What is ‘herd immunity’

6 min read

To different people, the term “herd immunity” means different things.

Immunity gained through vaccination or previous infection provides “herd immunity” or “population immunity,” which provides indirect protection from infectious diseases. Rather than allowing a disease to spread through a population segment, WHO believes that vaccination is the best way to achieve ‘herd immunity.’ This would result in an unnecessarily high number of disease cases and deaths.

Rather than exposing people to the pathogen that causes disease, herd immunity against COVID-19 should be achieved by immunizing them. 

Antibodies are proteins that our immune systems produce to fight disease, just as they do when we are exposed to a disease, but vaccines do not cause us to become ill, which is critical. People who have been immunized are no longer at risk of contracting the disease or spreading it to others, thereby breaking the transmission chain. Please see our dedicated page for more information on COVID-19 and vaccines.

To achieve herd immunity against COVID-19 and reduce the amount of virus that can spread throughout a population, a large percentage of the population must be vaccinated. One of the goals of working toward herd immunity is to keep vulnerable groups safe and protected from disease, such as those who cannot be vaccinated (e.g. due to health conditions like vaccine allergies). See our Vaccines and Immunizations Q&A for more information.

Herd immunity requires a certain percentage of the population to be immune to the disease. To achieve herd immunity against measles, for example, 95 percent of a population must be immunized. Measles will not spread among those who have been immunized, thereby protecting the remaining 5% of the population. In the United States, approximately 80% of people are at risk of contracting polio. We don’t know what proportion of the population must be immunized against COVID-19 before herd immunity develops. This is an important area of research, and the results will most likely vary depending on the community, vaccine, and target populations.

Vaccines that achieve herd immunity reduce disease spread and save lives, which is becoming increasingly important as the world’s population ages.

To learn more about the science behind herd immunity, watch or read this interview with Dr. Soumya Swaminathan, WHO’s Chief Scientist.

The significance of herd immunity

When the majority of a community (the herd) becomes immune to a disease, it is uncommon for it to spread from person to person. As a result, not only those who are immune are safe, but the entire community is as well.

A certain proportion of the population must be susceptible to a disease in order for it to spread. This is referred to as a threshold ratio. More immune people mean fewer people getting sick, which means fewer people getting sick means fewer people getting sick means fewer people getting sick means fewer people getting sick means fewer people getting sick means fewer people getting sick We’re talking about the herd immunity threshold here.

What percentage of a population must be immune to achieve herd immunity? A lot is determined by the illness. To halt the spread of a highly contagious disease, a larger proportion of the population must be immune to it. As an example, consider the measles, which is extremely contagious. According to current estimates, nearly a quarter of the population must be immune in order to break the transmission chain.

What safeguards are in place to protect the herd?

COVID-19 herd immunity can be achieved in two ways: infection or vaccination.

Infection brought about by something in the environment

Herd immunity can be achieved when a large enough percentage of a population has recovered from a disease and developed antibodies to protect them from re-infection.

However, relying on community infection to develop herd immunity to the virus that causes COVID-19 has significant drawbacks:

Reinfection

How long do you think you’ll be immune to getting sick again after recovering from COVID-19? Even if you have antibodies, you are still at risk of contracting COVID-19.

Influence on one’s health. According to experts, more than 200 million Americans, or 70% of the population, would need to recover from COVID-19 for the pandemic to be halted in the United States. With so many infections, millions of people could die, particularly as a result of serious complications and pre-existing health conditions. The healthcare system may become overburdened over time.

Vaccines

Herd immunity can be achieved by immunizing a large enough number of people against a disease and allowing those people to develop antibodies that will protect them from future infection. Vaccines, unlike natural infection, create immunity without causing illness or having any negative consequences. By utilizing the concept of herd immunity, vaccines have successfully controlled infectious diseases such as smallpox, polio, diphtheria, rubella, and many others.

Herd immunity can protect an entire population from a disease, even if some people are immune compromised or cannot be vaccinated.

The US Food and Drug Administration has approved one COVID-19 vaccine, and a limited number of COVID-19 vaccines have been authorized for emergency use.

However, a number of factors make achieving herd immunity through COVID-19 immunization more difficult. Consider the following example:

Unwillingness to get vaccinated

If you have religious or safety concerns about receiving the COVID-19 vaccine, you should think twice. If the proportion of vaccinated people in a community falls below the herd immunity threshold, a contagious disease may continue to spread.

Concerns about safety and security

Covid-19 vaccines are only effective for a short period of time. The ability of COVID-19 vaccines to reduce COVID-19 virus transmission will need to be studied further. Furthermore, some research indicates that COVID-19 vaccines may be less effective against some COVID-19 virus variants. The emergence of new vaccine-resistant variants happens on a regular basis.

Inequitable vaccine distribution. The distribution of COVID-19 vaccines has been extremely uneven across countries. If a high rate of COVID-19 vaccination is achieved in one community but not in the surrounding areas, outbreaks may occur if the populations in those areas mix.

When it comes to protecting against COVID-19-related serious illness that necessitates hospitalization or even death, vaccines approved and authorized by the FDA are extremely effective. Even if the COVID-19 virus cannot currently be stopped from spreading, vaccines are assisting people in dealing with the virus more effectively.

How can the spread of COVID-19 be slowed?

If possible, consider getting vaccinated against COVID-19. You can resume normal activities that you were previously unable to do due to the outbreak as long as you are fully protected against the pandemic and have received your vaccinations. This includes not wearing a mask or otherwise maintaining a safe distance from other people, unless otherwise required by a rule or law. If you live in an area where there have been a lot of new COVID-19 cases in the last week, the CDC recommends wearing a mask indoors and outdoors in crowded areas or when you are in close contact with unvaccinated people.

If you have not received the COVID-19 vaccine, take precautions to avoid infection.

Avoid contact with anyone who is sick or has symptoms (within about 6 feet or 2 meters).

Maintain your distance from others (within about 6 feet, or 2 meters). This is especially important if you have a family history of a life-threatening illness. Even if you have no symptoms, you could be infected with the COVID-19 virus and spread it to others.

Use an alcohol-based hand sanitizer containing at least 60% alcohol, or wash your hands frequently for at least 20 seconds with soap and water.

Wear a face mask both indoors and outdoors when there is a high risk of COVID-19 transmission (such as a crowded event or large gathering). Whether you are fully vaccinated or unvaccinated affects how you should wear your mask in the future. Surgical masks should be used if they are available. Only medical professionals should use N95 filter respirators.

When you cough or sneeze, cover your mouth and nose with your elbow or a tissue. The used tissue should be properly disposed of.

Do not use your bare hands to touch your eyes, nose, or mouth.

Use no one else’s dishes, glasses, or bedding if you’re sick.

Make it a daily habit to disinfect and clean high-touch areas.

If you’re sick, stay away from work, school, and other public places unless you’re going to the doctor. If you’re not feeling well, take a taxi or ride-sharing service.


Our stance on COVID-19 “herd immunity”?

Attempting to achieve ‘herd immunity’ by exposing people to a virus is unethical and problematic from a scientific standpoint. COVID-19 infection will cause suffering and death in people of any age or health status.

For the time being, the vast majority of people in most countries can be infected by this virus. According to seroprevalence studies, COVID-19 infects less than 10% of people in most countries.

We don’t know why some people are immune to COVID-19. However, we don’t know how strong or long-lasting the immune response is in those infected with COVID-19, or how it varies between individuals. People who have been infected with COVID-19 for the second time have also been documented.

It will be impossible to estimate how many people are immune and how long that immunity will last without a better understanding of COVID-19 immunity, let alone make predictions for the future, without a better understanding of COVID-19 immunity. Allowing people to become infected should be avoided as a result of these difficulties.

Others, such as the elderly and those with pre-existing conditions, are also at risk.

Finally, while the majority of infected people develop mild or moderate COVID-19 symptoms and some do not become ill, many become critically ill and require hospitalization. People who have had COVID-19, including what is known as ‘Long COVID,’ have long-term health effects that we are only now learning about. WHO is collaborating with clinicians and patient advocacy groups to learn more about COVID-19’s long-term effects.

Do we know if people are immune to COVID-19 yet?

COVID-19 infection usually triggers an immune response within a few weeks.

The study of the strength and duration of this protection is still ongoing. Furthermore, WHO researchers are investigating whether the severity of an individual’s infection affects the strength and duration of their immune response: whether it is asymptomatic, mild, or severe. Immune responses appear to develop even in those who do not exhibit symptoms.

According to data from seroprevalence studies, the vast majority of the world’s population is still susceptible to this virus, with less than 10% of those surveyed globally infected.

SARS-CoV-1, Middle East Respiratory Syndrome (MERS), and other coronaviruses, like the common cold, gradually weaken the immune system. Individuals infected with SARS-CoV-2 develop antibodies and immunity, but the duration of this immunity is unknown.

Listen to this discussion between Dr. Mike Ryan and Dr. Maria Van Kerkhove to learn more about immunity.

What, if any, position does the WHO take on COVID-19 ‘lockdowns’?

COVID19 transmission can be slowed by limiting contact between individuals through large-scale physical distancing measures and movement restrictions, commonly referred to as “lockdowns.”

These measures, on the other hand, have the potential to be disastrous for individuals, communities, and entire societies, as they can effectively halt social and economic activity. These policies disproportionately affect vulnerable groups such as the poor, migrants, internally displaced people, and refugees. These groups are more likely to live in overcrowded, under-resourced areas and rely on daily labor to survive.

In the extra time provided by ‘lockdown’ measures, security forces and government agencies must use all available resources to build their capacity to detect and isolate all cases, trace and quarantine all contacts, engage and empower the population to drive societal response, and much more.

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