Influenza B and Severe Disease

By Amesh A. Adalja, MD, FACP, February 3, 2012

Currently, 2 lineages of influenza B circulate in the population—Victoria-like and Yamagata-like—though only 1 is covered by the trivalent seasonal influenza vaccine. In general, influenza B is not paid as much attention as the common A strain, notwithstanding influenza B’s ability to cause fulminant disease, precipitate Reye’s Syndrome, and result in fatal illness. In fact, influenza B caused 38% of pediatric influenza deaths in the 2010–11 influenza season. To better understand the pathology of fatal cases of influenza B, a CDC team analyzed autopsy data from cases submitted to the Centers during the decade between May 2000 and February 2010.

Shorter Time to Death

Samples from 45 patients were included in the study. Important demographic data from the case patients includes the following:

• 29 females and 16 males
• Median age of 11 years
• 76% of patients were younger than 18 years of age
• 43% had preexisting medical conditions
• Of the B strain types, 25 were of Victoria lineage and 17 were from the Yamagata lineage

Underscoring the sometimes fulminant nature of severe influenza B, the median number of days from onset of symptoms to death was 3—a time period shorter than was seen with the 1918, 1967, 1968, and 2009 influenza A pandemics.

An important inflection point in the data occurred with age, as those older than 18 years were statistically more likely to have bacterial pneumonia present in lung tissue. Of those with bacterial pneumonia, Staphylococcus aureus was the cause in 76% of patients.

Cardiac Pathology

Probably one of the most important findings of the study is the characterization of myocardial injury that developed in case patients. Of the 29 cases with cardiac tissue available, 20 revealed myocardial injury. Injury patterns included myocyte damage in 17 patients, myocarditis in 10 patients, and myocardial infarction in 1 patient. No viral antigens were present in myocardial tissue. Those with myocardial damage were also noted to be statistically less likely to have concomitant bacterial pneumonia.

Influenza Preparedness for Influenza A and B

Given the potential for pediatric deaths and hospitalizations caused by influenza B to impose a serious burden on the healthcare system, influenza planning and preparedness should broaden its scope to consider influenza B. Granted, influenza B does not have strong pandemic potential—antigenic drift occurs more slowly than in type A, antigenic shift does not occur, and B strains infect only humans and seals—it does have strong potential for causing a formidable burden of illness. With 2 strains of this virus circulating and only 1 strain covered by annual vaccines, a substantial hole in the population’s immunological defense will continue to be exploited by this virus.

References

Paddock CD, Liu L, Denison AM, et al. Myocardial injury and bacterial pneumonia contribute to the pathogenesis of fatal influenza B virus infection. J Infect Dis 2012. http://jid.oxfordjournals.org/content/early/2012/01/28/infdis.jir861.short?rss=1. Accessed January 31, 2012.

McCullers JA, Hayden FG. Fatal influenza B infections: time to reexamine influenza research priorities. J Infect Dis 2012. http://jid.oxfordjournals.org/content/early/2012/01/28/infdis.jir865.short?rss=1. Accessed January 31, 2012.