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Interpreting measures of vaccine efficacy from malaria vaccine trials

Source: TropIKA

A change is proposed in the way researchers report efficacy in clinical trials, in order to help policymakers get a better picture of how effective a vaccine actually is.

Author: Tatum Anderson

Paul Milligan, a researcher at the London School of Hygiene & Tropical Medicine, says researchers need to change the way they report efficacy in clinical trials to help policymakers get a better picture of how useful an intervention is.

Today, efficacy – particularly when expressed in vaccine trials – is usually defined as the percentage reduction in the proportion of children who have had an episode of malaria within a given period of time after being vaccinated.

In other words, when a trial vaccine has an efficacy of 30%, it means the proportion of children who have an episode of malaria after a given amount of time drops by 30% if they received the vaccine. If a vaccinated child is bitten by an infected mosquito, their chance of getting malaria is lower by 30%.

However, speaking to TropIKA.net, Milligan said he intends to propose a subtle but important change at this year’s MIM. He suggests that vaccine efficacy should instead be expressed in terms of the percentage reduction in the number of malaria episodes during the period of the trial after vaccination.

In other words vaccine efficacy should take into account several episodes of malaria rather than just one because children acquire immunity slowly, and experience malaria many times before they acquire immunity. Expressing efficacy as a function of just one episode of malaria means that scientists are not giving policymakers a true picture of just how good – or bad – a vaccine is at reducing the likelihood of contracting malaria.

“It will be a simple talk, to try to encourage people to include all episodes when they estimate vaccine efficacy.” He says. “That’s really what the policymakers are interested in because that’s the measure of the impact on disease burden.”

Statistical methods can seem complicated and impenetrable to many researchers he says. But determining efficacy is actually a relatively simple process. He says: “I will show how results can be biased if you don't do that and then to show that there are quite simple ways of doing it.”

Indeed many researchers do measure the incidence of malaria over a longer period of time, including several episodes, but often only present results in terms of the first episode. It has become a talking point amongst researchers he adds. “There's been a lot of confusion about what the commonsense interpretation of this is,” he says.

The talk will focus on probability models used to calculate vaccine efficacy from time-to-first-event analysis, as well as analysis of all events in different scenarios, such as sustained efficacy, waning efficacy and when the vaccine is assumed to interfere with acquisition of immunity.

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