[9] There are two known circulating lineages of influenza B virus based on the antigenic properties of the surface glycoprotein hemagglutinin.

[17] The influenza B virus genome is 14,548 nucleotides long and consists of eight segments of linear negative-sense, single-stranded RNA.

Also in 1936, Macfarlane Burnet made the discovery that influenza virus may be cultured in hen embryonated eggs.

[25] In today's current world, even while some technology has advanced and flu vaccines now cover both strains of influenza A and B, the science is still based on findings from almost a century ago.

[28][failed verification] However, the B/Yamagata lineage might have become extinct in 2020/2021 due to COVID-19 pandemic measures,[12] and there have been no naturally occurring cases confirmed since March 2020.

Based on calculations of the rate of amino acid substitutions in HA proteins, it was estimated that IBV and IAV diverged from one another around 4000 years ago.

[4] However, the mechanisms of replication and transcription, as well as the functionality of the majority of viral proteins, appear to be largely conserved, with some unusual differences.

Small changes in the HA and NA of influenza viruses caused by antigenic drift result in the creation of novel strains that the immune system of humans might not be able to identify.

[23] Annual influenza outbreaks are caused by antigenic drift and declining immunity, when the residual defenses from prior exposures to related viruses are incomplete.

[33] Hemagglutination inhibition assays were used to examine the antigens of the influenza B virus primary and reinfection strains that were isolated from 18 children between the years of 1985 and 1990, which encompassed three epidemic periods.

The findings revealed that reinfection occurred with the viruses recovered during the 1984–1985 and 1987–1988 influenza seasons, which belonged to the same lineage and were antigenically close.

[33] Today, the B/Yamagata lineage might be extinct as a result of COVID-19 pandemic measures,[12] and there have been no naturally occurring cases confirmed since March 2020.

[13][14] Although this development has resulted in updated recommendations regarding vaccine composition,[13][14] continued surveillance is required to assess this conclusion fully, as pauses in IBV circulation have been observed before.