Quiet season: Where is the flu?

Illness, death much reduced

At the end of February, typically the peak of the influenza season, there were only regional and sporadic cases of flu in most of the United States. In fact, the quiet left some infectious disease experts wondering, "Where is the flu?"

"One year ago, a vast majority of states were reporting widespread activity," Lisa Grohskopf, MD, MPH, a medical officer with the Influenza Division of the Centers for Disease Control and Prevention, told a federal advisory panel.

Grohskopf and her infectious disease colleagues were reluctant to declare the flu season over before it began. In some years, the flu season has a delayed start, they said.

As of mid-February, only 14% of influenza-like illness tested positive for influenza, and only three pediatric deaths had been attributed to influenza in 2011-2012. By contrast, there were 122 flu-related pediatric deaths reported in 2010-2011 and 282 such deaths in 2009-2010.

"I've just been totally amazed not only in terms of flu surveillance, but in flu morbidity and mortality," said Jonathan Temte, MD, PhD, a family medicine physician, professor at the University of Wisconsin School of Medicine and Public Health in Madison and a member of the Advisory Committee on Immunization Practice (ACIP).

This year, Temte said he has seen fewer hospitalized patients with chronic obstructive pulmonary disease and congestive heart failure, which may indicate fewer complications from influenza.

The 2011-2012 vaccine was identical to the one in 2010-2011, as the same strains continued to circulate. Meanwhile, ACIP had recommended universal vaccination of everyone over the age of 6 months, and retail pharmacies heavily marketed the flu vaccine. The resulting high vaccine coverage may have resulted in a very mild flu season, the infectious disease experts suggested.