You’ve probably heard the term “herd immunity” used in relation to the COVID-19 pandemic.
Some leaders — for example, Boris Johnson, the prime minister of the United Kingdom — suggested it might be a good way to stop or control the spread of the new coronavirus, known as SARS-CoV-2. Herd immunity is also called community immunity and herd or group protection.
Herd immunity happens when so many people in a community become immune to an infectious disease that it stops the disease from spreading.
This can happen in two ways:
- Many people contract the disease and in time build up an immune response to it (natural immunity).
- Many people are vaccinated against the disease to achieve immunity.
Herd immunity can work against the spread of some diseases. There are several reasons why it often works.
There are also many reasons why herd immunity won’t yet work to stop or slow the spread of SARS-CoV-2 or COVID-19, the disease caused by an infection of the new coronavirus.
When a large percentage of the population becomes immune to a disease, the spread of that disease slows down or stops.
Many viral and bacterial infections spread from person to person. This chain is broken when most people don’t get or transmit the infection.
This helps protect people who aren’t vaccinated, or who have low functioning immune systems and may develop an infection more easily, such as:
- older adults
- young children
- pregnant people
- people with weakened immune systems
- people with certain health conditions
For some diseases, herd immunity can go into effect when 40 percent of the people in a population become immune to the disease, such as through vaccination. But in most cases, 80 to 95 percent of the population must be immune to the disease to stop its spread.
The more transmissible a disease is, the higher percentage of immune people in a population is needed to achieve herd immunity. A disease like seasonal flu requires a smaller percentage of immune people than measles, which is far more transmissible.
For example, 19 out of every 20 people must have the measles vaccination for herd immunity to go into effect and stop the disease.
This means that if a child gets measles, everyone else in this population around them will most likely have been vaccinated, already have formed antibodies, and be immune to the disease to prevent it from spreading further.
The goal of herd immunity is to prevent others from contracting or transmitting the germ that causes an infectious disease like measles.
However, if there are more unvaccinated people around the child with measles, the disease could spread more easily because there’s no herd immunity.
To visualize this, picture someone without immunity as a red dot surrounded by yellow immune dots. If the red dot can’t connect to any other red dots, there’s herd immunity.
The percentage of people that must have immunity to safely slow or stop an infectious disease is called the “herd immunity threshold.”
Natural immunity occurs when you become immune to a specific disease after contracting it. This triggers your immune system to make antibodies against the germ causing the infection inside of you. Antibodies are like special bodyguards that only recognize certain germs.
If you’re exposed to the germ again, the antibodies that dealt with the germ previously can attack it before it spreads and makes you ill. For example, if you had chickenpox as a child, you most likely won’t get it again, even if you’re around someone with it.
Natural immunity can help create herd immunity, but it doesn’t work as well as vaccinations. There are several reasons for this:
- Everyone would have to develop the illness once to become immune.
- Developing an illness can have health risks, sometimes serious.
- You may not know if you’ve had the illness or if you’re immune to it.
Herd immunity does work for some illnesses. People in Norway successfully developed at least partial herd immunity to the H1N1 virus (swine flu) through vaccinations and natural immunity.
Similarly, in Norway, influenza was projected to cause fewer deaths in 2010 and 2011 because more of the population was immune to it.
Herd immunity can help stop the spread of illness, such as swine flu, and other outbreaks within an entire country. But it can change without anyone knowing. Also, it doesn’t always guarantee protection against any disease.
For most healthy people, natural immunity is never a good alternative to getting vaccinated.
Not every illness that has a vaccine can be stopped by herd immunity. For example, you can get tetanus from bacteria in your environment. You don’t get it from someone else, so herd immunity doesn’t work for this infection. Getting the vaccine is the only protection.
You can help build herd immunity to certain diseases in your community by making sure you and your family have up-to-date vaccinations. Herd immunity may not always protect every individual in the community, but it could help prevent widespread disease.
Masks, vaccinations, rapid testing prior to gatherings, physical distancing and frequent handwashing are the most reliable ways to help prevent you and those around you from contracting and potentially transmitting SARS-CoV-2, the virus that causes COVID-19.
There are several reasons why herd immunity isn’t the answer to stopping the spread of the new coronavirus:
- People who contract SARS-CoV-2 and develop COVID-19 can experience serious side effects. Severe cases can lead to death.
- Doctors don’t yet know exactly why some people who contract SARS-CoV-2 develop severe COVID-19, while others do not.
- Vulnerable members of society, such as older adults and people with some chronic health conditions, could get very sick if they’re exposed to this virus.
- Otherwise healthy and younger people may become very ill with COVID-19.
- Hospitals and healthcare systems may be overburdened if many people develop COVID-19 at the same time.
Overall, herd immunity will likely never be achieved with COVID-19. The emergence of variants and the waning immunity after both vaccination and natural infection will mean populations across the globe will always be susceptible to a certain degree.
Outbreaks are likely to continue occurring. But the level of immunity present in a population will determine how bad the outbreak will be.
Almost all healthy adults, teens, and older children would need to be vaccinated to provide herd immunity for people who can’t get the vaccine or who are too ill to become naturally immune to it. This is even more true with the emergence of more infectious new variants like Delta and Omicron.
If you’re vaccinated and build immunity against SARS-CoV-2, you’re far less likely to contract the virus or transmit it. And if you do, the effects will be far less severe.
Herd immunity is community or group protection that happens when a critical number of the population is immune to a certain disease. It can help stop or slow the spread of an infectious disease like measles or swine flu.
All germs have ways to outwit and camouflage themselves from the immune system, and SARS-CoV-2 is no different.
The immunity obtained after one contracts SARS-CoV-2 is unreliable. The only type of immunity for this virus that you can count on being present is from a vaccine.
However both natural immunity (if present after infection) and vaccine-induced immunity have been shown to wane.
We’re still learning how to combat this contagious and rapidly mutating virus. What’s clear is that herd immunity is not an effective solution to COVID-19.