Saturday, February 4, 2012

Vaccine efficacy

Fear of vaccination is nothing new. This famous cartoon by James Gillray is from 1802 and refers to the fears people had against being vaccinated by Cowpox, which was used to give protection to the far more deadly smallpox.

Today the internet gives a voice to everyone. But you need to be critical of what you read. Here's a popular argument against vaccines you'll find. I copied this verbatim from an anti-vaccine website.

Meanwhile, in the real world vaccines are failing miserably. A recent outbreak of mumps in the New Jersey / New York area occurred almost entirely among children who had already been vaccinated against mumps. Clearly if vaccines really worked, then an outbreak should have only occurred among those who were NOT vaccinated against mumps, right? But as I reported, 77 percent of the children who got infected had already been vaccinated!

Sounds convincing? Can you work out what's going on?

Let's consider the numbers. Imagine we have a vaccination rate of 90%. Now, most vaccines only have a certain efficacy, in some cases they just don't work. Let's imagine our mumps vaccine has an efficacy of, say 80%. If you get the vaccine you have an 80% chance of getting immunity, and a 20% chance that you won't. (Which is why we actually need a vaccine coverage in excess of that predicted from R0 in order to establish herd immunity).

So in a population of 100 children we have 10 children who have not been vaccinated and are susceptible. But we also have 20% of the 90 vaccinated children who are susceptible, that's 18.

Assuming our outbreak hits these two groups equally then 18 out of every 28 of those infected (65%) will have been vaccinated. This is an argument for MORE vaccination to protect the fragile herd immunity and for MORE research to increase the efficacy of our vaccines. One of the reasons vaccines are given to children, (apart from giving them protection!) is that their efficacy is usually greatest. I believe this is the case with the HPV vaccines I mentioned. You can get it later in life but there is a greater chance it will not lead  to immunity.

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