Most areas of North America are in the throes of a flu season marked by an unusual early dominance of influenza B, a strain not typically seen in large numbers until the later months of the flu season.

"We had a paucity of influenza B last year, so we may have anticipated community immunity would be low," said Danuta Skowronski, MD, MHSc, an epidemiologist with the British Columbia Centre for Disease Control in Vancouver. In fact, a few months ago, Skowronski said she gave a Grand Rounds talk about the upcoming flu season and the risk of influenza B.

"My concern was we had not seen influenza B/Victoria make a strong showing since the 2015-2016 season," Skowronski told CIDRAP News. "So immunity to that virus would be low."

And that low immunity may be behind the season's more severe illness in children.

Influenza B comes in two primary strains—Victoria and Yamagata. So far this year 98% of flu B viruses subtyped have been from the Victoria lineage. And B strains have outpaced A strains 58% to 42%. Quadrivalent (four-strain) vaccines contain both B strains, whereas trivalent (three-strain) versions this year have only the Victoria strain.

Severity in children

In a special Morbidity and Mortality Weekly Reports (MMWR) report last week, researchers from the Centers for Disease Control and Prevention (CDC) and Louisiana detailed early flu activity at a large pediatric hospital in New Orleans.

The facility reported 1,268 laboratory-confirmed influenza B virus infections, including 23 hospitalizations from Jul 31 to Nov 21, 2019, a time when influenza activity is typically low. And clinic visits for influenza-like illnesses started even earlier, in mid-August, before most patients had the chance to get a flu vaccine.

"B/Victoria viruses did not circulate widely during the past three influenza seasons, accounting for <10% of influenza virus isolates reported during the 2016–17 to 2018–19 seasons," the authors of the MMWR report said. "Even though influenza B viruses are less common than influenza A viruses in most seasons, influenza B virus infection can be severe in children."

Last week the CDC also released a Health Alert Network (HAN) advisory, warning clinicians that in previous seasons influenza B has been associated with a higher proportion of influenza-related pediatric deaths. So far in the 2019-20 flu season, 32 children have died, with 21 of those deaths linked to influenza B.

Imprinting flu B

Richard Webby, PhD, from St. Jude Children's Research Hospital in Memphis, Tennessee, said the pediatric vulnerability to influenza B may have to do with immunologic imprinting, a complex field of study that suggests patients will be able to defend themselves against viruses they were exposed to early in life.

In the case of B/Victoria, which has not been the dominant strain in a United States flu season since 1992-93, children may have no immunologic memory of the virus.

Skowronski said some data show B viruses tend to skew toward younger patients, and the average B/Victoria patients may be up to 20 years younger than a patient infected by another strain.

"With B/Yamagata, we see older people getting sick, but with B/Victoria, we see more flu activity in schools, less in long-term care facilities," Skowronski said.

Both Webby and Skowronski explained that, unlike A viruses, which can change via introductions from the animal kingdom, B viruses infect only humans and change through deletions and changes in amino acid structures.

In the past 3 years, B/Victoria has seen a double- and now triple-deletion, which could be another reason for this notable flu season.

"It's almost like this virus is searching around for a way to take off in a population," said Webby. "We've seen the virus take different pathways to evolve, as though each virus deletion version is testing the waters."

Vaccination may be especially useful

Webby emphasized that the current quadrivalent flu shot offered in the United States covers both B/Victoria and B/Yamagata, strains that demonstrate cross-reactivity and cross-protection. He said he is still encouraging flu vaccines.

"Even if we have seen the peak of the season, flu will last several more weeks," Webby said. And the flu vaccine is typically more protective against B viruses than certain A strains, he said.

"B tends to do better even when there is a bit of mismatch between the vaccine and circulating strain," he said.

See also:

Jan 10 MMWR report

Jan 10 CDC HAN advisory

Jan 10 CIDRAP News story "Five more kids' deaths in season dominated by influenza B"