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