Vaccine makers have to guess because it will help prevent the sickness from forming or worsening into a more deadly form.
The bird flu like other flu is caused by a virus and this virus mutates very fast with time.
Mutation is a change in the genetic make up of an organism which in this case, makes them resistant to vaccines that was effective on them not too long ago.
Learn more about flu, refer to the link:
https://brainly.com/question/9200383
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