But discovering that particular protein has be elusive.
There are parts of the influenza virus that are relatively unchanging. The hemagglutinin protein--the "H" part in the influenza virus name--has domains that are almost invariant. However, those domains are invisible to the immune system. The structure of those domains makes them very slippery, so there is not much for a B or T cell to grab on to. For this reason, they are not antigenic--they do not induce an immune response in the form of specific antibodies. The parts of the hemagglutinin that are antigenic and cause a good immune response are also the same domains that change from year to year. So even though the vaccine always works, in that it always produces a good immune response, the immunity might be against a virus that does not even exist any more.
Many of the researchers trying to make a universal flu vaccine try to target the invariant domains of the hemagglutinin--but they have had little luck.
Other influenza virus proteins are mostly conserved, but they are shielded inside the virion where the immune system cannot detect them. The killed virus vaccines probably induce immunity against those proteins, too, but that immunity is useless, since the antibodies would not "see" those proteins in a circulating virus.
Wow!
Thanks for the much better explanation of my thought.
Since we are on the subject I will ask you about this article
http://www.freerepublic.com/focus/f-chat/3631232/posts
Do you think that there is a better way to produce vaccines?