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Because of the guesswork involved, scientists tend to set a lower bar for flu vaccine. While childhood vaccines against diseases like measles are expected to be 90 or 95 percent effective, a flu vaccine that’s 60 to 70 percent effective in the U.S. is considered pretty good. By that standard, this year’s vaccine is OK.

For senior citizens, a flu vaccine is considered pretty good if it’s in the 30 to 40 percent range, said Dr. Arnold Monto, a University of Michigan flu expert.

A high-dose version of the flu shot was recently made available for those 65 and older, but the new study was too small to show whether that has made a difference.

The CDC estimates are based on about 2,700 people who got sick in December and January. The researchers traced back to see who had gotten shots and who hadn’t. An earlier, smaller study put the vaccine’s overall effectiveness at 62 percent, but other factors that might have influenced that figure weren’t taken into account.

The CDC’s Bresee said there is a danger in providing preliminary results because it may result in people doubting _ or skipping _ flu shots. But the figures were released to warn older people who got shots that they may still get sick and shouldn’t ignore any serious flu-like symptoms, he said.

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Online:

CDC report: http://www.cdc.gov/mmwr