#4881
Late yesterday the media began reporting on a new study appearing in JAMA on the severity of the pandemic H1N1 virus with headlines like:
2009 H1N1 flu not more serious than other seasonal influenza: study
H1N1 Swine Flu No Worse Than Seasonal Flu
Swine Flu in Children Found to Be No More Severe Than Seasonal Viruses
Swine flu less serious than other influenza: US study
While the relative mildness of the novel H1N1 virus isn’t exactly a new revelation, this study provides us with a unique and useful perspective of the impact of the pandemic.
But as you’ll see, there is more than one way to measure the severity of a pandemic.
The full article is behind a pay wall, but the link to the abstract is listed below.
Edward A. Belongia, MD; Stephanie A. Irving, MHS; Stephen C. Waring, DVM, PhD; Laura A. Coleman, PhD; Jennifer K. Meece, PhD; Mary Vandermause, BSMT; Stephen Lindstrom, PhD; Debra Kempf, BSN; David K. Shay, MD, MPH
JAMA. 2010;304(10):1091-1098. doi:10.1001/jama.2010.1277
This study comes from Dr. Edward A. Belongia and his colleagues doing influenza research at the Marshfield Clinic, located in central Wisconsin. Although the town has a population of only about 20,000 the clinic draws patients from surrounding areas as well.
These doctors began keeping detailed records of all patients presenting with ILIs (influenza-like-illnesses) back in 2007, as part of a study to measure the effectiveness of the seasonal flu vaccine.
They had detailed records on 545 pandemic H1N1 (Swine flu) patients, 221 patients with the old seasonal H1N1 flu during the 2008-2009 season, and 632 people with seasonal H3N2 flu during the 2007-2008 season.
They came up with the following numbers:
Children hospitalized:
1.5% with pandemic H1N1 flu
3.7% with old seasonal H1N1
3.1% with seasonal H3N2
Adults Hospitalized:
4.0% with pandemic H1N1 flu
2.5% with old seasonal H1N1
4.5% with seasonal H3N2
Children with pneumonia:
2.5% with pandemic H1N1 flu
1.5% with old seasonal H1N1
2.0% with seasonal H3N2
Adults with pneumonia:
4.0% with pandemic H1N1 flu
2.3% with old seasonal H1N1
1.1% with seasonal H3N2
Additionally, patient surveys indicated that most of those who contracted pandemic H1N1 felt their symptoms were less severe than they’d experienced with seasonal flu in previous years.
None of this comes as a great surprise, since from the start of the pandemic, the H1N1 virus was widely perceived as causing a high morbidity-low mortality illness.
Hospitalization rates for children in this study were actually less than with seasonal flu, although the rates for adults were about the same.
The incidence of pneumonia in adults and children were a bit elevated over those seen with seasonal flu in recent years.
Painting a picture of a virus not significantly worse (and in some respects, less severe) than seasonal flu.
There are some factors that may have affected these numbers, however.
The total number of children treated for swine flu in the six months between May-Nov 2009 (n=395) was roughly equal to the total of kids with seasonal flu (n=390) seen in the clinic in the previous two flu seasons.
Granted,we know that children were more likely to contract pandemic H1N1.
But given the public concern over the swine flu virus last summer, it is also possible that some milder cases of pandemic flu showed up at the clinic that in previous years might not have bothered.
And that has the potential, at least, to skew some of the percentages.
An unknowable factor, obviously. But one that makes direct comparisons of patient pools somewhat problematic.
And of course, this was a relatively small sampling in a single geographic location. Standard caveats for practically all studies.
While the percentage of pandemic H1N1 cases hospitalized was low, it should be noted that anecdotal reports from ICUs around the world often portrayed those who were hospitalized as being sicker than normally seen with seasonal influenza.
We also have this stark graphic showing a four-fold increase in pediatric deaths from influenza during the pandemic, caused at least in part, by the higher rate of infection among children.
There are many ways to measure the severity of an influenza outbreak. No single study is going to be able to take them all into account.
If you look at simple hospitalization rates, then yes, H1N1 appears to be a weak sister to seasonal strains of the past.
But the mean age of death from the novel H1N1 virus has been calculated to be half that of seasonal flu, or 37.4 years (see Study: Years Of Life Lost Due To 2009 Pandemic)
In terms of years of life lost (YLL), the average 2009 pandemic flu death had a many-fold greater impact than the average seasonal flu fatality.
As the saying goes; It’s complicated.
That said, the Marshfield clinic study provides us with an important view of data from the 2009 pandemic, one that shows the virus as being roughly comparable to previous seasonal flu strains.
But it isn’t the only view we must take into account.
