A majority of the infections during measles outbreaks are unvaccinated people. People often do not vaccinate their children, as vaccination is understood by many as an individual choice, when science makes clear that the choice – to vaccinate or not to vaccinate – can affect an entire community. When a child is immunized, it is not only immunizing the child, but also contributing to the control of the disease in the population, resulting in a sheltering effect called herd immunity. A population that is highly immunized makes for a virus that can’t spread easily, providing protection to the community – or the herd – as a whole.
The graphic below presents 6 hypothetical communities with different vaccination rates. Only very highly vaccinated groups are able to consistently block measles infection from their community, showing just how important high rates of immunization can be. All the dots represent people who are the same or similar ages, such as school children. There are no babies or non-immune elderly folks in our examples, but there are some children who can’t get the vaccine for medical reasons.
The sample children above are in close contact and they mix randomly and they each have an equal chance of coming into contact with someone from the outside world who is infected with measles – the incoming red dots. Measles is extremely infectious - the simulation below shows how it spreads within a community.
PROTECTED
58.5% vax rate
similar to Okanagan County, WA
When most people in a community are not vaccinated against measles, it is free to propagate out to its neighbors. As a result almost every unvaccinated person in the community gets infected.
PROTECTED
68.9% vax rate
similar to Thurston County, WA
As the percentage of vaccinated people increases, the likelihood of the disease entering a community decreases. However, if anyone within the community does get infected, measles will still spread rapidly. Because measles is highly infectious, the susceptibles will have a 90% chance of getting infected. Among the vaccinated, there's a 1% chance of vaccine failure.
PROTECTED
86% vax rate
similar to Santa Cruz, CA
There are certain people, primarily children, who are vaccinated but still vulnerable, or children who cannot be vaccinated because they are suffering from certain other diseases or undergoing treatment, such that vaccination can result in complications. Unvaccinated people put these people at risk.
PROTECTED
90% vax rate
similar to Los Angeles County, CA
Some states, such as California, allow parents to skip vaccination based on personal beliefs. If such unimmunized kids congregate, it results in pockets of under-immunization. As a result, several school districts in California have a vaccination rate below or around 90%. In comparison, the national average is 92%.
PROTECTED
95% vax rate
similar to Providence, RI
As the number of vaccinated people increases, some people are protected even if they are not vaccinated by the virtue of people around them who are - the disease is unable to propagate.
PROTECTED
99% vax rate
similar to Gadsden County, FL
In the case of measles, vaccination (two doses of MMR vaccine) rate has to be greater than 95% to comepletely prevent the disease from propagating.