Study: Compared to recent flu strains, 2009 H1N1 infection had lower risk of most serious complications

Sep 07, 2010

An analysis of data from influenza cases in Wisconsin indicates individuals with 2009 H1N1 infections were younger than those with H3N2 (2007-2008), and that the risk of most serious complications was not higher in adults or children with 2009 H1N1 compared with recent seasonal strains, according to a study in the September 8 issue of JAMA.

"The 2009 A(H1N1) virus caused widespread transmission in the United States and other countries. The (CDC) estimates that 43 million to 89 million infections occurred in the United States from April 2009 through April 10, 2010, with mid-range estimates of 274,000 H1N1-related hospitalizations and 12,470 deaths. Children, young adults, pregnant women, and individuals with underlying chronic medical conditions appear to have a higher risk of hospital admission and critical illness when infected with the pandemic virus," the authors write. "The clinical characteristics of pandemic 2009 influenza A(H1N1) infections have not been compared directly with illnesses caused by other influenza A strains."

Edward A. Belongia, M.D., of the Marshfield Clinic Research Foundation, Marshfield, Wisc., and colleagues conducted a study to compare the characteristics of pandemic and A infections occurring in a defined population. The study consisted of active surveillance with 30-day follow-up for influenza cases among children and adults living in a 14-zip code area in Wisconsin. Patients with subjective fever, chills, or cough of fewer than 8 days' duration were screened for eligibility during an outpatient or inpatient encounter. Consenting patients were interviewed and tested for influenza A during the 2007-2008 and 2008-2009 influenza seasons and from May to November 2009; 6,874 patients (70 percent-86 percent of eligible patients) agreed to participate. Medical records were reviewed to assess outcomes.

The researchers identified 2009 H1N1 influenza in 545 patients; 221 cases of seasonal H1N1; and 632 patients with H3N2 infection. The median (midpoint) ages of infected participants were 10, 11, and 25 years, respectively.

In children, 2009 H1N1 infection was not associated with either hospital admission or pneumonia compared with seasonal H1N1 or H3N2. Hospital admission occurred within 30 days for 6 of 395 children with 2009 H1N1 (1.5 percent), 5 of 135 with seasonal H1N1 (3.7 percent), and 8 of 255 with H3N2 (3.1 percent). Among adults, hospital admission occurred in 6 of 150 with 2009 H1N1 (4.0 percent), 2 of 86 with seasonal H1N1 (2.3 percent), and 17 of 377 with H3N2 (4.5 percent).

Among adults, pneumonia occurred in 4.0 percent of those with 2009 H1N1 infection; 2.3 percent of patients with seasonal H1N1; and 1.1 percent of those with H3N2 infection. Pneumonia occurred in 2.5 percent of children with 2009 H1N1; 1.5 percent of children with seasonal H1N1; and 2.0 percent with H3N2. There were no significant differences by strain in the proportion of children or adults with any serious outcome (pneumonia or hospital admission) during the 30 days after onset.

"Our results suggest that the clinical manifestations and risk of hospital admission are similar for 2009 H1N1 and other seasonal influenza A strains among those presenting for medical care and documented to have influenza infection," the authors write.

"In summary, we found that children were disproportionately affected by 2009 H1N1 infection, but the perceived severity of symptoms and risk of serious outcomes (pneumonia or hospital admission) were not increased in with 2009 H1N1 infection relative to seasonal influenza A viruses. This study demonstrates the benefit of ongoing active influenza surveillance in a defined cohort with standardized testing criteria and uniform collection of clinical and epidemiologic data. The use of consistent enrollment and testing procedures offers the opportunity to directly compare illness patterns and outcomes over multiple seasons, and these results complement information obtained through traditional public health surveillance systems."

Explore further: Recorded Ebola deaths top 7,000

More information: JAMA. 2010;304[10]:1091-1098

add to favorites email to friend print save as pdf

Related Stories

H1N1 influenza hits older children

May 04, 2010

Children hospitalized with pandemic H1N1 influenza in 2009 were older and more likely to have underlying medical conditions than children hospitalized with seasonal influenza during prior flu seasons, according to a study ...

Of swine, birds and men -- pandemic H1N1 flu

Feb 01, 2010

Current research suggests that pandemic H1N1 influenza of swine origin has distinct means of transmission from the seasonal flu, yet does not result in the pathogenic severity of avian flu viruses. The related report by ...

Recommended for you

Recorded Ebola deaths top 7,000

17 hours ago

The worst Ebola outbreak on record has now killed more than 7,000 people, with many of the latest deaths reported in Sierra Leone, the World Health Organization said as United Nations Secretary-General Ban ...

Liberia holds Senate vote amid Ebola fears (Update)

22 hours ago

Health workers manned polling stations across Liberia on Saturday as voters cast their ballots in a twice-delayed Senate election that has been criticized for its potential to spread the deadly Ebola disease.

Evidence-based recs issued for systemic care in psoriasis

Dec 19, 2014

(HealthDay)—For appropriately selected patients with psoriasis, combining biologics with other systemic treatments, including phototherapy, oral medications, or other biologic, may result in greater efficacy ...

Bacteria in caramel apples kills at least four in US

Dec 19, 2014

A listeria outbreak believed to originate from commercially packaged caramel apples has killed at least four people in the United States and sickened 28 people since November, officials said Friday.

Steroid-based treatment may answer needs of pediatric EoE patients

Dec 19, 2014

A new formulation of oral budesonide suspension, a steroid-based treatment, is safe and effective in treating pediatric patients with eosinophilic esophagitis (EoE), according to a new study in Clinical Gastroenterology and Hepatology, the official clinical practice journal ...

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.