The flu comes back year after year, and every season we get vaccinated (well, some of us anyway). Why do we need to keep getting a new shot for the flu while for others, like measles, we got way back in childhood and are done with it for the rest of our lives?
Our immune response to influenza involves production of antibodies, large proteins that specifically bind to the virus and help clear it out or neutralize it. It seems like the key to influenza immunity is neutralizing antibodies, antibodies that bind to the virion and prevent it from attaching to the host cell. You can imagine this large protein just being physically in the way, preventing the virus from binding the host receptor. The immune response that develops from natural exposure or vaccination generates neutralizing antibodies to HA, a viral envelope protein that is necessary for attachment to the host. I’ve mentioned HA in a previous post about influenza. The problem is that last season’s neutralizing antibodies dont bind to this season’s virus. Although it may be nearly identical to the virus from a past season, the new strain’s HA is slightly different, and those differences are enough to evade existing neutralizing antibodies.
Now a new approach to vaccination has shown that it may be possible to develop a vaccine that illicits broadly neutralizing antibodies, that is, antibodies that will protect against influenza strains with slightly different HAs. They used a prime/boost approach, in which a DNA vaccine was used to induce an initial response against HA, and then boosted with a regular seasonal flu vaccine. The only difference between this and what is currently done is the addition of the DNA vaccine. However the response seems quite different. Neutralizing antibodies were generated that can neutralize a variety of different influenza viruses. It seems the vaccine induced antibodies to a different part of HA. Antibodies are so specific, the dont recognize the whole HA, but rather discrete parts of it. The part recognized by these antibodies, called the stem, is highly conserved, meaning it doesnt change season to season.
This raises many interesting questions and possibilities. Could we soon have a universal vaccine that will protect us for life or at least for many years? Why did the change in vaccine regimen induce antibodies to a different part of HA? Why does the current vaccine or natural exposure fail to develop antibidies to the conserved portion of HA? Will the conserved portion of HA eventually change too, if sufficient selective pressure is applied through mass vaccination?