Though there are many different vaccines in use today, each targeting a different disease, they all have one very important thing in common: they are designed to effectively protect recipients from the bacteria or virus in question. But just how do scientists prove that a vaccine is effective?
For starters, it’s important to understand the difference between efficacy and effectiveness. Efficacy is focused more on the numbers: scientists look for the exact percentage in disease incidence between vaccinated and unvaccinated groups. Effectiveness is more practical, focusing on the ability of the vaccine to function successfully in the ‘real world’. You need both to identify the proportion of vaccinated people that are expected to be protected by inoculation.
A number of factors influence the effectiveness of vaccines, particularly in those that need to be redesigned each year to keep up with mutating viruses (like influenza).
- Host: The patient’s age, comorbidity (the simultaneous presence of two chronic diseases or conditions), previous exposure, and time since vaccination can all impact vaccine effectiveness.
- Vaccine: In terms of the vaccine itself, the mode of delivery, type of virus used (live vs inactive), and composition play roles in its success.
Although no vaccine is 100% effective, a very small percentage of people remain unprotected even after vaccination, and others may become unprotected as time goes on. Herd immunity — which relies on the vast majority of a population being vaccinated — can keep these vulnerable people safe.
In the end, monitoring vaccine effectiveness does more than just save lives; it allows for the optimization of limited resources, demonstrates the impact vaccines have on health outcomes (which easily justifies the cost of production), encourages people to get vaccinated in the first place, and stimulates the development of improved vaccines.