--- Vaccines versus Viruses

Sunday, 14th of October 2012

VACCINES VS VIRUSES

New York Times, 23 July 2012

Q. Why didn’t viruses like polio and other diseases mutate to become immune to vaccines?

A. “Bacteria mutate very well, but vaccines don't give viruses much of a chance,” said Dr. Paul A. Offit, chief of infectious diseases at the Children’s Hospital of Philadelphia.

The way it works with the measles vaccine, for example, is that a live, weakened form of the virus is given, so that the body develops antibodies directed against all 10 of the virus’s proteins and all the sites where it might attach to the body’s cells. “When the virus tries to infect you, it really doesn’t have a chance,” Dr. Offit said. “You attack it at all possible points.”

Polio has three serotypes, or types of surface proteins, but they were all in the vaccine. Mumps, German measles and chickenpox all have single serotypes.

It is different with bacteria, he said, because there are already resistant bacterial strains out there, so vaccination provides a fertile ground by eliminating sensitive strains, allowing resistant strains to thrive.

“With a virus, this never gets started,” Dr. Offit said. “I suppose if you were making a vaccine against one part of one protein, it might allow mutation, but we don’t give it a chance.”

There are two important exceptions, he said: influenza, which mutates so quickly that immunization one year does not protect from the next year’s strains, and H.I.V., which can mutate during a single infection.

C. CLAIBORNE RAY

A version of this article appeared in print on July 24, 2012, on page D2 of the New York edition with the headline: Vaccine vs. Virus.