A strain of seasonal influenza has completely vanished globally since March 2020, and due to its disappearance, health officials in Canada are changing up the annual flu vaccine.
The B/Yamagata influenza strain, once a dominant force among influenza B viruses, may have vanished entirely due to the stringent measures imposed during the COVID-19 pandemic, according to the National Advisory Committee on Immunization (NACI). No cases have been confirmed across the globe since March 2020.
Due to the strain’s disappearance, NACI announced on July 26 that expert groups have endorsed removing the B/Yamagata component from influenza vaccine formulations. As a result, Canada’s flu vaccine will shift from a quadrivalent to a trivalent formula.
“As of March 2020, following the onset of the COVID-19 pandemic and implementation of measures to reduce the transmission of SARS-CoV-2, there have been no confirmed detections of naturally circulating B/Yamagata lineage viruses worldwide, including in Canada,” NACI said in the statement.
“Based on current epidemiological data, there is global expert consensus that B/Yamagata virus strains should be removed from influenza vaccine formulations and that manufacturing should transition to trivalent vaccines exclusively,” it added.
Before the strains disappeared, there were four strains of seasonal influenza circulating: two from influenza A (H1N1 and H3N2) and two from influenza B (B/Victoria and B/Yamagata).
Since the 2014 to 2015 flu season, Canada has used quadrivalent influenza vaccines containing strains from both influenza B lineages (B/Victoria and B/Yamagata) to offer protection against circulating influenza B viruses.
Although influenza A is usually the dominant strain, both influenza A and B contribute to seasonal epidemics. Despite its lower overall prevalence, influenza B can still pose a significant burden in Canada, accounting for up to 44 per cent of reported laboratory detections in some seasons, according to NACI.
“In November, December and into January, we mostly see influenza A. And then in the latter part of the season, January, February and tailing off through March, we’ll see a little bit more influenza B,” explained infectious diseases specialist Dr. Isaac Bogoch.
“If you look at the absolute number of cases, we usually have way more influenza A compared to influenza B, but B starts to show up a little bit later on in the season.”
He said that while the diagnosis, prevention, and treatment of influenza A and B are the same, what differs is their epidemiology. This difference in epidemiology might explain why B/Yamagata disappeared while other strains persisted.