There are several reasons not to get both vaccines at once, but I don't think reduced efficacy is one of them. The most obvious reason not to get multiple vaccines at once is side effects. If you just get the typical sore arm, your arm will be twice as sore, or both arms will be sore if you do it that way. But if you get an unexpectedly bad side effect you won't know which vaccine caused it. And I don't know about you, but if I had an anaphylactic response to a vaccine I sure would want to know which one it was. If you have had both shots often enough that you are sure that isn't going to happen, it's probably not a huge risk. But it just seems like bad practice to get two vaccines at the same time unless it really is the only way you can or will get them both.

But the main reason not to get these 2 shots together right now is that it is simply too early to get the flu shot. The flu vaccine is not a very effective vaccine in the first place, and the effect wears off pretty quickly. If you get the vaccine 3 months before the flu season actually starts, half the efficacy will have dissipated by the time flu actually appears. Anyway, that is what Dr. Osterholm says, and he is not only a great epidemiologist, he has been specifically working with the flu vaccine for decades. He says he usually gets his flu shot in late November unless there are indications of an early flu season (which is what happened last year, but not this year).