Herd immunity and the herd immunity threshold:
Herd immunity is a type of “immunity” or protection against infection that occurs when so many members of the population have been vaccinated that nonvaccinated individuals have reduced risk of infection. In earlier posts, I’ve talked about R0, and how parasites/pathogens cannot persist in a closed population if Ro < 1. Ro depends on the number of susceptible individuals in the population. Therefore, if we reduce the number of susceptible individuals below some threshold – alternatively, if we increase the proportion of vaccinated individuals to the herd immunity threshold – then R0 < 1, and the parasite/pathogen cannot persist. The herd immunity threshold varies for each parasite/pathogen, and you can find lists of herd immunity thresholds for common parasites/pathogens on the web (see the wikipedia page, for instance).
Herd immunity explanation from NIAID.
If you click through to wikipedia, you’ll see that the herd immunity thresholds are pretty high for our bigname parasites/pathogens; 75-94% of the population must be immunized for things like polio, measles, and smallpox. I’m going to stand on my soapbox for eight sentences:
In 1998, a very unethical man by the name of Andrew Wakefield published a paper where he fraudulently linked the MMR vaccine and autism in children. Unfortunately, this and the following media frenzy made people question vaccine safety, which made people refuse to get themselves and their children vaccinated. Even though there is proof that Wakefield fabricated data and was being paid to make this fraudulent link, public uncertainty about vaccination is still high.
When people do not get vaccinated, two things happen. First, those unvaccinated people are the ones who will be infected by the parasite/pathogen. Second, the proportion of vaccinated individuals may drop below the herd immunity threshold, allowing the parasite/pathogen to spread through the population and persist. That means that those individuals who are physically unable to be vaccinated – the very young, the very old, and the immunosuppressed – are no longer protected against infection because individuals who can be vaccinated chose not to be. So, if any readers of this blog were thinking about forgoing vaccination, please remember that it is not only you and your children at risk, but also all of the people who cannot be vaccinated for medical reasons (like lack of a functioning immune system).
Herd Immunity, Economics, and International Cooperation:
The rest of this post is about a really cool talk I saw at EEID 2013. It was presented by Petra Klepac, and was titled, “Free ride or vaccinate? Cooperation in control of immunizing infections.” If you’re interested in the economic/political side of vaccination, you should check out some of her publications.
Typically, when we think about vaccination programs, we think about things at the national level. Nations need to balance two things when it comes to developing vaccination programs. They need to try to vaccinate their target proportion of the population, which may mean vaccinating up to the herd immunity threshold, or it may mean vaccinating enough people to go from a “severe” to a “small” epidemic. That requirement is often influenced by the second requirement: the need to minimize cost. Sometimes, vaccinating to the herd immunity threshold also minimizes cost. This is especially true if a disease is particularly harmful economically. But other times, countries may find that minimum costs can be found when vaccinating less than the herd immunity threshold. This may occur when a nation is not wealthy enough to invest in high vaccination coverage, when vaccination for a given parasite/pathogen is particularly expensive, when the herd immunity threshold is particularly high, etc.
What if you’re a relatively wealthy nation that can afford to vaccinate to the herd immunity threshold, but a neighboring nation can’t/won’t vaccinate to the herd immunity threshold? Your vaccination coverage insures that the parasite/pathogen will not persist in your population, but in less you vaccinate 100% of individuals, people may still get sick if travelers carry the parasite/pathogen into your nation. What to do?!
Petra used game theory models to evaluate how nations should cooperate or cheat when it came to vaccination coverage. She found that 1) by forming international coalitions that agree on some level of vaccination coverage, nations could reduce costs. 2) Sometimes, the most cost-effective way to spend vaccination funds was to pay for vaccination programs in a neighboring nation!!!! 3) The bigger an international coalition became, the cheaper it got to be a “cheating” nation which did not adhere to the same vaccination coverage goal. Cool stuff!