Vaccinating a Nation: a Guide to Achieving Herd Immunity

Flu season is here, and the media is filled with news about the new Covid vaccines, so this is a good time to have a sort of primer explaining the profound importance of vaccination for communities as well as individuals. Nursing@Georgetown, the online MSN program from the School of Nursing and Health Studies at Georgetown University, has kindly allowed DailyNurse to share their guide to the essentials of herd immunity and tips for addressing vaccine-hesitant patients and loved ones.

What is Herd Immunity?

Also referred to as “community immunity,” herd immunity is a public health term used to describe a case in which the potential for person-to-person spread is significantly reduced due to the broader community’s resistance against a particular pathogen.

A Glossary of Important Vaccination-Related Terms

• Active immunity: Immunity as a result of the body’s antibody creation after exposure to disease-causing pathogens, either through natural infection or vaccination.
• Herd immunity threshold: Also called the “critical vaccination level,” this is the approximate percentage of a population that needs to be vaccinated in order to reach herd immunity status.
• Immunity: Resistance to a particular pathogen, or disease-causing bacteria, through antibodies.
• Inactivated vaccine: A vaccine using a killed form of the disease-causing germ. This vaccine usually requires multiple doses over time to form immunity. Examples include the DPT and Hepatitis A vaccines and the flu shot.
• Live attenuated vaccine: A vaccine using a weakened form of a germ to produce an asymptomatic infection and generate an immune response similar to a natural infection, without sickness. Examples include the MMR and chickenpox vaccines.
• Natural infection: Contraction of a disease through person-to-person transmission or interaction with disease-causing bacteria.
• Passive immunity: Immunity after receiving disease-fighting antibodies from an external source.
• R0 (Pronounced “r-naught” or “r-zero”): The reproductive number of a disease that describes the average number of additional cases a single infected person creates.
• Subunit vaccine: A vaccine using a component of the germ (such as a specific protein) to produce an immune response. This vaccine does not contain a live germ. Examples include the shingles and HPV vaccines.
• Vaccine: A controlled simulation of natural infection meant to trigger antibody creation that helps fight against the disease later, without sickness.

Why is Herd Immunity So Important?

When enough community members are immune to a virus so that it inhibits spread, even those who are not vaccinated will be protected. The “herd” collectively provides insulated safety to all members, which is important for those who are too high-risk for certain vaccinations.

For example, the yellow fever vaccine should not be administered to individuals who are pregnant, have a weakened immune system, or have specific allergies, according to the CDC guidelines on who should not be vaccinated.

For these individuals, it is important that their community has built an immune response to the yellow fever so they are not at risk of infection and transmission. Individuals who are too high-risk to get a vaccination are often more likely to contract the illness and experience serious symptoms.

Some other reasons why people cannot get certain vaccinations include:

• Age
• Allergies
• Pregnancy
• Recent blood transfusion
• Underlying medical conditions like lung or heart disease
• Weakened immune system

Elke Jones Zschaebitz, DNP, ARPN, FNP-BC, and faculty member at Georgetown University School of Nursing & Health Studies, likens herd immunity to a perimeter fence that ensures the safety of the broader community.

“It’s like a little nuclear circle,” she said. “So that our babies that can’t get vaccinated yet, or our pregnant mothers, or our elderly with immune systems [that] are not as robust, or people who have certain kinds of conditions that they don’t have the correct immunity, will have protection from the active pathogen that could possibly harm or kill them.”

HITs and R0s: the Science Behind Herd Immunity

In a community where no one is immune to a virus, a disease can spread rapidly and lead to an outbreak. As individuals acquire immunity, either through infection or a vaccine, the disease spreads more slowly because fewer people can pass it on.

In a community where enough members are vaccinated, the disease will stop spreading because the virus will not be able to find susceptible hosts.

The herd immunity threshold (HIT) varies depending on a variety of epidemiological factors. The primary consideration is the infectiousness of a disease. Infectiousness is measured by the Basic Reproduction Number or reproductive ratio—often referred to as R0 (“r-zero”). R0 refers to the number of cases expected to occur on average in a susceptible population as a result of infection by a single individual at the start of an epidemic before widespread immunity starts to develop. So if one person develops the infection and passes it on to two others, the R0 is 2. Herd immunity helps R0 drop below 1.

Here are some examples of R0s for a few well-known infectious diseases:

• Measles R0: 12-18
• Polio R0: 5-7
• Ebola R0: ~2

Numbers for COVID-19’s R0 vary —The Atlantic reported a range of about 1.5 to 5.5 in February, while more recent estimates from WHO place COVID-19’s R0somewhere between 2 and 4 (PDF, 2 MB). The herd immunity threshold (HIT) varies depending on a variety of epidemiological factors. The primary consideration is the infectiousness of a disease, often referred to as R0; herd immunity helps R0 drop below 1.

“The higher R0 is, you’ll have to have really, really high [levels of active immunity], or it’s still transmissible and the vulnerable population will still get it,” Thompson-Brazill said.

A September 2020 report in Nature Reviews Immunology expects COVID-19’s herd immunity threshold to equal 67%, if R0 is 3. However, the Government Accountability Office cautioned researchers against reaching any conclusions regarding herd immunity for COVID-19 (PDF, 276 KB), as much is still unknown about the contagiousness of the disease.

The Role Clinicians Play in Herd Immunity and Vaccination

Zschaebitz and Thompson-Brazill have years of experience in various health care settings— from travel clinics and trauma units to international research. Some of the ways that clinicians increase herd immunity and vaccination include:

• participating in global research to genotype communities and help produce viable vaccines.
• administering vaccines so patients can stay up-to-date on immunization schedules and travel requirements.
• educating patients on the importance of vaccination and specific information related to different vaccines.

How to Talk to Patients, Family, and Friends About Vaccination

Thompson-Brazill shared her experience speaking with patients who are vaccine-hesitant and said she has learned that one of the roles clinicians play in herd immunity is educating patients about why vaccines are safe and normative for society.

Zschaebitz also shares the importance of herd immunity with patients — often through the retelling of her own experiences genotyping Maasai tribal women for an HPV vaccine.

“We were interested in preventing deaths of cervical cancer because in certain countries women just die,” she said. “Declining a vaccination is sort of a first-world problem because people in other nations would walk for miles to get what we have and what we take advantage of.”

Thompson-Brazill and Zschaebitz’s tips for speaking to vaccine-hesitant friends and family:

Tip #1: Avoid pointed questions that could make someone defensive.

Instead of: “Why wouldn’t you get the shot?” Try: “What about the shot worries you?”

Tip #2: Use storytelling as a way to share your experience.

Instead of: “I can’t believe you are not going to vaccinate your children” Try: “I chose to vaccinate my kids because…”

Tip #3: Refer people to credible, reliable sources.

Instead of: “Why would you believe that? That’s just a hoax!” Try: “The CDC has a lot of useful information about vaccines and potential risks. Have you read what they have to say?”

Tip #4: If you do not know how to respond, recommend an expert who does.

Instead of: “I can’t talk to you about this; we will never agree.” Try: “Have you shared your concerns with your clinician? They will be able to answer your questions.”

A Herd Immunity FAQ

Is it possible to achieve herd immunity without a vaccine?

Because the herd immunity threshold is so high, achieving herd immunity without a vaccine is unlikely, according to the American Lung Association. The likelihood depends on the disease’s R0, because a lower threshold would be easier to reach naturally. However, the severity of a disease could also mean that many individuals in a community would succumb to a disease before broader immunity is obtained.

Is herd immunity effective against all diseases?

No, not all diseases can be overcome with herd immunity. For example, herd immunity cannot be achieved for Clostridium tetani, the bacteria that causes tetanus. Although infection is avoidable via individual vaccination, the “herd” cannot provide protection. For example, the immunity of others will not prevent an individual from contracting tetanus after stepping on a rusty nail.

Can you still get the flu after a flu shot?

It is possible to become sick with a strain of influenza that the vaccine was not developed to protect against. However, the CDC says that the flu vaccine will minimize the severity of symptoms among those who do get sick. Other reasons someone could experience flu-like symptoms after getting a flu shot include contracting a different respiratory illness and exposure to the flu virus shortly before vaccination.

Why do I need to get vaccinated for diseases we already have herd immunity for?

Some immune responses weaken over time, which is why booster shots are so important. Additionally, vaccine refusal can lead to waning community immunity. For example, in 2019, measles outbreaks in New York and Oregon threatened the United States’ 20-year measles elimination status, according to a press release from HHS on measles outbreaks in 2019.

Is immunity from a natural infection stronger than immunity from a vaccination?

Yes. Natural immunity, the result of antibody production after natural infection, is often more effective and longer-lasting than acquired immunity via a vaccine. However, acquired immunity is safer. Some cases of natural infection can be accompanied by symptomatic illness and severe outcomes like pneumonia, liver cancer, and even death, according to an article on vaccine safety from the Children’s Hospital of Philadelphia.

Reliable Information Sources on Vaccines and Vaccination

Both Zschaebitz and Thompson-Brazill recommend using nationally recognized, expert-led sources to learn more about vaccines, like the CDC, FDA, and NIH. Some of the additional resources available include:

• CDC Immunization Schedules External link : The CDC-recommended vaccination series and timing schedule for children, adolescents, and adults.
• The CDC Yellow Book External link : A set of travel health guidelines, including country-specific vaccine recommendations and requirements.
• Questions About Vaccines, FDA External link : The FDA’s collection of information regarding specific vaccines and approvals.
• The Power of Herd Immunity, TED Talk by Romina Libster External link : Health researcher Romina Libster tells the story of an H1N1 outbreak in her town and the role of herd immunity.
• Vaccines by Disease, HHS External link : Vaccine-specific answers to questions such as: “Why is this vaccine important?” and “What are the side effects of this vaccine?”

DailyNurse would like to thank the Georgetown School of Nursing and Health Studies and the Nursing@Georgetown site for allowing us to share this guide.