Herd Immunity

What Is Herd Immunity?

Herd immunity means that enough people in a group or area have immunity to a germ (for instance, a virus) that it no longer spreads easily. Immunity means your immune system has learned to fight off a specific infection so it doesn’t make you sick.

Herd immunity — also called population immunity — often results from a combination of people getting vaccinated and people getting infected with the germ (natural immunity). Both natural and vaccine immunity cause your body to make antibodies and immune cells that recognize a specific germ. Your body can then fight the germ off more easily the next time it sees it. It usually takes a large percentage of people with immunity to achieve herd immunity.

The benefits of herd immunity include the potential to:

• Stop a disease from spreading in a particular area (elimination)
• Stop a disease from spreading worldwide (eradication)
• Protect people from serious illness and death
• Protect people who can’t get vaccinated or don’t develop immunity when they get vaccinated or infected

Herd immunity isn’t permanent. Unless a disease no longer exists anywhere in the world, it can start spreading again. This usually happens if someone without natural or vaccine immunity gets infected while traveling to an area where the disease still spreads. They can then spread it to others who don’t have immunity.

What are some examples of herd immunity in history?

Herd immunity has been achieved in the past for:

• Diphtheria
• Measles
• Polio
• Rubella
• Smallpox

Most of these have been eliminated in many countries worldwide thanks to vaccination and other public health efforts. But until a disease has been eradicated, it can begin spreading again to people who haven’t been infected or vaccinated.

Some of these diseases have started spreading again after achieving herd immunity. Smallpox is the only disease that’s been eradicated worldwide.

Challenges in reaching herd immunity

Herd immunity isn’t possible with every infectious disease. Some reasons herd immunity might not be possible for certain diseases include:

• The germ changes (mutates) frequently: This means you can get immunity to one version, but that may not protect you from a newer variant. There can also be multiple different versions of a germ that circulate at one time (like norovirus).
• Immunity to the germ doesn’t last long enough: Immunity has to last a long time to prevent spread. For example, immunity from both infection and vaccination for some viruses lasts only a few months. This usually isn’t long enough to achieve herd immunity.
• The germ can spread when you have no symptoms: Germs spread more easily when people don’t know they’re infected. This makes it much less likely to get herd immunity without vaccination.
• The germ is found in nature: Herd immunity only works for diseases that spread from person to person. Some germs naturally live in soil or water (think tetanus bacteria or infectious fungi). Vaccines can protect you personally from some of these infections, but population immunity can’t be achieved.

Flu, RSV and COVID-19 are all viruses that we might not be able to get herd immunity to because they mutate too often, they spread without symptoms or immunity doesn’t last very long (or a combination of these). Tetanus shots, on the other hand, last a long time. But we can’t reach herd immunity because the bacteria live in the environment, not exclusively in people.

Can herd immunity be achieved without vaccines?

Without vaccination, herd immunity might only be achieved if a very large number of people get sick — and potentially die — very quickly. Otherwise, not enough people would be immune all at once. The disease would just keep spreading as new people got infected. It’s unethical to allow new and potentially deadly diseases to spread this way on purpose.