Because flu viruses mutate extremely rapidly, so a vaccination against any current strain will be outdated very quickly. Therefore a mass vaccination would be a waste of time until we know exactly what form of the virus we need protection from.
Before the bird or swine versions of the flu can transfer to us, or for it to be transferable from human to human, it has to mutate, so we don't know exactly what form of the virus may become a problem if it crosses species. That's the problem with animal viruses when they transfer, we they've gone through multiple rounds of mutation and adaption (eg: bird to swine, swine to human) and may change significantly each species jump.
Basically, the virus changes too rapidly and moves too unpredictably between species for us to know exactly what to raise a vaccine against until it's already in humans. Therefore making/using vaccines in advance is very difficult and unreliable.
Then again, the current flu may not even make the jump from birds/swine to humans and we may never have to deal with it. We just don't know (until it's too late).
It's unfortunately much more complicated then just replacing the missing or damaged cells with new healthy cells. The neurons are useless unless they have synapses with the correct neurons both for input and output. One could treat neurons to make them more likely to make synapses and stimulate synaptic formation, but the cells would have to find and make new connections before they could possibly be actual replacements. How neurons are able to make and maintain these connections with the right cells is mostly unknown, so this is still far from a comprehensive treatment.
This is especially true in older people, plasticity is generally higher and more universal in younger animals. Plasticity is present in older animals but it's generally very localized (for example the olfactory system has massive neuronal turnover throughout one's life).
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Because flu viruses mutate extremely rapidly, so a vaccination against any current strain will be outdated very quickly. Therefore a mass vaccination would be a waste of time until we know exactly what form of the virus we need protection from. Before the bird or swine versions of the flu can transfer to us, or for it to be transferable from human to human, it has to mutate, so we don't know exactly what form of the virus may become a problem if it crosses species. That's the problem with animal viruses when they transfer, we they've gone through multiple rounds of mutation and adaption (eg: bird to swine, swine to human) and may change significantly each species jump. Basically, the virus changes too rapidly and moves too unpredictably between species for us to know exactly what to raise a vaccine against until it's already in humans. Therefore making/using vaccines in advance is very difficult and unreliable. Then again, the current flu may not even make the jump from birds/swine to humans and we may never have to deal with it. We just don't know (until it's too late).
It's unfortunately much more complicated then just replacing the missing or damaged cells with new healthy cells. The neurons are useless unless they have synapses with the correct neurons both for input and output. One could treat neurons to make them more likely to make synapses and stimulate synaptic formation, but the cells would have to find and make new connections before they could possibly be actual replacements. How neurons are able to make and maintain these connections with the right cells is mostly unknown, so this is still far from a comprehensive treatment. This is especially true in older people, plasticity is generally higher and more universal in younger animals. Plasticity is present in older animals but it's generally very localized (for example the olfactory system has massive neuronal turnover throughout one's life).