Early Season Pediatric Influenza B/Victoria Virus Infections Associated With a Recently Emerged Virus Subclade — Louisiana, 2019

Daniel Owusu, DrPH; Julie Hand, MSPH; Mark W. Tenforde, MD, PhD; Leora R. Feldstein, PhD; Juliana DaSilva, MA; John Barnes, PhD; Grace Lee, MD; Juliet Tran, MD; Theresa Sokol, MPH; Alicia M. Fry, MD; Lynnette Brammer, MPH; Melissa A. Rolfes, PhD


Morbidity and Mortality Weekly Report. 2020;69(2):40-43. 

In This Article

Abstract and Introduction


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. Nationally, to date, B/Victoria viruses are the most commonly reported influenza viruses among persons aged <25 years.[2] Of the 198 patients in the investigation, 95% were aged <18 years. 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.[3] Antiviral treatment of influenza is recommended as soon as possible for all hospitalized patients and for outpatients at high risk for influenza complications (including children aged <2 years and persons with underlying medical conditions).[4]

In November 2019, a field investigation was conducted to characterize the early influenza B virus–associated illnesses in Louisiana and to determine the influenza B virus subclades responsible for the outbreak. Medical chart abstraction, using a standard case report form, was conducted for 198 persons with laboratory-confirmed influenza B virus infection who had respiratory specimens submitted to the Louisiana Public Health Laboratory, including 173 outpatients and 25 hospitalized patients, from May 24 to November 21, 2019. Among 198 completed medical chart abstractions, 181 patients (158 outpatients and 23 inpatients) were from facility A; 17 were from other facilities in Louisiana.

The percentage of health care visits for influenza-like illness in Louisiana began to increase in mid-August 2019, corresponding to surveillance week 33 (Figure). Illness onset among the 198 patients occurred during May 24–October 29, 2019 with median onset during surveillance week 38 (ending September 21, 2019). The median age of patients was 6 years (range = <1 month–29 years); 95% were aged <18 years, reflecting both the increased circulation of influenza B viruses in children and the general patient population of facility A. None of the 198 patients had received the 2019–20 seasonal influenza vaccine before becoming ill, likely at least in part because influenza activity began early, before influenza vaccine campaigns start. Most patients reported subjective fever (95%), cough (68%), and runny nose (61%). Among the 173 outpatients, 41 (24%) had an underlying medical condition, the most common of which was asthma (Table); 17 (10%) had a complication associated with their infection, and 122 (71%) were prescribed influenza antivirals. Among 25 hospitalized patients, 14 (56%) had an underlying medical condition, 23 (92%) were prescribed influenza antivirals, 11 (44%) had complications associated with their infection, and six (24%) were admitted to intensive care units.


Percentage of visits for influenza-like illness* reported by sentinel clinics, by surveillance week — Louisiana, influenza seasons 2015–16 to 2019–20
*Defined as fever (temperature of ≥100°F [≥37.8°C], oral or equivalent) and cough or sore throat, without a known cause other than influenza.

Among 83 influenza B viruses sequenced from the 198 patients, 81 (98%) belonged to the influenza B/Victoria V1A.3 subclade, which began circulating in the United States in the latter half of the 2018–19 influenza season.[5] One of the detected viruses in Louisiana belonged to subclade V1A.1, which is the subclade of the influenza B/Victoria component (B/Colorado/06/2017) of the 2019–20 Northern Hemisphere vaccine. One of the 83 viruses could not be classified.