What to Know About Herd Immunity

Medically Reviewed by Zilpah Sheikh, MD on December 06, 2023
5 min read


Herd immunity, or community immunity, is when a large part of the population of an area is immune to a specific disease. If enough people are resistant to the cause of a disease, such as a virus or bacteria, it has nowhere to go.

Though every person may not be immune, the group as a whole has protection. This is because there are fewer people who can catch the infection overall. The infection rates drop, and the disease gradually disappears.

Herd immunity protects at-risk populations. These include babies and people whose immune systems are weak and can’t get resistance on their own or can't tolerate a vaccine.

Herd immunity happens in two ways.

Infection. You can develop resistance naturally. When your body is exposed to a virus or bacteria, it makes antibodies to fight off the infection. When you recover, your body keeps these antibodies and uses them to defend against another infection. This is what stopped the Zika virus outbreak in Brazil. Two years after the outbreak began, 63% of the population had been exposed to the virus. Researchers think the community reached the right level for herd immunity.

Vaccination. Vaccines can also build resistance. They make your body think a virus or bacteria has infected it. You don’t get sick, but your immune system still makes protective antibodies. The next time your body meets that bacteria or virus, it’s ready to fight it off. This is what stopped polio in the United States.

Herd immunity threshold

A community reaches herd immunity (or threshold) at different times for different infections. Each virus or bacteria has its own reproduction number, or R0. The R0 tells you the average number of people that a single person with the virus can infect if those people aren’t already immune. The higher the R0, the more people need to be resistant to reach herd immunity.

In 2020, researchers estimated the R0 for COVID-19 to be between 2 and 3. This meant that one person could infect 2-3 other people. It also meant that 50%-67% of the population would need to be resistant before herd immunity would kick in and infection rates started to go down. By the time the Omicron variant was spreading, it was more contagious, with an R0 estimated to be between 15 and 21, or requiring a threshold of 75%-80% of the population to be immune to reach herd immunity.

Other conditions that spread more easily will have a higher percentage of people who need to be immune to keep the infections under control. The threshold for polio is 80%, and measles, which is very contagious, is 95%.

The main obstacle to herd immunity when COVID began spreading in 2019 was the virus that causes the disease was “novel,” or new. That meant that it hadn’t infected humans before and everyone was at risk of infection. There was no existing immunity to build on.

There are now four vaccines for COVID that have been given in more than 13 billion doses and more than 770 million people have been infected worldwide. But we haven't reached herd immunity with COVID. Reasons the threshold is difficult to meet include: 


COVID has mutated and created new variants including Delta and Omicron. These variants have been more contagious than the original virus, spreading more quickly. To have herd immunity, vaccines have to be created to fight each variant. Because the disease keeps changing, researchers don't know exactly how long a vaccine is effective in fighting COVID.  

Vaccine effectiveness

The COVID vaccine has been effective at slowing the infection rates and making infections less severe in most people. But not enough people have been vaccinated to cause herd immunity. Not everyone who can get a COVID vaccine has received one because of lack of access or decisions not to take them. People who have compromised immune systems can't receive the vaccine. And not everyone has taken the vaccines as they are most effective -- when you receive the required dose and booster shots.  


For COVID, you should get vaccinated and keep up with your boosters so you are protected from any new COVID variants. It’s the best way to protect yourself and others from infection from COVID. And it appears to be the best path to help stop community spread and keep you from getting severe disease.

For other infections, there are a number of things you can do to stop, or slow, the spread including:

  • Get a vaccination if one is available (including hepatitis A and B, the flu, and measles).
  • If you are indoors with other people, open windows to let fresh air circulate.
  • Wash your hands with soap and water (or use hand sanitizer) regularly, especially before eating and after using the bathroom.
  • Keep areas you touch often (like door handles, computer keyboards, and remote controls) clean and disinfected.
  • Cough and sneeze into your elbow to cover your face and keep your hands germ-free.
  • Wear a mask if you are indoors with other people.
  • If you feel sick or have symptoms of a contagious infection, stay home so you don't infect other people.




Herd immunity has been effective in slowing or stopping the spread of a number of diseases including smallpox, measles, and polio. Because the number of people who have to be immune to a disease to reach herd immunity is so high, it's hard to count on infection and recovery to reach the threshold. The most important tools for creating herd immunity against many conditions are vaccinations and practicing good hygiene. 

  • What is an example of herd immunity? Measles. More than 90% of children in the U.S. are vaccinated for the measles by the age of 2. Because 9 out of every 10 people have been vaccinated against the disease, it makes it difficult to spread, even when there is a pop-up infection in a community.
  • Can you have herd immunity to the COVID virus? No country has reached the threshold for herd immunity of the COVID virus. The virus mutates quickly, it can be spread by people who don't have symptoms, and no one knows exactly how long the vaccination protects from the infection. For these reasons, traditional herd immunity may not be reached, but COVID may be kept under control through annual vaccinations, like we do with the flu.