Actually Tracy, its sheer blood mindedness that has kept me at this thread.
This morning it took 4 minutes and 42 seconds to get the index page of this forum to come up. A comparable amount of time, to get whatever page it was, to come up. Then to read. Then, unless you use the quick response form, a similar amount of time to get the reply to thread to come up. So potentially 15 plus minutes can be "wasted" going up the wall, while you have formulated your reply half a million times, and feel like throwing the computer and whole caboodle out the window.
Of course, "you" don't do that, since this is the only internet board for me, which is a slow as a dead dog's hind leg.
However, to post here, intrudes so much on whatever else goes on in a day, that I suspect a time will come when it simply isn't worth while by any stretch of the imagination.
Now, to answer Ishmael.
Even though she 'says' she won't come here, I suspect she won't resist the temptation to view the results of her quitting post:
|The fact that the attenuated strain of measles is so genetically close to wild measles means nothing in itself. Consider the fact that an alteration in one of the ~3 billion DNA base pairs that make up the human genome is enough to cause a chloride channel to malfunction, and another single base pair change will make hemoglobin misfold. If a child inherits one copy of these changes from each parent, she will have cystic fibrosis or sickle cell disease, respectively.
Okay, that's a wonderful example of how a single genotypic change can result in a big phenotypic change. But it is not appropriate to just suppose that this indeed has happened. If so, then I can just suppose that while the neighbor has a severe case of wild-type measles, it won't infect me because that virus will certainly mutate while in his body and there is a chance the mutation will will render it non-infectious. The reality is that the vaccine virus is the progeny of a wild-type strain. It is a wild-type strain that was propagated in chick eggs so that it would accumulate a few mutations. All of those mutations are known. The only characteristic that was selected for in the progeny virus (the vaccine) was high infectivity.
|So how does the attenuated measles virus differ from the wild one? I'll assume that everything posted by MT is correct, but I'd like to qualify one thing: infectivity is certainly necessary for the vax virus to work, but that's something that can be influenced by the way the virus is delivered.
The wild-type measles virus infects the pharynx through respiratory droplets and spreads from there. The vaccine virus is injected, produces a viremia (spreads all over the body) and winds up infecting the pharynx where it can be coughed onto other people. The observation that the vaccine virus infects the pharynx of the vaccinee means that it will also infect the pharynx of other humans. Incidentally, since the vaccine is injected, it has the potential to infect peripheral nerves from whence it travels through the axons into the brain.
|The attenuated measles virus contains a mutation in its hemagluttinin protein. This is the protein that allows the virus to bind and gain entry to the body, so it's conceivable that the attenuated strain would have a harder time getting into the body by a natural route.
Conceivable, but not true. These mutations have been characterized. The mutation in the hemagluttinin protein allows the vaccine virus to bind with high affinity to CD46, a cell surface protein. CD46 is a cell receptor present on the surfaces of all nucleated cells in your body. So while the wild-type measles virus is tissue-restricted, the vaccine virus is not: it can infect any cell. The safety implications of this have not been explored. Although it is assumed that the vaccine virus is quickly cleared from the body, the reality is that the virus could hide anywhere, causing persistent infections. The asymptomatic nature of the vaccine (with respect to classical measles symptoms) may make persistent infections very hard to identify. For example, measles virus has been identified in the bowels of IBD patients who received MMR but never had measles. Also, the vaccine virus has been shown to grow in neurons and travel through the axons to other neurons - all the while invisible to the immune system because no extracellular virus is shed.
Scary stuff - and mostly ignored.
These are things, Ismael, that as a future paediatrician, you might like to consider very carefully.