Ineffective vaccines usually don’t get approved in the first place or at least most countries have regulatory processes in place to ensure approved vaccines first pass some effectiveness checks. Thus, we know a vaccine’s effective the same way it was determined to be effective in the first place, if it prevents the infection/disease it was designed to protect against.
How are vaccines approved in the first place? In clinical trials, volunteers either get the vaccine or not. Vaccine’s considered effective if fewer vaccinated volunteers get the infection/disease (see infographic below from). So vaccine effectiveness is assessed on population averages, not on individual basis, i.e., on the basis that statistically significantly fewer vaccinated get the infection/disease compared to the unvaccinated. As well, this is biology so numerous factors influence vaccine effectiveness including age, gender, infection history, chronic conditions, to name a few. Thus vaccine approvals can often be segmented by age or gender, for example.
Unlike drugs, vaccines are tested on healthy people. Obviously, it’s hard to predict when and how many will get an infection/disease within a population. This means monitoring for years on end which can be prohibitively expensive so as knowledge of immune function improved,began to be used. These are immune responses proven over years and multiple studies to correlate with protection against a specific infection. Problem is proven correlates of protection currently exist for few infections. Thus far, as an example, circulating antibody levels against are among the best-accepted correlates of protection. Since different types of immune response are effective against different infections, different vaccines have different correlates of protection. Some match a vaccine’s ability to prevent infection better than others.
Best source for current vaccine correlates of protection is, creator of the rubella vaccine and a living vaccine encyclopedia. He’s likely forgotten more about vaccines than most people could ever learn in their lifetimes. He’s listed and explained vaccine correlates of protection in many of his peer-reviewed scientific publications (see tables below from ).
1. The Journey of Your Child’s Vaccine, CDC.
2. Plotkin, Stanley A. “Correlates of protection induced by vaccination.” Clinical and Vaccine Immunology 17.7 (2010): 1055-1065.