Herd immunity, also known as community immunity, describes the indirect protection of at-risk individuals against infectious diseases when the remaining population has been vaccinated. The logic behind this concept is that by minimizing transmission between individuals because they have acquired resistance by vaccination or previous illness, a disease’s path of infection will be obstructed and thus has less likelihood of spreading to unprotected members of the community (the “herd”).
A person may be considered at-risk if they are unable to receive vaccines. Ineligibility for vaccination can be due to factors that include pregnancy, immunocompromised status from chronic illness, allergy to components of the vaccine, and age – infants and the elderly have underdeveloped and weakened immune systems, respectively. Additionally, an individual may choose not to be vaccinated due to personal beliefs.
For every disease there exists a minimum percentage, or “threshold”, of the population that must be immune in order for herd immunity to be most successful. This critical percentage depends on several factors such as the properties of the bacterium or virus that causes the disease and the distribution of susceptible persons within the community in question. While the threshold varies amongst diseases, it usually falls within the range of 80-95% of the total population. For example, the threshold for measles is upwards of 94% while for smallpox, it is around 85%.
Despite improved vaccination efforts and solid evidence that vaccines significantly reduce illnesses and deaths by infectious diseases, our current state of herd immunity remains insufficient and the repercussions have recently manifested through alarming resurgences of diseases like mumps (2009) and whooping cough (2012). This year the United States has reported a historically high number of whooping cough/pertussis cases, particularly in specific states such as Washington and Minnesota. (For more information: http://www.cdc.gov/pertussis/outbreaks.html)
What has been keeping us from our goal?
1. Vaccination schedules – Communities with higher numbers of non-medical exemptions for school vaccine requirements are most susceptible to disease outbreaks. Additionally, deviation or failure to comply with vaccination schedules can greatly endanger a community, as many vaccines require multiple shots within specific time frames. The importance of vigilant and closely regulated vaccination schedules is well demonstrated in the mumps outbreaks of 2010, where cases sprang up in New York and New Jersey. According to the Center for Disease Control (CDC), 30% of those afflicted had not received the required shots. Some parents choose to put off vaccinations because they believe their children are too young, while others may not see any urgency to vaccinate their children. The fact that today’s society has little experience with once prevalent diseases like measles and mumps (thanks to the development of their effective vaccines) contributes to a false sense of security and unawareness of the potentially devastating effects of missed vaccinations.
2. Low adult vaccination rates – For most vaccines, a single administration is not sufficient to confer lifelong protection. Additional boosters and adult vaccinations, even if the vaccines were received during childhood, can greatly contribute to the herd immunity effect. One of the greatest examples of the importance of adult vaccinations for herd immunity is whooping cough. Whooping cough, which is most dangerous in children and infants and can often be fatal, is most commonly spread from parent to child. The current U.S. epidemic of whooping cough emphasizes the importance of adult vaccinations not to protect themselves, but rather their children and any others that they may come into contact with.
3. Anti-vaccine movement – The anxiety and mistrust towards childhood vaccinations are remnants of the Wakefield paper, which falsely associated childhood vaccinations with autism (see previous blog posts). Despite repeated studies that fail to prove any correlation between childhood vaccinations and autism, there exists a strong crusade against vaccinations that has been spread by unreliable celebrity ambassadors and media.
Herd immunity reminds us that we are all in this together. Protect not only yourself but those around you by staying informed and up-to-date with your vaccination schedules. Keep in touch with doctors, share information and reminders with friends and family, and be conscious of the fact that your health decisions have significant impact on your loved ones as well as the rest of your community.