percent of the U.S. population each year, with an average of 2.8 work days lost per ill individual. Although anyone can get a flu shot, the Centers for Disease Control recommend vaccination only for specific groups, including the elderly, those with weakened immune systems and health-care workers. Most healthy adults choose not to get flu shots.

In the study, published in Tuesday's issue of the journal Annals of Internal Medicine, Lee and colleagues gathered previously published data on the costs and benefits of flu vaccination and treatment of flu patients with antiviral medication in healthy adults ages 18 to 50. These included costs of the vaccine and drugs, lost work time due to illness, and duration of symptom relief from antiviral medications.

The researchers entered this information into a computer model that subtracted the costs from the benefits to yield a net benefit or cost. They ran the model 1,000 times while systematically changing the data within known limits. For each combination of costs and benefits, the computer determined which of eight prevention and treatment strategies was most cost-beneficial. The strategies included either vaccination or non-vaccination and, for those who became ill, treatment with one of three antiviral medications, or no treatment.

Using the average values for all costs and benefits, the researchers found that the four strategies that included vaccination had an overall savings of about $30 each compared with no vaccination and no treatment with antivirals.

Lee said using a computer model may appear less rigorous because randomized clinical trials are usually the gold standard of medical research. But two previous clinical trials on the costs and benefits of flu vaccinations for healthy adults yielded widely different results. One found a savings of $46.85 per vaccination, while another found costs of $65.59 and $11.17 per vaccination over two seasons.

"Clinical trials are snapshots in time. They look at one flu season and one flu vaccine," Lee said. "In a clinical scenario where variables change over time, randomized clinical trials are not the best way to answer questions."

Using their computer model, Lee and colleagues found that a strategy that included vaccination was optimally cost-beneficial in 95 percent of their 1,000 runs. Non-vaccination became optimal when the probability of contracting the flu was less than 6.3 percent -- a very mild flu season.