How does antigenic shift increase the risk of an influenza pandemic?

Antigenic shift increases the risk of an influenza pandemic by creating a new influenza A subtype that is immunologically distinct from strains previously circulating in the human population.12

This process often occurs through genetic reassortment, where an intermediate host—such as a pig—is simultaneously infected with different influenza A viruses, such as human, swine, and avian strains. Because the RNA genome of influenza A is segmented, these viruses can exchange genetic material to produce a new version with novel surface antigens, specifically hemagglutinin and neuraminidase.1342

Because the human population possesses little to no pre-existing immunity or antibodies against these novel antigens, the virus can spread rapidly and efficiently, leading to a pandemic. This mechanism is believed to have been responsible for the influenza pandemic (H1N1) of 2009, as the virus contained a combination of genetic material from human, swine, and avian influenza strains.1243