Flu season starts late but may be just as virulent: CDC

By Dr. Ananya Mandal, MDFeb 27 2012

This year the flu season begins later than any year in the past three decades. But experts caution it is on the rise and expected to increase in the coming weeks.

Dr. Joseph Bresee, chief of the epidemiology prevention branch at the influenza division at the Centers for Disease Control and Prevention (CDC), said for the third consecutive week this season the percent of respiratory specimen testing positive for flu surpassed 10 percent, which is generally a marker to indicate the flu season has begun. “This is the latest start to a flu season in the past 29 years,” said Dr. Bresee.

Bresee said it's unclear exactly why the flu has been so sluggish this year, but a few theories are floating around. For one, the warmer weather in most parts of the country may make it harder for flu viruses, which thrive in cold, dry climates, to survive. More pleasant weather could also mean people are spending less time indoors in close quarters, transmitting viruses to one another.

Bresee also noted that the lower numbers of flu cases point to the success of flu vaccines. More people were vaccinated by November 2011 than by November 2010, although it's hard to count exactly how many have gotten a flu shot, he said.

Based on CDC flu surveillance data, the only other time this has occurred was the 1987-1988 season, Bresee said. Despite flu activity being low from early October through January, influenza viruses have been reported from all 50 states and is widespread in California and Colorado.

Influenza A, H3N2 viruses, 2009 Influenza A H1N1 viruses and Influenza B viruses have all been identified this season in the United States. “Regional influenza activity was reported in 13 states and increased from 12 last week. And 20 states reported local influenza activity and increased from 17 last week,” Bresee told reporters on a conference call. In the past 35 years, the flu season has peaked in March four times and in April twice he said. “We can't predict the timing of peak activity in the United States nor when the season will end, nor can we predict how severe the season will ultimately be. So if you haven't gotten the flu vaccine yet or your loved ones haven't gotten theirs yet, get your vaccine now.”

Also, the strains of the virus are relatively the same this year as in 2011. Most of this season's flu cases have been caused by H3N2 and H1N1. The H1N1 virus has been circulating for the past three years, said Dr. Jon Abramson, professor and chairman of pediatrics at Wake Forest Medical School. “The virus is essentially unchanged,” Abramson said. “More people have been exposed to it so fewer people are susceptible to it.” “If you haven't been vaccinated yet, you should be,” Abramson said. “It is likely there will be some peak in the flu in March.”