Covering for co-workers who came down with the flu this year, even though they got their flu shots? You're not alone. A new report released Thursday by the U.S. Centers for Disease Control and Prevention indicates that this year's influenza vaccine was on average just 56 percent effective, USAToday reported.

In the past, the CDC has estimated that flu shots were 70 to 90 percent effective in preventing the illness. The new estimate for the vaccine's effectiveness is slightly lower than the 62 percent estimated effectiveness the CDC gave in January.

The vaccine was found to be as little as 9 percent effective for seniors in preventing the flu. Experts say drug companies feel little pressure to develop a more effective vaccine because public perception has been that the current shots are adequate.

CDC officials also advise that moderate protection by getting immunized is better than no protection at all.