Changes in the ANTIGEN population by slow and minor (antigenic drift) or sudden and major mutations (antigenic shift). Accumulation of minor mutations in antigenic drift over time may require development of new vaccine (e.g., seasonal FLU VACCINES). Antigenic shift on the other hand is less common but may result in emergence of a new subtype or species and sometimes pandemics (e.g., H1N1 pandemics in 2009 and 2018 and COVID-19) due to lack of HERD IMMUNITY to the newly emerged novel pathogen.
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