Malaria vaccines: fifty percent of what?
This week there was substantial news coverage about the effects of a new malaria vaccine. This is what Fergus Walsh of the BBC said, “The first phase three results, published online in the New England Journal of Medicine, show that the vaccine cut the risk of malaria infection and several malaria - by about half.” The Guardian’s Sarah Bosely chipped in with “The vaccine had been classed as around 55-60% effective,” and allAfrica.com reported “Malaria Vaccine Will Reduce Infection in Children By Half.”
Did you interpret the result to be meaningful, you and nearly everyone else?
Surely a 50% reduction in malaria is such a big effect that the vaccine is rolling of the production line right now. There is no doubt that a vaccine for preventing malaria would be a major health advance but the question immediately on mind was, 50% of what?
Only by accessing and working your way around the original publication can you get to the real figures. Which, I have to say, it wasn’t easy to get to, and interept the data I wanted. Is debate stifled by the incisive way journals publish papers which seemingly skirt around the data that matters, leaving you ever so slightly confused and dazed?
I think it's one of the reasons most media reports just focus on the press release. The papers themselves are written in a code which is only decipherable to mere humans after numerous re reads of the article. It probably also takes too long for most to read through the studies partcularly in the modern media world of instant news.
So what does the data reveal? The results of the trial are published in the New England Journal of Medicine shows during 12 months of follow-up in the first 6000 children the incidence of the first or only episode of clinical malaria was 0.44 per person-year in the vaccine group and 0.83 per person-year in the control group giving a relative reduction of 56%.
However, when you look at severe malaria 57 of 2830 children (2.0%) in the vaccine group and in 56 of 1466 children (3.8%) in the control group had at least one episode. A relative reduction of 47%.
As you can see the two figures for the same relative effect have very different absolute effects based on the severity of the outcome.
Yet, for death there was no difference: 56 of 5949 children (0.9%) died in the vaccine group and 28 of 2974 children (0.9%) in the control group.
So are you a relative or absolute person. I’m both, but I seem to be getting more sceptical the more headlines I read. You should be too