Measles, MMR, and herd immunity

This morning the Health Protection Agency blamed the rapid increase in the incidence of measles on the parents in the 90s who refused to have their children immunised with the MMR vaccine. It’s not quite that simple. 

Prior to use of MMR a high proportion of children received a couple of single vaccinations against measles and the incidence of the disease was very low (I think 56 cases was the number reported, compared with 1,348 in 2008). So why change? Well, the reason was cost. This was masqueraded as a convenience to the parents and a mercy to the infants who would suffer fewer stabbings, but really, the thinking was it would be cheaper to deliver multiple vaccines at once. That’s undoubtedly true, but the policy makers and advisors were too slow to react to the events of the day.

When Doctor Andrew Wakefield raised his suspicions of a link between the MMR jab and the onset of autism, no doubt given maximum coverage in the media, parents were quite naturally hesitant to take the risk. Here was a new injection and somebody saying it might be dangerous. What would you do? Many parents decided not to take a risk that they didn’t have to and the uptake of MMR fell dramatically.

Herd immunity is achieved when a sufficiently high proportion of a population is immune to a disease so that an infected person dropped into that population suffers the illness themselves but is unable to pass it on. For measles, that proportion is 95%.

The trick that was missed by the authorities was the recognition that preserving the herd immunity of the 80s was more important than proving themselves right about the safety of MMR. They failed to take into account the attitude to risk adopted by parents who had to compare the possibly immediate consequence of the positive act of injection with the far off, uncertain consequences of a bout of measles. At the time, science was giving contradictory results in the way that medical statistics often do, so parents rejected MMR in droves.

Rather than digging in with their insistence that MMR was safe and as soon as it was realised that herd immunity was jeopardised, the government should have acted by offering the single measles jab, about which there were no comparable safety concerns. Herd immunity would have been restored and we wouldn’t have this upward trend in the incidence of measles.

The Health Protection Agency should not only blame the parents; they should also learn about their own mistakes and accept that they have the responsibility to act as an Agency to Protect Health – and to do so by whatever means are available.

Copyright © Colin Hazlehurst 2009