In my last post I spoke about how vaccines work from the point of view of the person receiving the jab or pill. In that case we were talking about immunological memory but vaccines also work in another very important way from the point of view of the community and it is referred to as ‘herd immunity’.
Last week the importance of herd immunity was highlighted in my home-town after the death of another child due to Whooping cough (Bordetella pertussis infection) in South Australia. Children under 6 months, as most of these children were, are too young to receive the vaccine and so have to rely on community based herd-immunity to avoid the disease.
So what is herd immunity? There have been a bunch of really nice articles coming out recently talking about the importance of herd immunity so if you want a little more detail I suggest looking here, or here. But I will give it a short write up here as well.
Herd immunity exists when vaccination becomes commonplace in the community. The basic theory states that as more and more people in the ‘herd’ are vaccinated it becomes less likely that the infectious particle that is the target of the vaccine will be able to get a foothold in that herd. As a result even if a few people in the herd are not vaccinated they will be passively protected by the immunity of others. In this way community wide vaccination prevents an infectious particle entering a population. Even if it does get in it will prevent its effective spread within that population. If an infectious particle has nowhere to go it can just ‘die out’ like smallpox did or become essentially dormant like Mycobacterium tuberculosis did. The eradication of smallpox was possible because the WHO was able to run a global vaccination campaign and smallpox was not able to adapt around it fast enough. It’s thought that for infectious agents that do not have a high rate of mutation we could eradicate them but for those that can and do mutate quickly, like bacterial species and the influenza virus, we can at least limit spread and disease.
Ensuring vaccine coverage throughout the community allows us to accommodate for the young who have not been vaccinated yet or the people who cannot otherwise be vaccinated, those who are allergic for example. This is why anti-vaccination people get on my nerves.
Using Whooping cough as the example we can essentially think of the disease as eradicated in the community if we maintain a vaccination rate of approximately 94% of the population but even coverage of 80% would significantly impair the disease’s progression within the community. The problem is that any drop below 80% of the population allows the disease in and then deaths start to occur.
WARNING: This is a video of real children suffering from Whooping cough. These children are in real distress so please do not watch if you believe you will find it too upsetting. Having said that if you can try to sit through it I hope it highlights what a horrible disease this really is.
Frustratingly, thinking about the community as a whole is also misleading as has been identified in California. In sunny Cali the rates across the state are high but in isolated pockets the rates fall well below the 80% threshold resulting in thousands of cases each year and 9 confirmed deaths this year.
This explains why some anti-vaccination advocates loudly proclaim their children were never vaccinated and turned out just fine, turns out your child profited of the goodwill of others in the community.
As has been the case since we understood what Jenner achieved all those years ago, vaccines are the cheapest and most effective means of disease prevention available to us, both as individuals but also as a community. All people should be vaccinated and get all their boosters to make sure that we can protect ourselves and those most in need within our community.
Glanz, J., McClure, D., Magid, D., Daley, M., France, E., Salmon, D., & Hambidge, S. (2009). Parental Refusal of Pertussis Vaccination Is Associated With an Increased Risk of Pertussis Infection in Children PEDIATRICS, 123 (6), 1446-1451 DOI: 10.1542/peds.2008-2150
Amanna, I., & Slifka, M. (2005). Public Fear of Vaccination: Separating Fact From Fiction Viral Immunology, 18 (2), 307-315 DOI: 10.1089/vim.2005.18.307
Fung KS, Yeung WL, Wong TW, So KW, & Cheng AF (2004). Pertussis--a re-emerging infection? The Journal of infection, 48 (2), 145-8 PMID: 14720490
. (2010). Increasing Immunization Coverage PEDIATRICS, 125 (6), 1295-1304 DOI: 10.1542/peds.2010-0743