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* is 6 percent. Historically, the average probability of success for vaccine candidates* is 6 percent.12 What this means is that if you start with 100 candidates, only 6 of them will make it all the way to full regulatory approval. The others will fail for any number of reasons--they might not give people enough immunity, or clinical trials might not give you the conclusive results you need, or they might have unintended side effects. Of course, this 6 percent figure is just an average. The odds for drugs and vaccines created using tried-and-true methods would be a few percentage points higher. And they're a few points lower when you're trying a new approach. First you have to prove that the basic approach works, then you might also have to prove that the vaccine you make using that approach also works. You have to run massive trials, which can involve hundreds of thousands of people, and later you'll have to watch for side effects in millions of people. There are hurdles everywhere. Fortunately, the COVID virus is relatively easy to target with a vaccine. That's partly because it has few of the tricks that make some viruses especially hard to go after--such as the ability of two strains to create a third one by swapping genes--and the spike on its surface is not as camouflaged as the proteins on some other viruses are. COVID is fairly good at replicating itself accurately, which means it hasn't produced as many variants as, say, HIV. As a result, the success rate for COVID vaccines has been unusually high. . * A vaccine candidate is just what you'd imagine: something that might turn out to be a safe, effective vaccine, but is still in the development stages. It's like a bill that is making its way through Congress or Parliament and may or may not become law.