Herd Immunity: Protecting One Another from the Flu

Aaron S. Weinberg MD, MPhil
September 21, 2021
4 min read

You might have heard the terms “herd immunity” or “community immunity” recently when it comes to getting the flu vaccine. But what exactly do these terms mean, and how does herd immunity protect others around you?

Herd immunity is a population’s resistance to being infected with a disease, due to a large portion of the population already being immune. It significantly decreases the chance of an infected person coming in contact with a susceptible person.

+ So how does someone become immune?

A person can become immune by having already been infected with the disease and recovering, or by getting vaccinated. While vaccines provide direct protection to people who receive them through the development of antibodies, they also indirectly protect the community by reducing contact between those who are infected with a disease and those who are susceptible.

+ Herd immunity is an altruistic concept.

When herd immunity is achieved, folks who aren’t immune or who didn’t get the vaccine for certain established medical reasons are indirectly protected by folks who are immune or who did get the vaccine. “Do it for the herd” is a common, and accurate, slogan around flu season, because getting a flu shot doesn’t just help keep you healthy, but also helps everyone around you. You will be doing your part to potentially protect your partner, sibling, parent, friend, neighbor, community, and country.

For example when you get the flu shot, you’re helping protect:

     • A baby who may be too young (under six months) to receive a flu shot and who would be vulnerable due to a not yet fully developed immune system. They could potentially die if they got the flu, and you would be protecting them with herd immunity.

     • A loved one who is immunosuppressed — for instance, due to cancer treatment or other causes — and may not be able to mount as robust of an immune response to a vaccine.

     • A friend who can’t get a flu shot due to a very rare severe reaction.

When you get vaccinated you help protect them all.

+ Herd immunity can be tricky to achieve.

The ability to achieve herd immunity depends on several factors. These include the way a virus spreads, the season we are in, the population’s age, its socioeconomic status, its typical social interactions, its living situations, and most importantly, the number of people in the population who are susceptible to the disease. Many of those characteristics change over time, including how and for how long people are in contact with one another, which makes herd immunity more of a continuum.

It’s very difficult to achieve herd immunity when there is a high number of susceptible people in the population. It’s like a game of probability, which is why vaccines are so important. The goal of immunization programs is to reduce the number of susceptible people in the population and therefore decrease the probability of spread. Of course, this depends on the effectiveness of the vaccine. 

We’ve all heard about flu seasons in which the flu shot was “not a good match” for the flu. Sometimes mismatches happen, because certain viruses, like influenza, can rapidly mutate. A vaccine that is well matched to the circulating virus is more likely to achieve herd immunity, as opposed to one that is not well matched. Still, experts recommend getting vaccinated, even if there is a mismatch — because some protection will still occur from that vaccine, which can still reduce the chances of severe disease if you become infected. Depending on the flu season and what model is used, it is estimated that 33 to 73 percent of the population needs to be vaccinated to reach herd immunity and stop the spread. Usually, less than 50 percent of the adult population gets the flu shot each year, making herd immunity a challenge.

+ Protect your community by getting the flu shot.

If you’ve gotten the flu shot this year, you have already contributed to herd immunity. You made it safer for everyone in your community, but especially for folks who are high risk and for folks who can’t get the flu shot for health reasons. That’s something to feel good about. After all, herd immunity boils down to caring about one another.

The seasonal flu is detected year-round, but it peaks in the fall and winter. In the U.S., flu cases start to rise in October, so try not to wait, especially this year. Get your flu shot at Carbon Health!

(Learn more about the flu vaccine in “5 Reasons to Get the Flu Shot.”)‍

Carbon Health’s medical content is reviewed and approved by healthcare professionals before it is published, but it is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Talk to your healthcare provider about questions you may have regarding your health or a medical condition, and before making changes to your healthcare routine.


Aaron S. Weinberg MD, MPhil

Aaron S. Weinberg, MD, MPhil, is Director of Program Development at Carbon Health and triple board-certified in Pulmonary, Critical Care, and Internal Medicine.