Vaccines and Herd Immunity: How We Can Potentially Protect Our Neighbors

By Dr. Kinjal Solanki

Why herd immunity through vaccination works for some infectious diseases but not for others

Herd immunity refers to the community protection against infectious diseases that occurs when a high percentage of people in a population are immune to a disease. It reduces the chances that the infectious disease can spread from person to person and become a widespread outbreak in the population.

Why it’s called “herd” immunity

It’s called “herd” immunity because in the early 20^th century, this concept was originally applied to the efforts to protect cattle in the United States against a contagious disease. Veterinarians noticed that when most livestock survived a disease, the whole herd was more resistant to it.

This concept was applied to humans when new vaccines raised the question of whether they could eradicate or control a contagious disease in an entire population.

Passive immunity and active immunity

Immunity occurs when the human body has antibodies that fight off attacks by a specific disease. It can occur in two basic ways: passive and active.

Passive immunity

Passive immunity is when antibodies against a disease enter one’s body. This mostly occurs through mothers passing their antibodies on to their babies before birth. It can also occur from antibodies entering the body via blood products. Passive immunity is fleeting and only lasts as long as one receives the antibodies.

Active immunity: natural immunity & acquired immunity

Active immunity, which is either natural or acquired (vaccine-induced), is more effective and applicable to the general population. In active immunity, the body remembers how to produce the antibodies to fight off the disease, retaining that ability.

Natural immunity

Natural immunity is attained when a person has a disease and survives, retaining the ability to reproduce the effective antibodies upon encountering the infectious agent again. To attain natural immunity at a large group or population level would involve large scale sickness and death. It is essentially not desirable.

Acquired immunity through vaccines

The development of effective vaccines in the 1950s and ’60s made acquired immunity possible. A vaccine applies dead or highly weakened versions of an infectious agent to the body, enabling it to produce antibodies against the disease when it arrives via contagious infection.

According to a study in the National Library of Medicine, acquired immunity through vaccination is the safest and most effective way to achieve herd immunity against a disease. Acquired immunity protects vulnerable groups, including infants, children, elderly, those with poor immune systems who are unable to get vaccinated, and those who do not get vaccinated for personal reasons.

The herd immunity threshold

Not every disease needs the same level of immunity in the population to result in herd immunity. The more contagious the disease is, the higher the percentage of people who need immunity to stop the spread.

Infectious diseases need susceptible hosts, or non-immune people, to survive and spread. So, when vaccination rates are high in a population, the disease cannot spread effectively, preventing a more serious outbreak.

Challenges in meeting immunity

Besides the rate of a disease’s contagiousness, other factors impede attaining herd immunity. Population structures with a high mix of cultures, ages and different mingling patterns make large-scale immunity difficult. Vaccines are not 100% effective, and immunity can wane over time. Additionally, some viruses mutate over time, reducing vaccine effectiveness and making it harder for immunity to remain strong across populations.

Different disease, different herd immunity threshold

Scientists calculate the level of a disease’s contagiousness using a formula that arrives at a basic reproductive number for the disease. This is called R_0, or R-naught. This number relates to the number of people, on average, a person with the disease will infect in a completely vulnerable population (where everyone is unvaccinated).

Measles is very contagious and has an R₀ ranging from 12-18. Measles requires that about 95% of the population be vaccinated to reach herd immunity. At this level of herd vaccination, at-risk populations, such as children under 12 months old who cannot get vaccinated, are protected against measles.

In comparison, flu (influenza), which is less contagious, has an R₀ ranging from 0.9-2.1. While the flu doesn’t typically cause massive outbreaks like measles, it persists year after year because of its ability to mutate into new strains requiring changes in the annual vaccine.

Why is herd immunity desirable?

Herd immunity reduces the number of people who get sick, seriously ill or die from an infectious disease. This population-level of immunity reduces suffering, mortality and the high costs of treating so many people.

This level of immunity is particularly important for the vulnerable

Herd immunity adds protection for those who are vaccinated and for those who are most vulnerable to contagious diseases. High vaccine coverage in the overall population is often the only shield against dangerous infectious diseases for many, including those who:

• Are unable to receive vaccines due to allergies, age or medical conditions.
• Choose not to be vaccinated.
• Are on chemotherapy treatment and have a weak immune system.
• Are pregnant and should not receive certain vaccines.
• Are HIV positive.
• Are very sick in the hospital.
• Are at higher risk for complications if infected (elderly & immunocompromised).
• Do not respond adequately to vaccines (elderly & immunocompromised).

For what infectious diseases is herd immunity possible?

We have had notable successes in attaining herd immunity through vaccination for some diseases. For others, attaining herd immunity can be possible, but not as likely.

Smallpox

Smallpox is the only human disease eradicated through global vaccination in 1980. Ring vaccination strategies – where people in close contact with an infected person (such as family and friends) were vaccinated – played a crucial role in halting transmission.

Polio

The U.S. has attained herd immunity against polio, which was at its highest rate in 1952. The Centers for Disease Control and Prevention (CDC) says no cases of polio have been transmitted in 40 years. Some isolated cases have shown up, such as a 2022 case in Rockland County, New York. Polio has been eliminated in most regions of the world, though only a few countries still experience small numbers of endemic cases.

Measles

For measles, we had achieved herd immunity before losing it again. In 2000, the CDC declared that measles had been eradicated in the U.S., primarily due to high vaccination rates. But it has made a comeback in several years since then, including in 2023 when the vaccination rate dropped from 95% (the threshold for herd immunity) to 92%. In 2025, West Texas has been a hotspot for outbreaks.

Mumps

Herd immunity is possible for mumps, but has yet to be reached. The MMR vaccine covers measles, mumps and rubella (German measles). The vaccination rate for mumps with two or more doses was 91.9% for adolescents 13-17 in 2019.

Even with these vaccination rates, which have decreased since 2019, the U.S. regularly has outbreaks of hundreds and thousands of mumps cases. This is likely related to waning immunity over time and high-density living situations, such as college dorms.

Infectious disease unlikely to attain population immunity

Herd immunity (population immunity) isn’t likely for many infectious diseases, including the following examples.

Flu

Flu is different from measles and mumps, which are caused by one organism. Many viruses cause the flu. Flu vaccines target the strains that research indicates will be most troublesome in the upcoming flu season. The threshold for immunity in the U.S. is projected to be 80% for healthy people. In the 2023-24 flu season, 57.4% of those age 6 months-17 years were vaccinated.

COVID-19

It is unlikely we will reach herd immunity for COVID-19, which has a vaccination rate threshold of 60%-70%. The CDC reports a COVID-19 vaccination rate of under 30% in April 2025. In 2025 the Food and Drug Administration restricted the availability of the newest vaccine to high-risk groups such as those over age 65 and pregnant women.

Whooping cough (pertussis)

Population immunity is unlikely for this infectious disease due to vaccination rates, decreasing immunity from the vaccines and from having had the disease, and asymptomatic transmission. The herd immunity threshold is 92%-94%. Vaccination rates for whooping cough were 80.4% for children by age 2; 90.2% for adolescents; and 39.1% for adults.

RSV (respiratory syncytial virus)

No immunity threshold has been set for RSV. An effective vaccine has only recently become available, and its use has not been widely accepted. RSV’s transmission patterns and immunity characteristics make herd immunity very unlikely.