Herd immunity and COVID-19: What you need to know

Herd immunity occurs when a large portion of a community (the herd) becomes immune to a disease. The spread of disease from person to person becomes unlikely when herd immunity is achieved. 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.

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, experts now believe it'll likely be difficult to achieve herd immunity for COVID-19.

Getting COVID-19 offers some natural protection or immunity from reinfection with the virus that causes COVID-19. It's estimated that getting COVID-19 and COVID-19 vaccination both result in a low risk of another infection with a similar variant for at least six months.

But because reinfection is possible and COVID-19 can cause severe medical complications, it's recommended that people who have already had COVID-19 get a COVID-19 vaccine. In addition, COVID-19 vaccination might offer better protection than getting sick with COVID-19. A recent study showed that unvaccinated people who already had COVID-19 are more than twice as likely as fully vaccinated people to be reinfected with COVID-19.

Recent research also suggests that people who got COVID-19 in 2020 and then received mRNA vaccines produce very high levels of antibodies that are likely effective against current and, possibly, future variants. Some scientists call this hybrid immunity. Further research is needed.

Natural infection

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

• Reinfection. It's estimated that getting COVID-19 results in a low risk of another infection with a similar variant for at least six months. However, even if you have antibodies, you could get COVID-19 again. Because reinfection can cause severe medical complications, it's recommended that people who have already had COVID-19 get a COVID-19 vaccine.
• Health impact. Infection with the COVID-19 virus 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 two COVID-19 vaccines and given emergency use authorization to a handful of COVID-19 vaccines.

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

• Vaccine hesitancy. Some people 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. Research suggests that COVID-19 vaccination results in a low risk of infection with the COVID-19 virus for at least six months. However, although COVID-19 vaccines are effective in preventing severe illness from current and possibly future variants, people who are vaccinated and up to date on their vaccines may still get breakthrough infections and spread the virus to others.
• Uneven vaccine access. 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.

Given the challenges, it's unclear if herd immunity to the virus that causes COVID-19 will be reached. However, the number of fully vaccinated adults continues to rise. In addition, more than 80 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.

Even if it isn't currently possible to stop transmission of the COVID-19 virus, 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. The vaccines are allowing people to better be able to live with the virus.

There are steps you can take to reduce your risk of infection. When possible, get a COVID-19 vaccine. Also stay up to date with COVID-19 vaccines, including getting recommended booster doses, to prevent serious illness. You're considered up to date with your vaccines if you've gotten all recommended COVID-19 vaccines, including booster doses, when you become eligible.

If you're up to date with your vaccines, you can more safely return to doing activities that you might not have been able to do because of the pandemic. However, if you are in an area with a high number of people with COVID-19 in the hospital and new COVID-19 cases, the CDC recommends wearing a mask indoors in public.

The CDC recommends following these precautions:

• Avoid close contact with anyone who is sick or has symptoms
• Keep distance between yourself and others when you're in indoor public spaces if you're not fully vaccinated. This is especially important if you have a higher risk of serious illness. Keep in mind some people may have COVID-19 and spread it to others, even if they don't have symptoms or don't know they have COVID-19.
• Avoid crowds and indoor places that have poor air flow (ventilation).
• 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 if you're in an area with a high number of people with COVID-19 in the hospital and new COVID-19 cases, whether or not you're vaccinated. The CDC recommends wearing the most protective mask possible that you'll wear regularly, fits well and is comfortable.
• Cover your mouth and nose with your elbow or a tissue when you cough or sneeze. Throw away the used tissue. Wash your hands right away.
• 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, regularly.
• Stay home from work, school and public areas and stay home in isolation if you're sick, unless you're going to get medical care. Avoid taking public transportation, taxis and ride-hailing services if you're sick.

If you have a chronic medical condition and may have a higher risk of serious illness, check with your health care provider about other ways to protect yourself.