What is the phenomenon called when a subtype of influenza replaces another, as seen with H1N1 and H2N2?

The phenomenon where a subtype of influenza replaces another, as seen with H1N1 and H2N2, is referred to as antigenic shift. This process involves a significant change in the influenza virus's surface proteins, specifically hemagglutinin (HA) and neuraminidase (NA), due to genetic reassortment. When two different influenza viruses infect the same cell, they can exchange genetic material, leading to the emergence of a new subtype with distinct antigenic properties. This major change can result in new viruses to which the population has little or no immunity, making antigenic shift a key factor in the emergence of pandemic strains of influenza.

The other processes, such as viral interference, phenotypic mixing, and antigenic drift, describe different mechanisms of viral evolution or interaction. Viral interference usually refers to a situation where the presence of one virus can inhibit the replication of another virus. Phenotypic mixing involves the exchange of surface proteins between different viruses without altering their genetic structure. Antigenic drift, on the other hand, describes small, incremental changes in the virus through mutations over time, which can lead to seasonal outbreaks but does not result in the emergence of entirely new subtypes.