Late Start to Influenza Season

Lara C. Pullen, PhD

February 24, 2012

February 24, 2012 — Today the US Centers for Disease Control and Prevention (CDC) reported that, for the third consecutive week, more than 10% of respiratory samples have tested positive for influenza. At this time, 14.4% of specimens are testing positive compared with 13.4% last week. This increase in influenza activity indicates that flu season is beginning.

In a press briefing today, Joseph Bresee, MD, chief of the Epidemiology and Prevention Branch of the CDC explained, "The US is experiencing a late start of the influenza season this year." Although levels of influenza-like illness remain relatively low nationally, further increases in activity are expected in the coming weeks, he said.

The late start to the influenza season is not unprecedented. In the 1987-1988, the season also started in February. The timing of influenza seasons is unpredictable and the start, peak, and end can vary widely.

For the 2011-2012 flu season, influenza viruses have been reported in all 50 states. Two states, California and Colorado, have reported widespread influenza activity. In addition, 2 of the 10 US regions (representing Midwestern states) have reported increases in influenza rates. Dr. Bresee explained that it is extremely variable and difficult to predict which states and regions will first report influenza activity, as well as which states are likely to next demonstrate a peak in influenza activity.

The viruses circulating this year are quite similar to the viruses circulating last year. So far, influenza A (H3N2) has predominated nationally, although 2009 H1N1 and a very few influenza B viruses have also been identified. Regional differences do exist, however, with 2009 H1N1 viruses most commonly reported Arkansas, Louisiana, New Mexico, Oklahoma, and Texas.

The majority of influenza subtypes this season are well matched to the vaccine. In addition, so far the antiviral drugs oseltamivir and zanamivir are effective against the current circulating strains.

The late start of the influenza season has naturally corresponded with relatively low morbidity and mortality. Mortality has not exceeded the epidemic threshold.

Although no influenza-associated pediatric deaths were reported last week, 3 such deaths have been reported so far in 2011-2012. This compares favorably with the 122 pediatric deaths reported last year and the 282 deaths reported in the 2009 pandemic.

Dr. Bresee noted the CDC's recommendation that everyone aged 6 months and older receive the influenza vaccine. So far, more than 132 million vaccine doses have been distributed in the United States, although it is not known how many of these doses were actually administered.

It is impossible to say exactly why the influenza season is so late, and possibly mild, this year, Dr. Bresse noted, "As vaccine coverage increases, we should see less disease."

Dr. Bresee has disclosed no relevant financial relationships.

CDC Summary of Weekly FluView.

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