Final answer:
The flu vaccine's effectiveness varies due to the influenza virus's rapid mutation and antigenic drift, which lead to new flu strains that the immune system may not recognize. This requires the development of new vaccine formulations each year, yet mismatches can still occur, causing vaccinated individuals to catch the flu.
Step-by-step explanation:
The flu vaccine is highly variable in its effectiveness because the influenza virus undergoes frequent mutations, leading to antigenic variation. This means that immune system may not recognize new strains that emerge, despite previous exposure or vaccination. Antigenic drift causes continuous changes in the virus's surface molecules, which affects the vaccine's ability to provide protection across different flu seasons. As such, new vaccine formulations must be developed annually to match the most prevalent flu strains of the year. However, this rapid evolution and adaptation of the flu virus can result in a mismatch between the vaccine and the strains circulating in the population, which happened to a person who received a flu vaccine in November but then caught influenza in January, likely due to a new strain that had enough differences to evade the previously developed antibodies.