Final answer:
Antigenic shifts, leading to significant changes in influenza viruses due to gene reassortment, most commonly occur when two different viruses infect the same host cell.
Step-by-step explanation:
Antigenic shifts are major changes in an influenza virus resulting from the combination of two different virus genomes, a process called gene reassortment. This event most often occurs when two different influenza viruses infect the same host cell. Antigenic shift can produce a novel virus against which the human population has little to no immunity, leading to more severe outbreaks or pandemics.
In contrast, antigenic drift refers to smaller, gradual changes in the virus due to point mutations. Understanding these mechanisms is crucial for developing influenza vaccines and anticipating potential pandemics.