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Severe H1N1 Influenza: Still a Risk

Amesh A. Adalja, MD, FACP, FACEP, FIDSA, February 5, 2016

Thus far, the 2015-16 influenza season has been relatively mild as judged by all aggregate statistical indicators. The lack of severity of flu this season is likely multifactorial, but it is partly due to the match of the strains included in the seasonal vaccine with those circulating. The season has been dominated by influenza A strains, with over half of typed viruses being H1N1 pdm09—the former pandemic strain of 2009. This dominance has been increasing over recent weeks.

However, in contrast to the tranquil season experienced thus far, clusters of severe infection have triggered a CDC Health Alert.1


Severe H1N1 Disease

According to the health alert, CDC has recently been notified of severe H1N1 pdm09 infections in young to middle-aged adults. Some of these patients have required admission to an intensive care unit (ICU), and some have died. Not surprisingly, given its poor sensitivity, rapid antigen diagnostic testing was negative in a proportion of these patients, requiring the use of PCR-based tests to make the definitive diagnosis. Most of the patients were unvaccinated. No evidence has been reported to date that the severe cases attributed to H1N1 pdm09 strains reflect any change in the virus. 

One cluster, likely part of the health alert, involves an outbreak of H1N1 at an Indiana prison in which 20 inmates were sickened, 1 fatally.2


Flu Season Is Not Over

The health alert serves as a reminder that the historical peak of the influenza season is in February, and as the month progresses it is expected that flu activity will increase nationwide. Already statistical indicators are showing mild upticks. The fact that the influenza vaccine is perfectly matched this season—and this cluster of severe disease occurred in largely unvaccinated individuals—attests to the efficacy of the vaccine in preventing severe disease. 

That severe disease has been reported in young to middle-aged adults, but not the elderly, is a well-established hallmark of H1N1 and underscores the need to be alert to the possibility of severe influenza disease in this demographic, even when the poorly sensitive rapid influenza test returns with a negative result. Such negative results should never prompt the withholding of antiviral therapy in cases of influenzalike illness that are severe or in those expected to have severe symptoms. As the flu season continues, attention to severe clusters of illness and the underlying mechanisms at work, such as lack of vaccination, absence of antiviral therapy, and membership in special populations, will be important to determine and target intervention.



  1. US Centers for Disease Control and Prevention. Flu season begins: severe influenza reported. CDC website. February 1, 2016. Accessed February 2, 2016.
  2. Influenza virus hits inmates in Indiana prison, officials say. CNN January 26, 2016 Accessed February 2, 2016.