What Is Herd Immunity?


Herd immunity isn’t a new concept, but it’s been getting more attention because of the coronavirus pandemic. It occurs when a large portion of a population – the herd – has immunity (protection against reinfection) to a contagious disease.

“Essentially, it’s a dead end for the contagion since it can’t easily spread from person to person any longer,” says Annabelle de St. Maurice, MD, MPH, co-chief of infection prevention officers at UCLA Health. “There are two ways to gain herd immunity: widespread infection or vaccination.”

How does herd immunity work?

When you have an infection, your immune system mounts a response to eliminate the germs. It often creates long-lasting antibodies – proteins that fight off specific germs – to ward off future attacks.

Vaccination also triggers your immune system to create these protective antibodies. Once a significant portion of a population has immunity, the few that are still vulnerable are protected.

"For example," says Dr. de St. Maurice, "say you have someone who can’t get a vaccine because of a weakened immune system. If everyone else has immunity, that one person is protected. They’re surrounded by people who can’t carry or pass on the disease."

How long does herd immunity last?

With some infections, like measles, if you get the infection once you’re protected for life. With others, there’s a waning immune response. That means that over time, your immune response weakens and eventually you’re vulnerable to infection again. That’s why you need booster shots for some diseases (like whooping cough) years or decades later. It re-ups your protection.

The same is true for the herd. If enough people have lost immunity, there is potential for another outbreak. The virus can be reintroduced – say from a traveler – and people in the formerly immune group could get sick again. How long herd immunity lasts depends on the disease.

How far is the U.S. from herd immunity for COVID-19?

When it comes to the novel coronavirus, there are a lot of unknowns. First, not everyone who’s infected creates the antibodies. Also, scientists aren’t sure if having the antibodies provides protection and if it does, how long it lasts, says Dr. de St. Maurice.

How much of the population needs to be immune in order to have herd immunity depends on the contagiousness of a disease. Given that researchers are still learning about COVID-19, it’s impossible to say at this point how many people would need COVID-19 immunity for the U.S. population to be protected.

Why not just get exposure “over with” to gain immunity?

Everyone is yearning to get back to what life was like before COVID-19. But it’s not a good idea to intentionally expose yourself to coronavirus to gain immunity.

Even though most people who contract COVID-19 have very mild symptoms, for some people it can prove deadly. Older people and those with pre-existing medical conditions such as diabetes, heart disease or chronic lung disease are more vulnerable.

“By exposing yourself to the illness, you risk getting very sick and putting other lives in danger. And you would be doing that without even knowing if the antibodies you develop will actually protect you,” says Dr. de St. Maurice.

If experts find that antibodies do offer protection against COVID-19, vaccination is the safest path to building herd immunity.

Find out if antibody testing is right for you, and get the latest coronavirus information from UCLA Health.