Slide 1: Influenza B was once thought to only infect humans. Which animal was subsequently discovered to be an additional host for the virus?

Slide 2: Answer: Seals!

Slide 3: Influenza B was discovered in 1940. At the time, it differed from influenza A in that humans were its only reservoir. However, in 1999, seals were discovered to also harbor influenza B. [Diagram showing: 1933 – Influenza A is isolated from humans, 1940 – Influenza B discovered, 1999 – Influenza B discovered in seals]

Slide 4: Influenza can only undergo antigenic shift to create pandemics though animals. Without an animal reservoir, it can only undergo drift to cause milder epidemics. Antigenic drift involves small changes in surface proteins due to change mutation. In comparison, shift involves large changes due to mixing in animal hosts. [diagram on shift vs drift]

Slide 5: The current annual influenza vaccine contains two influenza A strains and either: One influenza B strain (trivalent vaccine) OR both influenza B strains (quadrivalent vaccine). The influenza A strains included are different every year based on the WHO and CDC’s predictions of the circulating strain.


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  • Osterhaus AD, Rimmelzwaan GF, Martina BE, Bestebroer TM, Fouchier RA. Influenza B virus in seals. Science. 2000 May 12;288(5468):1051-3. PMID 10807575.
  • Center for Disease Control and Prevention. Frequently Asked Influenza (Flu) Questions: 2019-2020 Season. Accessed Nov 2019 CDC Website.