Answer:
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. Also, available vaccines need to be tried clinically to determine their potency and also the possible side effects of using them. To counter the problem of fast mutation, scientists are working fast to study the genetic base of these viruses in order to learn how they mutate and what causes them to mutate and maybe get some answers to how they can counter this and produce a vaccine that will work for a longer time interval.
