# 3641
Robert Roos of CIDRAP takes a long and fascinating look at an immunological concept very few people have probably heard about, but which was first postulated by Thomas Francis, Jr. M.D. nearly a half a century ago.
Original Antigenic Sin.
'Original antigenic sin': A threat to H1N1 vaccine effectiveness?
Robert Roos
News Editor
Aug 18, 2009 (CIDRAP News) – Half a century ago, scientists reported evidence of some curious behavior by the immune system in humans and animals: If a host was exposed to an influenza virus and later encountered a variant strain of the same virus, the immune system responded to the second attack largely with the same weapons it used against the first one.
Like an army still fighting by the tactics of the last war, the host immune system mostly produced antibodies matched to the first virus instead of the second, resulting in a less effective defense. With a nod to theology, this phenomenon was labeled "original antigenic sin."
Today, in the face of the pandemic H1N1 flu virus, many countries are preparing to launch H1N1 vaccination campaigns this fall. Millions of people are in groups recommended to receive both seasonal flu immunizations and H1N1 vaccinations. Seasonal flu vaccine—which contains an H1N1 component, distantly related to the novel H1N1 virus—will be available sooner in most places.
This timing has caused some observers to wonder: If a person gets a seasonal flu shot and then an H1N1 dose a few weeks later, will original antigenic sin come into play and cause a poor response to the H1N1 vaccine?
Nobody knows the answer for sure, but leading flu and immunization experts say they aren't especially worried at this point. At the same time, they suggest the possibility bears watching.
Whether or not original antigenic sin plays a part in this year’s vaccination campaign is unknown right now, but it does highlight the fact that with this pandemic we are entering largely unexplored territory.
The good news in all of this is novel H1N1 hasn’t proven to be as deadly as 1918 or H5N1 bird flu. We are getting an opportunity to learn, and practice, with what has been (so far) a more moderate pandemic.
Hopefully we’ll take these lessons to heart, and use this knowledge to prepare for the pandemic-after-next. Because there are no guarantees that we will have another 40 years before the next pandemic strikes.
