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------------------------------------------------ Which of the following is a challenge in developing an annual influenza vaccine?

1) Limited effectiveness against new strains of the virus
2) Difficulty in predicting the dominant strains of the virus
3) Time-consuming manufacturing process
4) All of the above

Developing an annual influenza vaccine is challenging because of the virus's rapid mutation leading to new strains, difficulty in predicting prevalent strains, and the lengthy manufacturing process.

Explanation:

The challenges in developing an annual influenza vaccine include limited effectiveness against new strains of the virus, the difficulty in predicting the dominant strains of the virus, and the time-consuming manufacturing process. These challenges arise primarily due to the high rate of antigenic variation in influenza viruses, which includes both antigenic drift and antigenic shift. These mechanisms enable the virus to mutate rapidly, resulting in new strains that the immune system and previous vaccines may not recognize. Consequently, health organizations must regularly monitor flu outbreaks and recommend new vaccine formulations for each flu season.