Professor Evans, what is the difference between the trials for vaccines and clinical trials for medications? You are an expert in clinical trial analysis and, in particular, surveillance for the adverse drug effects of those medications that are already in the market. So how do you make that difference between conducting clinical trials for vaccines and medications?

There are two or three things that make vaccine trials different. Firstly, as I've already said, vaccines tend to be given to healthy people. And very often, Of course, they're given to healthy children. And you've very definitely do not wish to give healthy children adverse effects on so you are looking to find benefits with very minimal harms, we won't have no harm. If you give a vaccine, you can very easily get a rash, you can get a sore arm, and that's an adverse effect of the vaccine. Occasionally they cause more serious harms. And you need to be sure that those harms aren't occurring. But the problem is that they are there to prevent a disease. So you need to be sure that they will prevent that disease. Very often, the disease is fairly rare itself. And so you can't always be absolutely sure that they will prevent the disease. So you have to measure a different sort of outcome. And you very frequently measure what is called is immunogenicity in a vaccine and that is that you are measuring the The ability of the body itself to produce antibodies to the disease or the virus that you are trying to prevent causing significant harm in people. And so instead of demonstrating truly that you prevent the disease, you show that the body has got antibodies against that disease. And that is usually fairly convincing. Even then, you very often need very large numbers. So the key things are, we need large numbers in vaccine trials, we're giving them to healthy people. We can tolerate very few adverse effects on sometimes it's difficult to measure the outcome. And for drugs very often, when we're giving them for treatment to people, we're giving them to older people. We're not necessarily giving drugs to children; we're giving them to people with diseases. The occurrence of the disease or the outcome that you're interested in is sufficiently frequent that you do not need such large trials. For example, with trials of statins, where you give them to people to prevent heart attacks or death, you need very much larger trials of those drugs. And the Healthy People will not tolerate adverse effects nearly so easily.