The top U.S. public health official on Monday called Influenza "the largest single risk the world faces."
Even in a good year, when the flu is relatively mild, hundreds of thousands of people die from flu around the world, said Dr. Tom Frieden, director of the U.S. Centers for Disease Control and Prevention (CDC).
"And in a bad flu year, it goes into the millions," he said.
According to the agency, more than 200,000 people are hospitalized from flu-related complications each year in the U.S alone. The CDC's web site says the flu can exacerbate chronic health problems.
According to CDC-sponsored study on flu and pneumonia by researchers at Vanderbilt University Medical Center, of more than 2,000 pneumonia patients who visited U.S. hospitals over three flu seasons, those who received vaccination were 57 percent less likely to be admitted with flu-pneumonia than those patients who were not vaccinated.
The study also found vaccines were somewhat less effective for older patients and for those who had weak immune systems, but worked well for healthy people under the age of 65 and young children.
Because the flu vaccine isn't 100 percent effective, doctors also recommend frequent hand washing and staying away from those who have the flu.
In the meantime, staffers at the CDC are on alert as this year's flu season gets underway.
"A flu pandemic remains the nightmare scenario for public health," said Frieden, adding that CDC epidemiologists work around the clock to better detect flu as it spreads so public health officials can better respond to outbreaks of influenza and so medical scientists can work to produce a better vaccine.
The study appears in JAMA, The Journal of the American Medical Association.