Why is influenza so difficult to control from year to year?

Influenza is difficult to control from year to year primarily due to the virus's high mutation rate, which leads to antigenic drift and shift. These genetic changes can alter the virus's surface proteins, making it less recognizable to the immune system, including responses from previous infections or vaccinations. Additionally, the virus can infect a wide range of hosts, including birds and pigs, facilitating the emergence of new strains. This variability requires annual updates to vaccines, complicating efforts to achieve widespread immunity.