Influenza B/Victoria Strain Predominating 2019-20 US Flu Season Thus Far


Between September 29-December 28, 2019, influenza B viruses accounted for 59.2% of influenza-positive results reported nationwide. Of these isolates, 97.9% belonged to the B/Victoria lineage.

For the first time since the 1992-93 influenza season, influenza B viruses are the predominant circulating flu virus in the United States. Typically, influenza B viruses circulate near the end of the flu season. In fact. in the previous 3 flu seasons B/Victoria viruses accounted for <10% of influenza isolates.

This year appears to be quite the contrast. Between September 29 through December 28, 2019, influenza B viruses accounted for 59.2% of influenza-positive results reported nationwide. Of these isolates, 97.9% belonged to the B/Victoria lineage.

According to a report in the US Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report, B/Victoria viruses appear to be the most common strain affecting individuals under the age of 25 years, while influenza A(H1N!)pdm09 viruses are most commonly reported among individuals 25 years and older.

Authors of the article also report that 70% of influenza-associated hospitalization among children and 18 of 27 influenza-associated deaths among pediatric populations were associated with influenza B viruses.

The report also discusses the severe nature of the 2019-20 influenza season in the state of Louisiana. The state’s flu season has been “usually early and intense” and health officials have noted a high number of illnesses of the B/Victoria lineage.

In one particular pediatric health care facility a total of 1268 laboratory-confirmed influenza B virus infections, including 23 hospitalizations, were documented between July 31 through November 21, 2019.

An investigation was conducted using data from medical and vaccine records from 198 patients. In total, 158 individuals were treated at the aforementioned facility and 17 were from other facilities in the state. The research found that none of the patients had received the seasonal influenza vaccine, which is likely attributable to the fact that flu activity began before annual flu vaccination is recommended.

Among 83 influenza B viruses that were sequenced from the 198 individuals, 98% belonged to the influenza B/Victoria V1A.3 subclade, which first began circulating in the later part of the 2018-19 US influenza season.

Most individuals reported fever (95%), cough (28%), and runny nose (61%). Among the 173 outpatients, 24% had underlying medical conditions, 10% had a complication associated with their infection and 71% were prescribed influenza antivirals. Among the 25 hospitalized patients 56% had an underlying medical condition, 92% were prescribed antivirals, 44% had infection-related complications, and 24% were admitted to intensive care units.

“Although most illnesses were uncomplicated, the number of hospitalizations, clinical complications, and the reported pediatric death in Louisiana serve as a reminder that, even though influenza B viruses are less common than influenza A viruses in most seasons, influenza B virus infection can be severe in children. All persons aged ≥6 months should receive an annual influenza vaccination if they have not already received it,” the authors of the report wrote.

The authors concluded the report by reminding the public that influenza activity is expected to continue for many weeks in the United States and that antiviral treatment is a critical tool for reducing the duration of flu symptoms and complications.

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