Types of Influenza: A, B, C and D
Four distinct types exist, but only two matter for seasonal workplace outbreaks.
Type A
Responsible for most seasonal epidemics and all past pandemics. Infects humans and animals, and mutates most frequently — the primary reason vaccines are reformulated annually.
Type B
Circulates seasonally alongside Type A, generally less prone to major mutation, and only infects humans.
Type C
Typically causes milder illness and doesn’t drive seasonal epidemics.
Type D
Primarily affects cattle and is not currently known to infect or cause illness in humans.
For workplace planning purposes, Types A and B are what drive the seasonal absence patterns discussed throughout this hub. See vaccine effectiveness for how well annual formulations track these circulating strains.
This page provides general information and is not medical advice.
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