Understand what's known about herd immunity and what it means for coronavirus disease 2019 (COVID-19).

Curious about progress toward herd immunity against the coronavirus disease 2019 (COVID-19)? Understand how herd immunity works, its role in ending the COVID-19 pandemic and the challenges involved.

Herd immunity occurs when a large portion of a community (the herd) becomes immune to a disease, making the spread of disease from person to person unlikely. As a result, the whole community becomes protected — not just those who are immune.

Often, a percentage of the population must be capable of getting a disease in order for it to spread. This is called a threshold proportion. If the proportion of the population that is immune to the disease is greater than this threshold, the spread of the disease will decline. This is known as the herd immunity threshold.

What percentage of a community needs to be immune in order to achieve herd immunity? It varies from disease to disease. The more contagious a disease is, the greater the proportion of the population that needs to be immune to the disease to stop its spread. For example, the measles is a highly contagious illness. It's estimated that 94% of the population must be immune to interrupt the chain of transmission.

There are two main paths to herd immunity for COVID-19 — infection and vaccines.

Natural infection

Herd immunity can be reached when enough people in the population have recovered from a disease and have developed protective antibodies against future infection.

However, there are some major problems with relying on community infection to create herd immunity to the virus that causes COVID-19:

  • Reinfection. It’s not clear how long you are protected from getting sick again after recovering from COVID-19. Even if you have antibodies, you could get COVID-19 again.
  • Health impact. Experts estimate that in the U.S., 70% of the population — more than 200 million people — would have to recover from COVID-19 to halt the pandemic. This number of infections could lead to serious complications and millions of deaths, especially among older people and those who have existing health conditions. The health care system could quickly become overwhelmed.

Vaccines

Herd immunity also can be reached when enough people have been vaccinated against a disease and have developed protective antibodies against future infection. Unlike the natural infection method, vaccines create immunity without causing illness or resulting complications. Using the concept of herd immunity, vaccines have successfully controlled contagious diseases such as smallpox, polio, diphtheria, rubella and many others.

Herd immunity makes it possible to protect the population from a disease, including those who can't be vaccinated, such as newborns or those who have compromised immune systems.

The U.S. Food and Drug Administration has approved one COVID-19 vaccine and given emergency use authorization to a handful of COVID-19 vaccines.

But reaching herd immunity through vaccination against COVID-19 might be difficult for many reasons. For example:

  • Vaccine hesitancy. Some people may object to getting a COVID-19 vaccine because of religious objections, fears about the possible risks or skepticism about the benefits. If the proportion of vaccinated people in a community is below the herd immunity threshold, a contagious disease could continue to spread.
  • Protection questions. It’s not clear how long the COVID-19 vaccines will protect you from COVID-19. Further research is needed to see how much the COVID-19 vaccines reduce transmission of the COVID-19 virus. Also, research suggests that COVID-19 vaccines may have lower efficacy against some of the variants of the COVID-19 virus. New variants, which could be more resistant to vaccines, are regularly emerging.
  • Uneven vaccine roll-out. The distribution of COVID-19 vaccines has greatly varied among and within countries. If one community achieves a high COVID-19 vaccination rate and surrounding areas don’t, outbreaks can occur if the populations mix.

The U.S. is currently making progress toward herd immunity through a combined approach. The number of fully vaccinated adults continues to rise. In addition, more than 31 million people in the U.S. have had confirmed infections with the COVID-19 virus — though, again, it’s not clear how long immunity lasts after infection.

Given the challenges, it’s not clear if or when the U.S. will achieve herd immunity.

However, the FDA-approved and FDA-authorized COVID-19 vaccines are highly effective at protecting against severe illness requiring hospitalization and death due to COVID-19. Even if it isn’t currently possible to stop transmission of the COVID-19 virus, the vaccines are allowing people to better be able to live with the virus.

When possible, get a COVID-19 vaccine. If you’re fully vaccinated, you can return to doing activities you might not have been able to do because of the pandemic, including not wearing a mask or social distancing in any setting — except where required by a rule or law. However, if you are in an area with a high number of new COVID-19 cases in the last week, the CDC recommends wearing a mask indoors in public and outdoors in crowded areas or when you are in close contact with unvaccinated people.

If you haven’t had a COVID-19 vaccine, take steps to reduce the risk of infection:

  • Avoid close contact (within about 6 feet, or 2 meters) with anyone who is sick or has symptoms.
  • Keep distance between yourself and others (within about 6 feet, or 2 meters). This is especially important if you have a higher risk of serious illness. Keep in mind some people may have the COVID-19 virus and spread it to others, even if they don't have symptoms.
  • Wash your hands often with soap and water for at least 20 seconds, or use an alcohol-based hand sanitizer that contains at least 60% alcohol.
  • Wear a face mask in indoor public spaces and outdoors where there is a high risk of COVID-19 transmission, such as at a crowded event or large gathering. Further mask guidance differs depending on whether you are fully vaccinated or unvaccinated. Surgical masks may be used if available. N95 respirators should be reserved for health care providers.
  • Cover your mouth and nose with your elbow or a tissue when you cough or sneeze. Throw away the used tissue.
  • Avoid touching your eyes, nose and mouth.
  • Avoid sharing dishes, glasses, bedding and other household items if you're sick.
  • Clean and disinfect high-touch surfaces, such as doorknobs, light switches, electronics and counters, daily.
  • Stay home from work, school and public areas if you're sick, unless you're going to get medical care. Avoid public transportation, taxis and ride-sharing if you're sick.
Aug. 28, 2021