Herd immunity: Definition, threshold, and examples

Once a population has herd immunity, even people vulnerable to an infection are less likely to get it, as few people around them could pass it on.

Read on to learn more about herd immunity, including the definition, how it works, real-life examples, and its role in vaccine programs.

Herd immunity refers to how immunity from infections not only helps individuals, but whole “herds” or groups of people. To understand this, it is necessary to understand how infections spread.

Once a germ is inside someone’s body, it begins reproducing. New copies of the germ then spread to others, and the process continues.

This is how an infectious illness can spread rapidly through a group of people. However, if most of that group develops immunity to the germ, it can no longer spread so easily, meaning even people with no immunity are less likely to get sick.

With immunity at high enough levels, it may be difficult for a germ to spread at all.

There are two ways humans can develop herd immunity: infection or vaccination.

Vaccination “teaches” the immune system how to respond to an infection without a person needing to contract it. Vaccines:

Recovering from an infection can also produce herd immunity under the right circumstances, but it is less predictable. For this to happen, most people would need to:

However, not everyone will develop strong and lasting immunity to every infection. Immunity can vary in duration and may wane over time.

Additionally, the infection could still circulate among children who do not yet have natural immunity. Viruses can also evolve ways of evading the immune system over time.

Smallpox is the only human disease that herd immunity has eradicated. Through much of its 3,000-year history, it had no treatment and a high mortality rate, killing around 1 in 3 people it affected.

In those who survived, smallpox could cause scarring, vision loss, or infertility. However, in some parts of the world, people practiced an early type of inoculation that protected people from the smallpox virus.

In the 19th century, the scientist Edward Jenner developed a vaccine for smallpox. After a global effort to eradicate the disease in the 20th century through vaccination programs, the World Health Organization (WHO) declared “smallpox is dead”.

Measles has existed since at least the 9th century. Before vaccination became available, it caused around 30 million infections and killed around 2 million people worldwide per year.

There are no treatments for measles, although antibiotics can treat some of its complications.

Scientists developed a highly effective measles vaccine in the 1950s and 1960s. In 1971, it became part of the measles, mumps, and rubella vaccine (MMR). Today, the MMR vaccine is available alone or with the addition of the varicella or chickenpox vaccine (MMRV).

In 2016, the Americas became the first region in the world to become measles-free due to herd immunity. However, measles returned two years later due to gaps in vaccination coverage.

Polio, or poliomyelitis, caused epidemics in the mid-20th century. The condition causes no symptoms in most people, but in some, it causes limb weakness or muscle paralysis, which can be lifelong. It especially affects children under 5.

Scientists in the United States and the United Kingdom developed polio vaccines in the 1950s. By the end of June 1960, 77% of children had received a vaccination.

According to Action Medical Research for Children, the number of polio cases has reduced by 99% since 1988. There were only 6 cases in 2021, and only two countries in which one type of polio is still endemic: Pakistan and Afghanistan.

“Endemic” means that a disease has a constant presence in a specific geographical area. As long as polio still exists, it can still affect children who have not had a polio vaccine.

This varies depending on the disease. For example, the herd immunity rate for measles is 95% or above. This is because measles spreads very easily.

However, because the measles vaccine is very effective, scientists can be certain that vaccination programs work if enough people get them.

The threshold is less clear for other infections. For example, at the beginning of the COVID-19 pandemic, some scientists speculated that humans would reach herd immunity with mass infections.

But as of 2024, this has not happened, and scientists do not know the level of immunity that would prevent COVID-19 from circulating.

This is partly because respiratory viruses and their vaccines often produce temporary immunity. While some vaccines do not lead to herd immunity, though, they can still significantly reduce the risk of severe illness, complications, and death.

If a population’s herd immunity declines, people will begin getting infections again that may have been rare previously.

This is happening in numerous places worldwide due to gaps in vaccinations. For example, measles cases rose by 18% between 2021 and 2022, and deaths from measles rose by an estimated 43% during the same period.

From 2000 to 2019, measles vaccination coverage had increased to 86%, but after the COVID-19 pandemic began, it declined to 81%.

There are several reasons for this, including vaccine hesitancy, misinformation, and lack of availability or access. Decreasing herd immunity can put vulnerable people at risk, especially if they cannot have a vaccine for medical reasons or if they are young children.

However, people can still take steps to protect themselves and their families.

Vaccinations can still offer protection even without herd immunity, so it is important to get them as soon as possible.

Herd immunity can occur when enough people in a group or “herd” have immunity to a specific infection.

The people with immunity prevent the infection from spreading person-to-person as easily, meaning that even if some people have no immunity, they are less likely to get sick.

Through vaccination programs, humans have been able to reach or almost reach herd immunity for several conditions, including smallpox and measles.

Vaccinations are an effective way to create herd immunity because they protect many people simultaneously. However, inconsistent or waning immunity for any reason can allow infections to spread again.