New CDC Study Compares Severity of Illness Caused by Flu A and B Viruses

Influenza B virus infections can be just as severe as influenza A virus infections

As late-season influenza B viruses currently predominate in the United States, a new study published by CDC and partners highlights the comparative severity of illness associated with influenza A versus influenza B virus infections. The results of the study showed that among hospitalized adults, flu B viruses caused equally severe disease outcomes and clinical characteristics as flu A viruses. This contradicts a common misconception that flu B viruses are associated with milder disease than flu A viruses.

The study looked at the disease characteristics – including the severity of illness – associated with flu A and flu B viruses among hospitalized adults over eight flu seasons (2005-06 through 2012-13). The study identified 21,186 flu A and 3,579 flu B-associated hospitalizations during the study period. Flu A viruses were the predominant flu virus in circulation during all of the seasons studied, particularly during the 2009 H1N1 pandemic.

The study found no significant difference in the overall proportion of hospitalizations with an ICU admission by virus type for each season. Among hospitalized adults with flu A or B infection, length of hospital stay and the proportion of patients admitted into an Intensive Care Unit (ICU) were comparable. Results also showed that flu B virus infections caused a similar proportion of deaths as flu A virus infections among hospitalized adults during the study period.

Study findings prompted the authors to conclude that clinicians should not regard flu B infections as less severe than flu A when considering treatment options. These findings support CDC’s existing antiviral treatment recommendations for the treatment of all hospitalized and high risk patients with suspected or confirmed flu infection. The type of flu virus infection (A or B) should not influence treatment decisions.

For most of the 2013-14 flu season, the 2009 H1N1 virus has been the predominant virus. However, over the past month and a half there has been a late season wave of flu B virus activity. At this time, flu B viruses are the predominant virus, with the highest levels of activity primarily focused in the Northeastern United States. Second waves of influenza B virus activity are not uncommon. About 70% of recent B viruses have been like the B virus component of the trivalent flu vaccine. The remaining flu B viruses have been like the second B vaccine component in the quadrivalent flu vaccine.

This study is available online from the Clinical Infectious Diseases journal website.