What Is Herd Immunity?

Herd Immunity Explained: Let’s do a thought experiment:

Think of every person you interact with on an ordinary day. Every person whose hand you shake or every person you hug. The people you live with and work with. Then try to imagine every person those people interact with, too. Every person you meet is a person you could pass an infectious disease to. Oppositely, every person you meet could pass an infectious disease to you.

Granted, this depends on how infectious the disease is and how the infectious disease spreads, and there are many ways this can happen. But for our purposes let’s stick to the flu. According to the CDC, “[p]eople with flu can spread it to others up to about 6 feet away.”

If you have the flu, and your coworkers and family members are vulnerable, you can spread the flu to them. They in turn can spread it to their coworkers and family members. But if one of your coworkers has gotten the flu vaccine, your colleague may be immune and your  coworker won’t pass the flu along.

Now imagine every one of your coworkers has received the yearly flu vaccine. Even though you have the flu, they will likely not pass it on to their not-yet-vaccinated family members. In other words, by getting their flu shot your coworkers have helped protect other people, as well as themselves.

To be certain, this is not a novel idea. Many people suggest vaccination protects not only the vaccinated, but everyone, especially since some people are not able to be vaccinated. Our thought experiment illustrates what experts call herd immunity (or “community immunity”) at work.

What is Herd Immunity (or Community Immunity)?

The CDC glossary defines herd immunity or community immunity as:

“A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community…”[1]

Other’s explain herd immunity as “a tipping point — once enough people are vaccinated, the flu can’t spread quickly because it encounters people who are protected against it.”[2] But herd immunity applies to more infectious diseases than just the flu. In theory, herd immunity could account for any infectious disease for which we have a vaccine, so long as the disease is spread by human-to-human contact.[3] For example, vaccines.gov  cites the case of pneumococcal vaccines. Another example is measles. However, because measles is so highly infectious herd immunity for the illness requires as much as ninety-five percent of the population to be immune at any given time to prevent the spread of infection. [4]

Because some diseases are more contagious than others, the number of people who must be vaccinated for herd immunity to exist can vary. Herd immunity against mumps, for example, requires between 75-86% of the population to be vaccinated, whereas herd immunity against pertussis requires 92-94% (see chart in link). There is in fact a mathematical formula for determining the exact proportion theoretically required. And if you relax the theoretical assumptions, the formula can even change.

As many of the sites I have linked to note, and as I observed several paragraphs up, some people are unable to receive certain vaccines. Likewise, this government site notes possible concerns that diseases can reenter the United States, after we have nearly eliminated them. Vaccination rates in America are not universal, even for “children aged 19-35 months.” This article notes, “…herd immunity… is not a good alternative to getting vaccinated.”

Herd Immunity and Vaccine Injuries

However, there are individuals who can’t have any or certain vaccines. Additionally, there are also risks of negative side effects with vaccines. Although ultimately whether to vaccinate is a question you should ask your physician, in the majority of cases the benefits of vaccines may outweigh the risks. As we’ve seen with herd immunity, vaccines can protect not only the people who receive them but also the people those vaccinated people interact with. “The United States has the safest, most effective vaccine supply in history,” according to the Health Resources & Services Administration

Most people suffer mild side effects, at worst, after a vaccination. Nonetheless, some people do suffer exceptionally rare, though serious, injuries as a result of vaccines. In these very unusual instances, victims can seek compensation through the National Vaccine Injury Compensation Program. In all, the program has paid out “approximately $4.1 billion” in awards.

The National Vaccine Injury Compensation Program is a no-fault non-adversarial program where the Secretary of Health and Human Services is named as the respondent in a case before the United States Court of Federal Claims Office of Special Masters. So long as you can prove your injuries were, more likely than not, the result of a covered vaccine, you may be eligible for compensation. Even if you are not found eligible, the program may still cover your attorneys’ fees and costs for bringing the matter before the court.

If you are thinking of pursuing a case in the National Vaccine Injury Compensation Program, seek help from a thorough, steadfast ally. At Sands Anderson, we are a true team of legal professionals who can use our knowledge and experience to help you. Contact the Sands Anderson Vaccine Injury Legal Team today to discuss your potential claim.


[1] Some experts have advocated using herd immunity to refer to “‘the proportion of subjects with immunity in a given population.’” They suggest using “‘herd effect’” for the “the reduction of infection or disease in the unimmunised segment as a result of immunising a proportion of the population.” I have not followed their advice and will be using the CDC definition instead.

[2] In the case of the flu specifically, this abstract suggests “[t]he objectives of vaccination coverage proposed in the United States – 80% in healthy persons and 90% in high-risk persons – are sufficient to establish herd immunity…” However, the abstract cautions, “[t]he percentages of vaccination coverage registered in the United States and Europe are not sufficient to establish herd immunity.”

[3] In his interview, Dr. Manish Sadarangani notes, “Herd immunity only works for diseases that are spread directly between people (i.e. are ‘contagious’), like measles. One example where it would not work is tetanus…” He goes on to explain, “The bacteria which cause tetanus lives in the soil, so anyone who is not vaccinated…could easily be infected if they were exposed …even if everyone else around them was vaccinated and protected.”

[4] The two sources linked to cite the possible number as 93-95%. This article puts the figure at 90-95%.