An ‘unusual’ start to flu season: Influenza B predominates early
The proportion of outpatient visits attributed to influenza-like illness climbed above the national baseline for the first time this season, comprising 2.5% of visits during the week ending Nov. 16, according to CDC FluView data.
Overall, more than half of influenza-positive samples tested at public health laboratories this season have been influenza B viruses. In the week ending Nov. 16 — the most recent week with available data — 63.9% of positive specimens were influenza B viruses.
According to Bernhard L. “Bud” Wiedermann, MD, MA, attending physician in infectious diseases at Children’s National Hospital in Washington, D.C., and professor of pediatrics at The George Washington University School of Medicine & Health Sciences, the early predomination of influenza B is “unusual” in the United States.
“Generally, we see it toward the tail end of the season,” he told Healio. “At present, it is just an anomaly but could mean that we are going to see another peak of influenza A later in the flu season. It’s just a sign to be on the lookout for an atypical year.”
The overall hospitalization rate due to influenza was 1.4 per 100,000 people during the week ending Nov. 16, typical for this time of year, the CDC said. Pneumonia and influenza caused 5.2% of deaths, still below the 6.2% epidemic threshold. The CDC reported one new influenza-related pediatric death, bringing the total to four this season.
To date, there have been 158,740 samples tested in clinical laboratories, of which 3.3% have been positive for influenza — 31.7% for influenza A and 68.3% for influenza B. In public health laboratories, 8,105 specimens have been tested, of which 1,676 were positive. Influenza A comprised 48.4% of samples and influenza B 51.6%.
Wiedermann noted that these trends should not affect treatment.
“We don’t need to treat influenza B any differently from influenza A,” he said. “Medications that treat influenza are generally active against both strains.”
However, he said vaccine efficacy may be impacted, particularly for older patients.
“This year, all the regular flu vaccines are quadrivalent: they contain protection for two different strains of influenza A and two different strains of influenza B,” Wiedermann said. “However, one of the high-dose vaccines, usually used in persons older than 65, is trivalent: it contains the two strains of influenza A but only one influenza B strain. If the current influenza B strain isn’t covered by the trivalent vaccine — we don’t know if this is the case yet — then some of our older individuals may not have good protection.”
According to Wiedermann, CDC data going back 15 years do not contain evidence of an influenza B predominant season.
“We haven’t seen this predominance of influenza B in the ‘modern’ era when we have had ready access to rapid molecular testing,” he said. – by Marley Ghizzone
CDC. Weekly U.S. Influenza Surveillance Report. https://www.cdc.gov/flu/weekly/index.htm. Accessed November 26, 2019.
Disclosures: Wiedermann reports no relevant financial disclosures.