New Thinking About Herd Immunity

An outbreak of measles at Disney this year caused a media event that had many discussing the effectiveness of vaccines and vaccine policy in the US. Throughout history infectious diseases have ravaged human populations. With the advent of antibiotics we have managed to tame many formerly deadly diseases. However, the rise of antibiotic resistant strains of bacteria and the increasing spread of viruses in our mobile world populations have health officials relying heavily on vaccination to prevent the spread of many diseases. From smallpox which has been effectively eradicated to Measles which is well contained to Ebola, a disease we hope to soon control, vaccines have led the charge in reducing infections and deaths for well over one hundred years.

While vaccination programs have been proven effective in either eliminating or greatly reducing the spread of many infectious diseases, they are only successful if enough people are immunized to prevent a widespread outbreak of disease. Essentially, the more immune individuals in a population the less likely a sick individual will pass the disease on to a susceptible individual.

Interestingly, it is not necessary to have EVERYONE in a population immune to the disease to prevent an epidemic (a widespread outbreak of disease beyond normal levels). As an infectious person interacts with other people in a group, only those who are not immune are at risk of infection. The concept of herd immunity allows for a few people in a population to lack immunity to a disease while still preventing a widespread outbreak or epidemic. If only a few people are at risk it’s highly unlikely that one sick individual will ever be near every single susceptible person in that population to pass on the disease. Our video explains the concept of herd immunity.

Typically, epidemiologists like to see herd immunity in a population to be 85% to 95%. At those levels the 10% who are not immune are not likely to become infected by single sick person in that population. Since epidemics only occur when one sick person infects many people, widespread outbreaks are prevented if most, but not necessarily all, individuals in a population are immune. When 90% of the people who are around the sick individual are immune they cannot catch or spread the disease to anyone else. This is vital since there are some individuals who cannot receive an immunization due to age or medical conditions. Therefore adding additional susceptible persons to the population puts those individuals at increased risk. This figure shows herd immunity levels to specific diseases in all 50 states and whether that level has been increasing or decreasing over time.

Vaccination programs have increased herd immunity to the point where many formerly common infectious diseases are now rare. The progress that has been made in this area has become undermined in recent years due to a lack of participation in vaccination programs. Typically, children undergo a battery of vaccines when young to prevent diseases that used to be common. The success of vaccination programs has ironically led some to believe they are no longer necessary. Since so many childhood diseases are so rare (due to vaccinations) some parents feel vaccination is no longer necessary. As a result there is an entire generation of people who are not being immunized and the herd immunity in that age group has been reduced. This has led to an increase in certain diseases that were once rarely, if ever, seen by most doctors.

A rather infamous study proposed a link between autism and childhood vaccinations. Since autism is typically diagnosed at the age of childhood vaccinations, some parents wondered about a connection between mercury based preservatives in vaccines and the onset of autism. This study has since been discredited and the journal in which it was published has issued a retraction. However, the damage has been done and even though mercury based preservatives have been largely removed from vaccines, some internet sites still claim vaccines are related to various forms of autism. For these and other reasons, some individuals are resisting medical advice and choosing to not vaccinate their children.

This phenomenon has an impact on herd immunity beyond the lowering the raw percentage of the population herd immunity. The protective effect of herd immunity relies on the fact that the small number of people who are not immune to a disease are uniformly distributed throughout the population. In other words if a sick person interacts with a healthy person, (assuming normal herd immunity levels) there is a 90% chance that the healthy person is vaccinated and thus immune. The sick person only has a 1 in 10 chance of infecting any single random person. Thus the odds of establishing a chain of spread are very low.

Some are now advocating that we need to strive for 100% herd immunity because it seems that the unvaccinated are not uniformly distributed in a random way throughout the population.

Those who oppose vaccination for their children often times tend to cluster together in various ways. They may send their children to a school that does not require immunizations or perhaps many parents living on the same street hold like minded opinions and choose not to vaccinate. In these cases, subgroups form where herd immunity is very low or nonexistent. This means a single case will cause a mini epidemic (see diagram to right) in that cluster of susceptible individuals.

For example, the Amish do not tend to participate in vaccination programs. As a result a single case of whooping cough, which is highly contagious, can cause an outbreak of a dozen or more cases in a small community. Normally an entire state may not see a dozen cases in one year, yet this small community could easily exceed the entire state total of cases of whooping cough.

While having the goal of 95% herd immunity seems like sound policy, it won’t work at all if the remaining 5% cluster together. In such a case even a single infected individual can spread an illness very rapidly when nearly everyone in the group is lacking immunity.

Additional Resources:

Why a few unvaccinated children are an even bigger threat than you think. Washington Post (Feb 2015)
Herd immunity and measles: why we should aim for 100% vaccination coverage: The Conversation (Feb 2015) – some excellent graphics and discussions here.
Fine, Paul, Ken Eames, and David L. Heymann. ““Herd immunity”: a rough guide.” Clinical infectious diseases 52.7 (2011): 911-916.
Infographic: Myths about Vaccinations: RicochetScience

Michael Troyan has spent 20 years teaching non-majors biology and microbiology and currently works as an online instructor at Penn State University. He is a regular contributor at RicochetScience and can be reached at [email protected]

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