WHO says prior immunity may offer partial protection against new flu variants

Immunity acquired through a previous influenza infection or vaccination may retain some protective value against new variants, WHO technical experts told RIA Novosti on October 10.

Dmitry Lioznov, director of Russia's Smorodintsev Research Institute of Influenza, had earlier described new strains against which Russians lacked immunity. WHO's experts explained that a change in the virus does not necessarily erase all previously developed protection.

Seasonal influenza viruses continually evolve and can partially evade immune responses. Even so, immunity from an earlier infection or vaccination may still help when the virus a person encounters differs from a strain they have encountered before.

WHO recommends vaccine compositions twice a year for the northern and southern hemisphere influenza seasons. Formulations are selected to match the variants expected to circulate in the coming season and provide the best available protection, including against severe illness and death.

WHO's general guidance notes that protection varies between seasons and depends on how closely circulating viruses resemble vaccine strains. Existing immunity therefore does not guarantee that a person will avoid infection.