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This year's flu vaccine appears to be about 62% effective against the currently circulating strains, according to CDC. This is "moderate effectiveness" according to CDC's report and slightly above the usual level of effectiveness for flu vaccine based on previous research.

Even if the vaccine is not 100% effective, vaccination is still crucial for not only personal but also public health. It offers substantial protection for the individual. In addition, people who have been vaccinated and later go on to get the flu appear to have much milder cases. And those same people also shed less flu virus, making it less likely they'll pass it along to those around them, said Arnold Monto, a professor of epidemiology at the University of Michigan in Ann Arbor, Mich., and a member of the Infectious Diseases Society of America's Influenza Advisory Group, which works with CDC.

This year about 35% of the U.S. population is on track to be vaccinated, said CDC spokesman Tom Skinner. "Certainly it is not perfect but it's a better defense versus not having flu vaccine."

Anecdotal reports from some medical centers show some people who had documented flu shots later coming down with the flu.

Robert Glatter, an emergency medicine physician at Lenox Hill Hospital in New York, is seeing indications that this year's vaccine may not be as protective as had been hoped, especially against Influenza B, one of three flu strains currently circulating. "This may be contributing to some patients' profound and severe symptoms associated with the 2013 outbreak" he said.

Michael Osterholm, director of the Center for Infectious Disease Research and Policy, says a careful analysis of the data shows that the flu vaccine's "match" has little to do with its effectiveness. Even flu vaccines that are well-matched for the flu strains in circulation may still offer poor protection, says Osterholm, also a professor at the University of Minnesota School of Public Health.

A report released four months ago by the the Center for Infectious Disease Research and Policy at the University of Minnesota found that while many studies put the efficacy of flu vaccine for healthy adults at between 70% to 90%, when they only looked at well-designed and rigorous studies it was closer to 59% for healthy adults between 18 and 64 years of age. The intranasal vaccine, sold as FluMist, is about 83% effective for children ages 6 months to 7 years, the report says.

The flu vaccine used in the United States is based on the mix of flu viruses circulating in the Southern Hemisphere the season before. Every year virologists at the World Health Organization look at the mix and attempt to predict what will appear in the Western hemisphere later in the year.

They announce their decision in April, just as the previous flu season in the northern hemisphere is wrapping up. Vaccine manufacturers have about five months to produce vaccine for the next season and vaccination begins in August and September, said Michael Jhung, an officer in the Center for Disease Control and Prevention's influenza division.

The best guess for this year's season was a mix of H3N2, H1N1 and Influenza B. The H3N2 and H1N1 in the vaccine "look a lot like" the flu currently in circulation and the Influenza B strain is about a 70% match, said Jhung. "That's not bad," said Jhung.

With such relatively low efficacy, the flu shot doesn't have much power to produce true "herd immunity," Osterholm says. Herd immunity is achieved when so many people are immunized with an effective vaccine that the community's viral load drops, protecting even the unvaccinated.

While Osterholm encourages people to be vaccinated, he says it's also important for people to realize how mediocre the current vaccines are. "The number one deterrent to getting new flu vaccines is the perception that the current ones are good enough," Osterholm says.

Osterholm says the world needs "next generation" vaccines to provide broader, longer-lasting protection from the flu. But very few of these "game-changing" vaccines are being developed, partly due to the expense and financial risk.

If the 177 influenza vaccines in clinical trials around the world, all but 13 are made in the traditional way, Osterholm says. And only one of those 13 is being developed by the U.S. government.

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