While influenza continued to range widely across the United States last week, several measures of flu activity dipped slightly, hinting that the season may have peaked.
Markers that declined, according to the Centers for Disease Control and Prevention (CDC), included the share of respiratory samples testing positive for flu, the percentage of outpatient visits due to influenza-like illness (ILI), the number of states with high flu activity, and the number of pediatric deaths.
Meanwhile, CDC officials and other medical experts yesterday reported a pair of trends that echo the 2009 H1N1 pandemic: a high (over 40%) prevalence of obesity in hospitalized adult flu patients and an unusual number of flu patients who have gastrointestinal symptoms.
The CDC's weekly FluView report said that 23.1% of 12,108 respiratory samples were positive for flu last week, down a bit from 25.1% of 10,841 samples tested the week before. The 2009 H1N1 virus continued to be the overwhelmingly dominant strain, accounting for 1,727 of 1,785 influenza A isolates that were subtyped.
The agency said ILI accounted for 3.4% of visits to sentinel medical providers, compared with 3.6% a week earlier.
At the state level, 13 reported high flu activity, 1 fewer than the previous week, while 7 states and New York City had moderate activity, versus 12 a week earlier. States with high activity were mostly south-central but extended east to Tennessee, Virginia, and North Carolina.
On the other hand, the count of states with geographically widespread cases rose to 41, from 40 the week before, while 8 states and Puerto Rico had regional activity.
The flu death toll among children last week was eight, or two fewer than the preceding week, the CDC reported. That raised the season's total to 28 pediatric deaths. Of the eight fatal cases last week, five involved H1N1 infections, and the rest were not subtyped.
The share of deaths attributed to pneumonia and flu, generally seen as a lagging indicator, rose to 8.1% from 7.5% the week before, staying well above the epidemic threshold of 7.2%, the CDC said.
The season's total for flu-related hospitalizations climbed to 4,615, up by 870 from the week before, according to the CDC figures. With that increase, the cumulative flu-related hospitalization incidence reached 17.0 cases per 100,000 population. The numbers come from the CDC's Influenza Hospitalization Surveillance Network (FluSurv-NET), which covers 70 counties in 10 states.
The CDC found 7 more 2009 H1N1 isolates that were resistant to antiviral drugs (neuraminidase inhibitors [NIs]), raising the total for the season to 20. That amounts to 1.1% of the 1,827 isolates tested. No H3N2 or influenza B isolates with resistance to NIs have been found.
Obesity as risk factor for severe illness
During a teleconference for clinicians yesterday, CDC flu expert Angela Campbell, MD, MPH, said obesity has been a common characteristic among adults hospitalized for flu this season, a finding that is reminiscent of the 2009 H1N1 pandemic.
"So far this season, 45% of adults and 13% of children hospitalized with flu have been obese," Campbell said. Obesity is the most common underlying condition in adult patients, followed by metabolic disorders, cardiovascular disease, and asthma, according to the FluView report.
(The CDC's latest graph showing underlying conditions in hospital patients, updated today, indicates that obesity has been reported in 43% of adults and 11% of children this season.)
Obesity—especially morbid obesity—was first recognized as an independent risk factor for severe flu illness during the 2009 pandemic, CDC spokeswoman Erin Burns noted today.
Sandra Chaves, MD, MSc, medical director of the CDC's FluSurv-NET, told CIDRAP News that roughly 40% of hospitalized patients during the 2009 pandemic were classified as obese. In the past couple of seasons—in which the 2009 H1N1 virus was far less common—about 30% of hospitalized flu patients have been considered obese, which is comparable with the general population prevalence of obesity, she reported.
She cautioned that the 45% figure for this season is based on a subset of about 25% of the hospital cases and therefore could change as more information is gathered.
GI symptoms cited
In other developments, Pauline K. Park, MD, an intensive care specialist from the University of Michigan who spoke during yesterday's CDC teleconference, reported that a number of hospitalized flu patients at her center have initially had gastrointestinal symptoms.
"They often have initial GI symptoms for a couple days of flu-like illness, and once they have respiratory symptoms, the progression [to severe illness] is very quick," Park commented. She is a professor of surgery and co-director of the surgical intensive care unit at the university.
Chaves told CIDRAP News that the CDC is not collecting data on GI symptoms in flu patients this season, but the suggestion that they are common is not surprising and echoes what happened during the 2009 pandemic.
"During the pandemic, 2009 H1N1 caused a lot of GI symptoms," she said. "It's quite consistent that during the pandemic, maybe 30% of cases, regardless of age, had some GI symptoms, like diarrhea or vomiting, which was unusual, because we usually see it more in kids."
In a normal flu season the prevalence of GI symptoms is very low—less than 5% in adults, but higher in children, she said.
"There is no clear correlation between GI symptoms and severity [of illness]," she commented. For example, a 2009 study of hospitalized flu patients in California found no difference in the frequency of diarrhea between those who died and those who survived, she said.
In other comments during the teleconference, Campbell indicated that this flu season is not as intense as last year's, at least so far. "We're not quite seeing as much flu activity as last season, but more than the year prior," she said.
And in other US developments, California health officials said today that 50 more flu deaths have been confirmed in the state this week, raising the seasonal total to 95, including 3 children. Another 51 deaths suspected to be due to flu are under investigation, the California Department of Public Health (CDPH) said in a press release.
Trends in Canada, Europe
In Canada, health officials reported mixed signs on the flu front today.
"In week 03, overall laboratory detections of influenza decreased slightly, reflecting decreased activity in some regions that experienced an earlier start to the influenza season, and increases in activity in other regions," the Public Health Agency of Canada (PHAC) said in its weekly update. "Prescriptions for antiviral medications to treat influenza have also decreased.
PHAC added that the number of people seeking treatment for ILIs and the number of flu-related hospitalizations in children continued to increase last week. The 2009 H1N1 still predominated, and working-age adults continued to be the hardest-hit age-group, the agency said.
In Europe, where the flu situation has been much quieter than in the United States so far this season, there was a hint of rising activity among mixed markers last week, according to today's update from the European Centre for Disease Prevention and Control (ECDC).
"Increasing numbers of hospitalised laboratory-confirmed and fatal influenza cases have been reported in the last few weeks," the statement said.
The agency said 15 countries reported rising ILI rates and increases in flu-positive respiratory samples. Overall, 32% of 1,401 sentinel respiratory specimens tested positive for flu, which is down slightly from the previous week. Influenza A accounted for all but five of the positive samples. Of type A viruses that were subtyped, 60% were 2009 H1N1 and 40% were H3N2, about the same as the previous week.
Medium-intensity flu activity was reported by the same four countries as the previous week: Bulgaria, Greece, Portugal, and Spain. All other countries had low-intensity activity. Also, Portugal, Spain, and England again reported that flu was geographically widespread, as they had the week before, the ECDC said.
The 2009 H1N1 virus was listed as the dominant strain in Bulgaria, Greece, Spain, Portugal, Sweden, and the United Kingdom, while H3N2 was the leading strain in Slovenia.
The ECDC said 205 hospitalized flu case-patients were reported last week, raising the seasonal total to 759. Noting that media reports in Spain and the United States have highlighted severe cases in previously healthy people, the agency said, "These cases among unvaccinated individuals are to be expected in light of the dominant H1N1pdm09 strain. However there are no indications of increasing mortality or an unusually high number of severe cases in EU Member States at this time."
CDC FluView report
CDC graph of underlying conditions in hospitalized flu patients
Jan 24 CDPH press release
Jan 24 ECDC update