Five fewer states reported high flu activity levels in the first week of January than the 29 that reported high activity levels in the last week of December, according to the CDC's weekly flu report. This week, 24 states reported high illness levels, 16 reported moderate levels, five reported low levels and one reported minimal levels, suggesting that the flu season peaked in the last week of December.
"It may be decreasing in some areas, but that's hard to predict," CDC Director Dr. Thomas Frieden said in a Friday morning teleconference. "Trends only in the next week or two will show whether we have in fact crossed the peak."
The flu season usually peaks in February or March, not December, said Dr. Jon Abramson, who specializes in pediatric infectious diseases at Wake Forest Baptist Health in North Carolina. He said the season started early with a dominant H3N2 strain, which was last seen a decade ago, in 2002-03. That year, the flu season also ended early.
Click here to see how this flu season stacks up against other years.
Because of the holiday season, Frieden said the data may have been skewed.
For instance, Connecticut appeared to be having a lighter flu season than other northeastern states at the end of December, but the state said it could have been a result of college winter break. College student health centers account for a large percentage of flu reports in Connecticut, but they've been closed since the fall semester ended, said William Gerrish, a spokesman for the state's department of public health.
The flu season arrived about a month early this year in parts of the South and the East, but it may only just be starting to take hold of states in the West, Frieden said. California is still showing "minimal" flu on the CDC's map, but that doesn't mean it will stay that way.
Click here to read about how flu has little to do with cold weather.
"It's not surprising. Influenza ebbs and flows during the flu season," Frieden said. "The only thing predictable about the flu is that it is unpredictable."
Dr. William Schaffner, chair of preventive medicine at Vanderbilt University Medical Center in Nashville, Tenn., said he was expecting California's seeming good luck with the flu to be over this week.
"Flu is fickle, we say," Schaffner said. "Influenza can be spotty. It can be more severe in one community than another for reasons incompletely understood."
Early CDC estimates indicate that this year's flu vaccine is 62 percent effective, meaning people who have been vaccinated are 62 percent less likely to need to see a doctor for flu treatment, Frieden said.
Although the shot has been generally believed to be more effective for children than adults, there's not enough data this year to draw conclusions yet.
"The flu vaccine is far from perfect, but it's still by far the best tool we have to prevent flu," Frieden said, adding that most of the 130 million vaccine doses have already been administered. "We're hearing of shortages of the vaccine, so if you haven't been vaccinated and want to be, it's better late than never."