By Julie Stachowiak, PhD
Multiple Sclerosis Guide; Guest blogger
It looks like some regions are becoming immune to widespread outbreaks of H1N1 swine flu, as they have developed immunity from previous waves of exposure.
"Herd immunity" is a term that describes a community being immune to (not able to catch) a certain disease. This immunity can be conferred either from exposure, meaning that people had the disease and recovered, or vaccination. It does NOT mean that no one in the community can get the disease, it just means that there will not be a large outbreak and that individual infection is therefore less likely.
Herd immunity is an interesting concept, as different diseases require different levels of immunity to protect the community, based on the transmissibility of the virus or bacteria. For instance, measles is one of the most infectious, easily transmitted viruses in people. To achieve herd immunity against measles, between 90 and 94% of the population must have been vaccinated. However, for smallpox, the immunity levels were between 80 and 85%, which is one reason that it was possible to eradicate that disease.
Few experts have tackled the idea of herd immunity to a pandemic flu like H1N1, as it was such a rapid wave and there are many unknowns. True herd immunity seems a little less likely than the fact that these types of viruses usually just "peter out" and move along. However, there is evidence that there has been enough exposure in some southern hemisphere countries that there is no longer widespread infection, but isolated cases of H1N1 occurring, even though the virus is still present.
If you are intrigued with the idea of herd immunity and H1N1, see what you can make of this article, which mentions some interesting assumptions and some difficulties in modeling pandemic influenza: Pandemic influenza dynamics and the breakdown of herd immunity