Nothing works quite as well in the real world as it did in the trial, but vaccines come close, as this article reminds us.
Of course we mustn't leave our critical faculties behind - if the claim is for an enormous reduction in pneumococcal disease, we need to know how cases were ascertained: how good was the surveillance system? Did it work equally well in all age groups? What about strains not covered by the vaccine - and in particular was serotype replacement a problem? The authors discuss all of the these issues.