The Centers for Disease Control and Prevention officially declared that the U.S. 2011-2012 influenza season had begun Feb. 24, based on data through Feb. 18, the latest opening to the influenza season in 29 years.
For the time being, the season remains in relatively low gear, although it’s gaining some momentum: Only California and Colorado are showing "widespread" flu activity, while 14 states continue to show "sporadic" activity, according to the latest CDC data.
"While seasonal flu activity is currently low, we expect it to increase in coming weeks," said Dr. Joseph Bresee, chief of the CDC’s Epidemiology and Prevention Branch in the Influenza Division. "The U.S. is experiencing a late start to the flu season this year, but activity has increased in the past few weeks."
That pattern "is unusual, but not unprecedented," he said. Over the past 35 years, peak U.S. flu activity occurred in March four times, and in April twice, Dr. Bresee noted.
"We could speculate as to the reasons" for the late and mild season so far, he added, "but flu is inherently unpredictable." Among Dr. Bresee’s possible explanations:
• This year’s flu strains mimic last year’s. The U.S. flu strains circulating so far have been predominantly A/H3N2, constituting more than 70% of infections, followed by the 2009 A/H1N1 pandemic strain and a B strain. That pattern closely mimics the strains that circulated last year. "So, it is possible there is higher immunity to these viruses than last year, and that leads to less transmission," he said.
• More people are vaccinated this year. The pattern of circulating flu so far closely matches the seasonal 2011-2012 vaccine, and vaccine coverage rates continue to trend upward this year. That should translate into less transmission and disease over time, especially as people at high risk for infection and complications continue to get vaccinated.
"I think that increased vaccine coverage is certainly playing a role," he said. As of mid-February, U.S. distribution of the 2011-2012 seasonal flu vaccine reached 132 million doses. While that number is slightly below the rate for all of last season, there is still time before the current season’s number wraps.
As of last November, U.S. flu vaccination rates were running ahead of November 2010. And while no high-risk populations have yet come close to the 80% vaccination rate that is the CDC’s goal, coverage rates in pregnant women and children have trended up "slowly but steadily over the past couple of years," Dr. Bresee said.
• Residual immunity may play a part. Many Americans continue to have high levels of antibody against the 2009 H1N1 pandemic strain, a factor that "probably plays a role in the [low] amount of H1N1 we’re seeing this year," he said.
• Mother Nature may be helping. Finally, the relatively mild winter that has occurred in much of the country is a factor that "might play a role," although Dr. Bresee cautioned that the influence that the weather has on flu transmission is poorly understood.
Other elements that together paint an upbeat picture of the current flu season are easier to document with data.
As of Feb. 18, three children had died as a result of influenza infection. To put that number in perspective, since the CDC first made flu-related pediatric death reportable in 2004, the full seasonal number ranged from 46 in 2005-2006 to 282 in 2009-2010, with 340 pediatric flu-related deaths during the entire 2009 H1N1 pandemic.
Another positive is that so far this season, not a single flu isolate tested has shown resistance to oseltamivir or zanamivir, data that "reinforce the value of these medications, especially for people with serious illness due to underlying health conditions," Dr. Bresee said.
And so far, hospitalization rates for patients with laboratory-confirmed influenza have been low. Since Oct. 1, the CDC tallied a national total of 347 laboratory-confirmed infections that required hospitalization, a rate of 1.3 cases/100,000 people.
The CDC declares the start of the annual flu season when its national surveillance statistics show 3 consecutive weeks when at least 10% of collected respiratory specimens are positive for flu. During the week ending Feb. 18, that percentage was 14.4%, following levels of 15.5% for the week ending Feb. 11, and 10.5% for the week ending Feb. 4.
Dr. Bresee had no disclosures.