Some people ask the question, “If vaccines work, why do unvaccinated people present a risk to those who have been vaccinated?”
An unvaccinated population poses a risk to everyone by increasing the opportunity for a disease to spread through the community. Once a disease is in a community, everyone is more likely to be exposed and, in the same way that rain finds the weak spots in your roof, pathogens find the weak spots in your community. The result: Both vaccinated and unvaccinated individuals may be affected during an outbreak.
When vaccinated people get ill during an outbreak, some people assume that to be evidence that vaccines do not work. The reality is vaccines work for most people but not all. In rare instances the person will not develop a detectable immune response even after having multiple doses. However, in most cases, when previously vaccinated people get ill, the vaccine was not effective enough to completely prevent infection. Most of the time, these people will not be as ill as those who have never been exposed to the infection before; however, that is not always the case. In addition, we typically cannot tell in advance for whom the vaccination was not completely protective, so we cannot know which previously vaccinated people will become ill.
Of course, completely unvaccinated people are at even greater risk during an outbreak, and while people often think of unvaccinated people as those who have chosen to remain unvaccinated, in actuality most unvaccinated people are those who cannot be vaccinated. Reasons why someone may not be vaccinated include: being too young; having a medical reason not to get a certain vaccine, such as an allergy to a vaccine component; or undergoing immunosuppressive treatments, such as chemotherapy for cancer or treatment following an organ transplant.
Because pathogens are so effective at finding susceptible people in communities, both vaccinated and unvaccinated people are at risk when a vaccine-preventable disease outbreak occurs. Indeed, studies have shown that vaccinated people in a relatively unvaccinated community are at greater risk than unvaccinated people in a highly vaccinated community; that’s because the more likely you are to be exposed to a pathogen, the more likely you are to get sick. Remember, although 95 of every 100 people who receive a measles vaccine will be protected, five of 100 won’t be protected. Therefore, the vaccination status of our communities is important whether or not we have individually been vaccinated.