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We Now Know Why Type B Influenza Has Been So Predominant This Season, the First Time in 27 Years


René F. Najera, DrPH

January 11, 2020

A report by the Centers for Disease Control and Prevention's (CDC) Morbidity and Mortality Weekly Report (MMWR) has finally clarified why this flu season has started so strongly with a type B influenza predominance. :
“Multiple genetically distinct influenza B/Victoria lineage viruses have cocirculated in the United States recently, circulating sporadically during the 2018–19 season and more frequently early during the 2019–20 season (1). The beginning of the 2019–20 influenza season in Louisiana was unusually early and intense, with infections primarily caused by influenza B/Victoria lineage viruses. One large pediatric health care facility in New Orleans (facility A) reported 1,268 laboratory-confirmed influenza B virus infections, including 23 hospitalizations from July 31 to November 21, 2019, a time when influenza activity is typically low. During this period, Louisiana also reported one pediatric death associated with influenza B virus infection. An investigation of the influenza B virus infections in Louisiana, including medical and vaccine record abstraction on 198 patients, primarily from facility A, with sporadic cases from other facilities in the state, found that none of the patients had received 2019–20 seasonal influenza vaccine, in part because influenza activity began before influenza vaccination typically occurs. Among 83 influenza B viruses sequenced from 198 patients in Louisiana, 81 (98%) belonged to the recently emerged B/Victoria V1A.3 genetic subclade.”
Two things stand out in this passage from the full report. First, 98% of the identified type B viruses are from a recently emerged subclade. This means that the genes inside the type B influenza have re-assorted themselves in a way that the virus is different from its predecessors. This could result in the virus being completely “new” to the immune system of most people, leading to the increased number of cases and outbreaks of influenza that we are seeing this season. Second, most of the hospitalizations occurred in patients who had not been vaccinated, although the report does mention that the hospitalizations happened early in the flu season when the vaccine was not yet widely available. Nowhere in the report does it mention that the current vaccine does not confer immunity against this new type B influenza virus. The report does recommend the seasonal influenza vaccine for those who have not yet received it. Historically, even if a virus is not included in the influenza vaccine, there is a good chance that the vaccine still provides some protection. For more information on the current influenza activity in the United States, visit . For the remainder of the season, we will post a widget with the most recent influenza activity map for the United States at the top of the right sidebar on the blog.


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