Why do vaccine makers have to guess which flu strains will be active months before the flu season begins?

Read the passage below and answer the question. somehow, it was hotter then: a black dog suffered on a summer’s day; bony mules hitched to hoover carts flicked flies in the sweltering shade of the live oaks on the square. men’s stiff collars wilted by nine in the morning. in the excerpt above, the phrase "stiff collars wilted by nine in the morning" suggests that despite the heat, men still dressed up the men in the town were vain men's clothing appeared its best on hot days the men were not accustomed to wearing nice clothes