Why do flu viruses keep changing: what are antigenic drift and shift?

Flu viruses keep changing because their surface proteins are what our immune system learns to recognise, and those proteins keep changing in ways that let the virus slip past it. Scientists call this antigenic variation, and for flu it happens in two ways. Antigenic drift is a steady trickle of small copying errors, called point mutations, that arise when the virus's genome is imperfectly replicated. Antigenic shift is a sudden swap of whole gene segments when two different viruses infect the same cell.

Drift is why a new flu vaccine is required every year. Flu viruses have a relatively high mutation rate, typical of RNA viruses, so the surface proteins slowly change. The immune system remembers the old version of the antigens, and a changed version can evade that memory.

Shift is rarer and bigger. The flu genome comes in eight separate RNA segments, so if two strains infect one cell, their segments can be mixed and matched into new viruses, a process called reassortment. Meaningful shifts tend to occur every couple of decades. It is thought that every flu A virus behind human outbreaks or pandemics since the 1900s began in wild aquatic birds and acquired its new traits through reassortment with other strains. The 2009 "swine flu" pandemic came from an H1N1 that was antigenically very different from earlier H1N1 strains.