Flu activity up but way behind 2009-10 season

The number of influenza-associated deaths, hospitalizations and outpatient physician visits are beginning to rise as the 2010-11 flu season progresses. But the number of cases is down considerably from the 2009-10 season marked by the arrival of the influenza A(H1N1) virus, according to a February report by the Centers for Disease Control and Prevention.

"When we look at outpatient visits for influenza-like illness, when we look at hospitalizations, the numbers are a bit higher than what was seen in the 2008-09 flu season and about the same as 2007-08," said Shikha Garg, MD, MPH, epidemic intelligence service officer in the CDC's Influenza Division. "Last year, influenza attack rates were very high because of the lack of population immunity, and the rate of outpatient visits for influenza-like illness were the highest seen since the CDC started influenza surveillance."

Through Feb. 20, 2010, 40,618 confirmed influenza-associated hospitalizations were reported to the CDC, compared with 8,227 as of Feb. 19, 2011.

The number of deaths also is down, with 115 flu-related fatalities reported to the CDC so far this season, compared with 1,994 deaths at the same point during the 2009-10 season.

Though this flu season is unlikely to match the severity of last year's, direct comparisons are difficult because of how the seasons unfolded, Dr. Garg said.

"Last year was extremely unusual in that activity peaked very early in September and October," she said. "Because of that, it is hard to compare activity in terms of the timing. One thing about this season is that it started very, very late. Activity hasn't peaked yet this season."

All 50 states and the District of Columbia have reported test results positive for influenza, with 32% of specimens testing positive as of Feb. 5, according to the Feb. 18 CDC Morbidity and Mortality Weekly Report. Nearly a majority of flu cases this season are due to influenza A(H3N2) viruses, and influenza A(H1N1) and B viruses also have circulated widely, said the CDC report.

Nineteen states saw "high" rates of outpatient visits for influenza-like illness, nine states had "moderate" activity, 10 states reported "low" activity and 12 states saw "minimal" activity. Thirty-seven states reported "widespread" influenza activity.

The flu hospitalization rate this season is worst among young children and seniors, in contrast with the last season, when many people in their 20s were hospitalized with H1N1.

Children younger than 4 have been hospitalized with influenza at a rate of 14.5 in 100,000 people, while adults 65 and older have a rate of 18.8 per 100,000 this season.

The influenza vaccine this year includes the H1N1 strain as well as two seasonal strains. That has made a difference, said Lisa Grohskopf, MD, MPH, an epidemiologist at the CDC's National Center for Immunization and Respiratory Diseases. The flu strains circulating "appear to be well-matched to the 2010-11 seasonal vaccine," she said.

Progress on vaccination

Manufacturers have distributed 163 million flu vaccine doses, and more people are getting vaccinated, according to CDC surveys conducted in November 2010. The proportion of adults who said they were already immunized or definitely intended to get vaccinated was 42%, up from 33% during the 2008-09 flu season.

The uptick is due to immunization guidelines adopted in February 2010, as well as the impact of last year's flu season, Dr. Garg said.

"There's a lot more awareness of the influenza vaccine, and the importance of vaccination, in the wake of the pandemic," she said. "And while several years ago only the very young and the very old were recommended to be vaccinated, now all people6 months of age and older are recommended to be vaccinated. That may be driving health care providers to offer vaccination more, as well as individuals to seek influenza vaccination."

Physicians should continue to offer the flu vaccine to patients, with the flu season expected to continue through May, Dr. Garg said.

More than 70% of physicians told the CDC in mid-November 2010 that they were vaccinated or intended to be immunized.

At a February meeting, the CDC's Advisory Committee on Immunization Practices strengthened its position on vaccinating health care workers, broadening the group to include custodians, orderlies, administrative staff and others at hospitals, clinics and other health organizations.

The committee did not call for mandating health worker vaccination, but did note that is one route health care organizations can choose to improve immunization rates.

"All health care personnel should receive an annual flu vaccination," said Harry Keyserling, MD, who served on the ACIP working group on health worker immunization. "There are a variety of strategies to increase immunization and that can include mandatory vaccination."

Meanwhile, the ACIP recommended that all adults who frequently come into contact with infants should be vaccinated against pertussis.