A yearly flu shot can sound like a hassle, but there are two clear reasons for it.
The virus changes every year
Flu viruses keep changing. Each year the World Health Organization uses surveillance data to predict the strains most likely to circulate next season, and the vaccine is updated accordingly. Last year's vaccine may not match this year's circulating strains.
Protection wanes
Even if the strains don't change, protection from a flu vaccine falls over time, usually lasting a few months. Vaccinating again each year restores protection to a higher level before the season arrives.
The Hong Kong recommendation
The Centre for Health Protection recommends a seasonal flu vaccine once a year. High-risk groups, including the elderly, young children, pregnant women, people with chronic illness and healthcare workers, should especially get it yearly, because they're at higher risk of serious complications.
When to get it
Ideally from September each year, to build protection before the winter peak (see "When is flu season in Hong Kong, and when should I get the flu shot?").