by Janet Grischy

Created on: November 03, 2009   Last Updated: February 06, 2010

Viruses change rapidly. They evolve so quickly that some vaccines must be reformulated every year to remain effective. Two main factors, combined with fast growth in successful viral populations, cause viral change to be rapid. One factor is antigenic drift and one is antigenic shift. Both these mechanisms permit viruses, particularly viruses with RNA in their genetic material rather than DNA, to mutate so fast that new vaccines can hardly keep up with them.

Genetic drift, the title of this article, is a different phenomenon, though somewhat related to antigenic drift. It is responsible for a huge part of the evolution that Charles Darwin so brilliantly observed and explained.

Antigenic Drift

Antigenic drift protects viruses from vaccines, and from the human body’s immune system. In effect, antigenic drift changes the way a virus “looks” to the immune system. The influenza A virus, for example, has two important molecules on its outer coat. One, hemagglutinin, is responsible for opening up host cells so that viral cells can enter. When a host organism, through vaccination or previous infection, recognizes the shape of the hemagglutinin, it will prepare a defense against the virus, and the viral attack will be weakened or fail.

The other molecule that the immune system may recognize is neuraminidase. It is responsible mainly for releasing viruses from used up host cells, so that the virus can spread through the organism. If a flu virus is described as H1N1, for example, the H refers to hemagglutinin, and the N to neuraminidase.

The H and N are antigens. That is, they are the part of the virus that the immune system recognizes and reacts to. Antigenic drift in influenza A (and in other viruses) happens because they change readily into other shapes that are not so easily recognized by the immune system.

Because the influenza A virus is single stranded RNA, it lacks the genetic safeguards against copying errors found, for instance, in human double-stranded DNA. It is more likely to make a mistake when it reproduces itself, because it has no proofreading enzymes to make sure it has produced a true copy. In fact, essentially every new copy of influenza A will contain a copying error, a mutation.

Once these mutations change the shape of the N and H on the outside coat of the virus, they escape detection by the host’s immune system. They enter cells, and begin killing them, thanks to the disguise they have evolved through antigenic drift.

Antigenic

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