CDC: Main flu marker dips below baseline

Mar 29, 2013 (CIDRAP News) – One of the key markers the US Centers for Disease Control and Prevention (CDC) uses to gauge flu activity returned to baseline last week for the first time since early December, signaling that the season is drawing to a close, the agency said today.

Doctor visits for influenzalike illness (ILI) last week dropped to 1.8%, putting it below the national baseline of 2.2%. Only 3 of the CDC's 10 regions reported that levels were above their regional baselines.

Another major indicator—the percentage of respiratory specimens testing positive for flu —also fell last week, dropping to 13.2% from 16.3% the previous week. The percentage of samples determined to be influenza B continued to surge; the strain accounted for 74% of all positive specimens and was the dominant strain in 9 of 10 CDC regions.

Five more pediatric flu deaths were reported, raising the season's total to 110. One was linked to H3N2 and occurred in mid January, and two were from undetermined influenza A subtypes and also occurred in earlier weeks. The two most recent deaths were from influenza B.

Last week the CDC posted an early analysis of the pediatric flu deaths so far this season. It reported that about 90% of children who died from flu had not been vaccinated against flu and about 60% had underlying conditions that put them at higher risk for flu complications.

Overall, the percentage of deaths from flu and pneumonia fell slightly last week to 7.5%, putting it at the epidemic threshold.

Nationally, the rate of hospitalizations for flu rose slightly, with the disease accounting for 41.8 per 100,000 population. The hardest-hit group is still seniors.

In a separate report today the CDC offered some of its impressions of the nation's 2012-13 flu season, which said the season began about 4 weeks early and was intense. The level of ILI rose quickly above baseline and stayed elevated for 15 consecutive weeks, making the season slightly longer than average.

Hospitalization levels started to rise in the middle of December, and increases were sharp among seniors. "While hospitalization rates have leveled off, this is the highest proportion of persons 65 and older hospitalized for flu that has been measured since this kind of record-keeping began during the 2005-2006 season," the CDC said.

Deaths from pneumonia and flu showed a similar pattern, with that marker rising above its epidemic threshold for the first time in early January and peaking the week of Jan 19 at 9.9%, its highest level in nearly a decade.

Last week in its assessment of pediatric deaths so far this season, the CDC said this season is similar to 2003-04, another early and intense season when 153 deaths were reported from 40 states.

For comparison, the CDC said last season was mild, with flulike activity elevated for only 1 week.

About 134.9 million doses of flu vaccine were distributed this season, the CDC estimated, adding that it expects to make a final uptake estimate in the fall.

Studies that monitored flu vaccine effectiveness this season showed moderate protection in most groups, with somewhat better performance against influenza B, the CDC said. The research found protection gaps in seniors this season, especially against H3N2.

Vaccination can still reduce the risks of getting sick, being hospitalized, or dying from flu, the CDC said. "While it can vary in how well it works, flu vaccination is the best tool currently available to protect against influenza infection."

In other parts of North America, Canada's flu markers continued to decline, with influenza B detections increasing, similar to the pattern seen in the United States, according to a Mar 28 flu update from the Pan American Health Organization (PAHO). It said seniors were the hardest-hit group for flu hospitalizations in both Canada and the United States.

In Mexico, H3N2 is still the most commonly detected strain, PAHO said.

Meanwhile, flu activity in Europe is significant, though levels have slowly been declining, the European Centre for Disease Prevention and Control (ECDC) said yesterday in its weekly update. The percentage of respiratory specimens that tested positive for flu last week was 45%, a slight drop from 46% the previous week.

Flu activity has declined or returned to baseline in all reporting countries, the ECDC said. It added that active flu transmission this season has occurred for more than 3 months, which is long compared with previous years.

See also:

Mar 29 CDC weekly flu update

Mar 22 CDC flu spotlight

Mar 29 CDC flu spotlight

Mar 28 PAHO flu update

Mar 28 ECDC flu update

Newsletter Sign-up

Get CIDRAP news and other free newsletters.

Sign up now»


Unrestricted financial support provided by

Bentson Foundation Gilead 
Grant support for ASP provided by


  Become an underwriter»