Herd Immunity

What is herd immunity?

Herd immunity means that enough people in a group or area have achieved immunity (protection) against a virus or other infectious agent to make it very difficult for the infection to spread. Immunity happens in multiple ways: through natural infection, vaccination or passive transfer. Vaccination is the best way.

Every person who has immunity makes it harder for the infection to spread to other people. If you’re vaccinated, it’ll be harder for the virus to use you to infect other people or to mutate into a new variant. Higher numbers of immune people are needed to stop the spread if a virus is very infectious.

Herd immunity may also be called community immunity or population immunity.

What are the types of immunity?

Immunity against a disease happens because of antibodies that your body produces to fight off invaders. Each antibody protects against a specific disease but only against that disease. We can have active or passive immunity.

Active immunity

Active immunity is either natural or vaccine-induced. Natural immunity comes from having the disease and surviving. Acquired immunity comes from vaccines that use weak or dead versions of the infectious organism to provoke your body into creating antibodies against the disease. In active immunity, your body produced “memory” that allows you to continue to produce antibodies.

Passive immunity

Passive immunity means that you get antibodies from outside your own body. This happens when antibodies are passed on to newborn babies from their mothers and when antibodies are passed on through blood products that contain antibodies. In passive immunity, your immune system does not produce “memory” and therefore it only protects you as long as you’re receiving the antibodies.

What is herd immunity in terms of COVID-19?

COVID-19, in its original form and in variants, has proven to be very infectious. At the start of the pandemic, researchers thought that having 60% to 70% of the people in the world immunized through vaccination or infection would equal the level of herd immunity needed for COVID-19. However, the contagiousness of the delta and omicron variant has made researchers rethink that number. Now that number could be as high as 85%.

Can we achieve herd immunity against COVID-19 without vaccines?

It’s very unlikely that we can achieve immunity against COVID-19 without vaccines. It would require huge numbers of people to get sick, which would result in a large number of deaths.

Currently, there are problems with reaching herd immunity even with vaccines.

• Some people aren’t willing to be vaccinated even though vaccines are safe.
• Some people are unable to be vaccinated for health reasons.
• Some people (immunocompromised) don't produce antibodies despite having the vaccines.
• We don’t know for sure how long immunity lasts from having COVID-19 or from vaccines.
• Vaccines are not equally available all over the world.

What have we learned about herd immunity from measles and influenza?

Vaccines have made our lives safer by providing effective protection against deadly diseases. We don’t think of measles as being one of those types of illnesses, but about 5% of children who get measles die from measles in areas of the world without good medical care. In the U.S., measles cases dropped by more than 99% after vaccines became routine.

However, despite being declared eliminated in 2000 in the U.S., there have been outbreaks of cases. In 2019, there were 1,282 cases of measles confirmed in the U.S., the highest number since 1992. People who aren’t vaccinated and travel outside of the U.S. are vulnerable, along with people who live within communities of unvaccinated people.

While herd immunity is possible with vaccinations against diseases caused by one organism (like measles or smallpox), diseases like influenza (flu) are different. There are many viruses that cause the flu. The flu shot is based on research about which strains will be most common in any given year. U.S. flu shots protect against four different viruses.

Getting the flu vaccine prevents deaths, especially in children, and reduces the rate of serious disease and hospitalizations. It may also help protect other people around us. However, despite the positives associated with having a flu vaccine every year, there are many people who don’t get the shot.

A note from Cleveland Clinic

Achieving herd immunity the ‘natural’ way against deadly diseases would mean that many people would die and many others get ill and very ill. We’ve used vaccines for years to protect us and to provide herd immunity for the people around us who need protection and can’t get a vaccine, aren’t able to get a vaccine yet or have a weakened immune system and don’t respond fully to the vaccine.