Two reports offer new data on severe H1N1 cases

Oct 8, 2009 (CIDRAP News) – Two reports published by the New England Journal of Medicine today filled in more details about severe cases of H1N1 influenza, generally confirming previous findings that most of them occur in non-elderly people who have chronic health conditions but that previously healthy people are also affected.

One report deals with 272 patients who were hospitalized with H1N1 in the spring wave in the United States. The researchers, from the Centers for Disease Control and Prevention (CDC), report that 73% had an underlying medical problem, 25% required intensive care, and 7% died. Close to half of the patients were under the age of 18; only 5% were 65 or older.

A second report covers all 722 patients who were treated in intensive care units (ICUs) for H1N1 in Australia and New Zealand from June through August. It says that ICU admissions for influenza A in that period were 15 times the number of admissions for viral pneumonia in recent years. About 65% of the patients needed mechanical ventilation, and about 14% of them did not survive.

Obesity showed up as a possible contributing factor for severe illness in both studies, occurring in 29% of the Australian and New Zealand cases and in 45% of US cases for which data were available. In line with previous reports, pregnant women were disproportionately represented in both groups.

US hospital-case study
The US report covers 272 hospital cases that occurred from May 1 to Jun 9, representing 25% of the 1,082 H1N1 hospitalizations during that period. The data, from 24 states, were the product of a CDC appeal to state health departments to collect and submit clinical information on each hospitalized H1N1 patient.

The median age of the patents was 21, with 44% of them younger than 18, 50% between 18 and 64, and 5% 65 or older. Of 19 patients (7%) who died, all had received mechanical ventilation. While all were given antibiotics and 90% received antivirals, none were started on antivirals within the recommended 48 hours after they got sick.

Asthma was the most common chronic medical condition in the patients, found in 28%; other common conditions were diabetes (15%), immunosuppresson (15%), cardiovascular disease (13%), and renal disease (9%). Eighteen patients (7%) were pregnant, a fraction well above the expected 1% prevalence in the general population.

Height and weight data were available for 70% of the patients, and 45% of these were obese or morbidly obese, the report says. Most of these (81%) had an underlying condition known to increase the risk of flu complications.

Few bacterial coinfections were found, but not all of the patients were tested for these infections, and most patients were on antibiotics, which could have reduced test sensitivity, the report says. The CDC reported last week that bacterial infections were found in 22 of a series of 77 patients who died of H1N1 in the past 4 months, but the agency cautioned that the series was not a statistically valid sample of all fatal cases.

The authors say their findings suggest that antiviral treatment is beneficial, especially when started early, and should not be withheld when patients present more than 48 hours after they get sick.

ICU cases from Australia and New Zealand
In the other report, a large team gathered data on all the patients with confirmed H1N1 infections who were admitted to ICUs in Australia and New Zealand during the peak Southern Hemisphere flu season from June through August.

The 722 cases represent an incidence of 28.7 per million inhabitants, the report says, adding, "The number of ICU admissions due to influenza A in 2009 was 15 times the number due to viral pneumonitis in recent years." The data indicate that the peak effect on ICU resources in a region comes about 4 to 6 weeks "after the first confirmed winter ICU admission and that the extra workload lasts several weeks."

As of Sep 7, 103 patients (14.3%) had died, while 114 (15.8%) were still in hospitals, the report says. The authors state that this death rate is no higher than that previously reported for influenza A patients who required critical care.

Of the 722 patients, 669 (92.7%) were younger than 65, and 66 (9.1%) were pregnant women, the report states. Among the 601 patients with available data, 172 (28.6%) had a body mass index greater than 35, indicating obesity.

H1N1 patients found to be at particular risk for needing critical care were infants up to 1 year old and adults between 25 and 64, the report states. Others at increased risk were pregnant women, the obese, and indigenous Australians and New Zealanders.

While the rate of ICU admissions among elderly people was low, "the risk of death increased with increasing age," the authors note.

Proof of H1N1's potential seriousness
William Schaffner, MD, an immunization expert and chair of the Department of Preventive Medicine at Vanderbilt University in Nashville, said he found no major surprises in the two reports, but they showed once again how serious H1N1 flu can be.

"We still have many people who regard H1N1 influenza as, quote, mild, and of course, most patients get a mild case and recover," he said. "However, these papers demonstrate that H1N1 influenza can strike and create a grave illness, and I mean that in the root sense of the word.

"And although most of the patients who were admitted to ICUs had underlying illnesses, not all did. So even previously healthy people can be very seriously affected."

Schaffner added that the best way to limit the impact of the virus is through widespread vaccination. Because the virus has remained genetically stable, the vaccine should be "maximally effective," he said.

Jain S, Kamimoto L, Bramley AM, et al. Hospitalized patients with 2009 H1N1 influenza in the United States, April-June 2009. N Eng J Med 2009; early online publication Oct 8 [Full text]

The ANZIC Influenza Investigators. Critical care services and 2009 H1N1 influenza in Australia and New Zealand. N Engl J Med 2009; early online publication Oct 8 [Full text]

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