Why isn't influenza considered a pandemic? It kills over 500,000 people per year worldwide.

Answer 1

Pandemics, epidemics, outbreaks, and clusters, typically refer to situations when a number of cases of the same strain of the same disease spread to more and more people. The number of people and the geographic coverage determine which word is used (although the latter three can be argued to be synonymous). These days, we use whole genome sequencing and other genetic analysis to determine if two pathogens are related enough to determine that they are from the same source. If they ARE deemed from the same source, and there are 2+ cases, then it is an outbreak.

Following the same rules, seasonal influenza is not a pandemic because influenza varies greatly from one individual to the next and from one geographic area to the next. You may have even heard of things like H1N1 flu or H3N2 flu. These letters that describe proteins on the surface of the influenza virus are one way we classify the broad diversity within influenza strains. Hypothetically, if someone contracted bird flu from a farm or a state fair and then spread it to family, who spread it to acquaintances, who spread it across the globe, then THAT would be a pandemic, because it came from a single source and is genetically related (at least initially).