What causes variation in viruses?

Variation in viruses is driven by a process known as antigenic selection. This occurs when the immune response, induced by either natural infection or vaccines, puts pressure on the virus. This selection pressure favors the emergence of virus particles with mutations that allow them to escape immune detection.

According to Vincent Racaniello on the Lex Fridman Podcast, viruses are constantly mutating. The mutations necessary for creating variants are often already present in the viral population at low frequencies. When the immune system targets the virus, those mutations that enable the virus to evade the immune response become more prevalent and advantageous, leading to the emergence of new variants 1.

This process is evident in viruses like influenza, where annual variations in the virus necessitate changes in the vaccine each year 1. Thus, both natural infection and vaccination play roles in selecting for these viral variants.