Our immune systems get less responsive the older we get. Just like our muscles become less responsive. Bones become less responsive. Just about every system becomes less responsive as we age. You want to know 'why'? Most aging researchers want to know the same thing.

So I'm not sure if she's referencing a specific study. But I know that there are studies that have increased the amount of vaccine to elderly in order to boost their immune system even more since they are at higher risk since they are somewhat immune compromised.