A US study of flu cases in adults and children living in Wisconsin concluded that the risk of serious complications from 2009 H1N1 swine flu was no higher than the risk of serious complications from recent seasonal flu strains.

You can read about the study in the 8 September issue of the Journal of the American Medical Association, JAMA.

Researchers from the Marshfield Clinic Research Foundation, Marshfield, Wisconsin, and the Influenza Division at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, also found that patients who contracted the 2009 swine flu tended to be younger than those who contracted seasonal flu.

The CDC estimates that the pandemic 2009 A(H1N1) swine flu virus infected between 43 and 89 million people throughout the US between April 2009 and April 2010, resulting in around 274,000 H1N1-related hospitalizations and 12,470 deaths.

The authors wrote in their background information that:

“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.”

However, until now, no studies have made a direct comparison of the clinical characteristics of pandemic H1N1 swine flu infections and those caused by other influenza A viruses.

Influenza A viruses are the types that cause the most disease and complications in humans.

For their study, lead author Dr Edward A. Belongia, a Senior Epidemiologist and Director of the Epidemiology Research Center at Marshfield, and colleagues used data taken from active surveillance with 30 days of follow up of flu cases in children and adults living in a particular zip code area of Wisconsin.

A first screening included only outpatients and inpatients presenting with subjective fever, chills or coughs lasting less than 8 days.

The screened patients were then asked for their consent to be interviewed and tested for influenza A during the 2007-08 and the 2008-09 flu seasons, and also from May to November 2009. In total 6,874 patients, more than 70 per cent of those eligible, agreed to take part in the surveillance.

Among this population, they identified 545 patients of median age 10 years had been infected with 2009 H1N1 swine flu, 221 of median age 11 years with seasonal H1N1, and 632 patients of median age 25 years with H3N2, another serotype influenza A virus.

When they compared these case numbers to outcomes in the patients’ medical records, Belongia and colleagues found that:

  • During the 30 days of follow up, compared with seasonal H1N1 and H3N2, the 2009 H1N1 swine flu was not linked to higher rates of hospital admission or pneumonia among children.
  • 6 of 395 children (1.5 per cent with 95 per cent CI ranging from 0.6 to 3.1 per cent) infected with 2009 H1N1 were admitted to hospital compared with 5 of 135 children (3.7%, with CI from 1.4 to 8.0%) with seasonal H1N1, and 8 of 225 children with H3N2 (3.1%, with CI from 1.5 to 5.9%).
  • 10 children (2.5%, with CI from 1.3 to 4.5%) with 2009 H1N1 contracted pneumonia compared with 2 (1.5%, CI from 0.2 to 5.2%) with seasonal H1N1, and 5 with H3N2 (2.0%, CI from 0.7 to 4.3%).
  • In adults, a similar picture emerged, with no significantly higher rates of hospital admissions or pneumonia for 2009 H1N1 swine flu than for seasonal flu (note in the following figures you can see this more easily by comparing the 95% confidence intervals: there is a lot of overlap in each of the three sets of comparisons, suggesting statistical insignificance in the differences).
  • 6 of 150 adults (4.0 per cent, with 95 per cent CI ranging from 1.6 to 8.1 per cent) infected with 2009 H1N1 swine flu were admitted to hospital compared with 2 of 186 (2.3%, CI from 0.3 to 8.1%) adults with seasonal H1N1 and 17 of 377 (4.5%, CI, from 2.7 to 7.0%) with H3N2.
  • 6 adults (4.0%, CI from 1.6 to 8.1%) with 2009 H1N1 swine flu contracted pneumonia compared with 2 (2.3%, CI from 0.3 to 8.1%) with seasonal H1N1 and 4 (1.1%, CI from 0.3 to 2.7%) H3N2.
  • Also, during the 30 days of surveillance follow up, there were no significant differences by strain among the proportion of children or adults with serious outcomes (ie hospital admissions or pneumonia).

The researchers concluded that:

“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.”

Overall, they found that while more children appeared to contract 2009 H1N1 swine flu, the perceived severity of complications, as measured by counting hospital admissions and cases of pneumonia, were no worse in swine flu cases than in seasonal flu cases of the A viruses.

They stressed that the study shows the benefit of continuing to monitor defined groups with standard enrollment, screening tests and ways of collecting and clinical and population data, so that disease patterns can be compared on an equivalent basis over several seasons.

Such results are a useful accompaniment to the more traditional public health surveillance reports, they added.

“Clinical Characteristics and 30-Day Outcomes for Influenza A 2009 (H1N1), 2008-2009 (H1N1), and 2007-2008 (H3N2) Infections.”
Edward A. Belongia; Stephanie A. Irving; Stephen C. Waring; Laura A. Coleman; Jennifer K. Meece; Mary Vandermause; Stephen Lindstrom; Debra Kempf; David K. Shay.
JAMA, Vol. 304, No. 10: pp 1091 – 1098, September 8, 2010.

Written by: Catharine Paddock, PhD