When we hear the word “herd,” we tend to think of a group of cattle huddled close together. This imagery is useful for understanding how diseases affect humans.

Herd immunity (also called herd effect, community immunity, or mass immunity) is a form of indirect protection that applies only to contagious diseases. Once a significant percentage of the population—around 85 percent—becomes immune to an infection, the pathogen causing that infection will not be able to maintain itself. That means the disease we have been vaccinated against will not affect us and will have a very hard time finding an unvaccinated person to infect. 

Think of a crowd of people together at a function. All are immunized, except one standing in the middle of the crowd. That one person might be protected because everybody else in the crowd has been immunized for that disease. However, when the herd immunity for that disease falls below 85 percent, that unvaccinated individual becomes more susceptible.

I’ll give you an example. Influenza (or “the flu”) is on the rise in New York State. Doctors expect the increase to continue. There were 16,000 lab-confirmed cases of influenza in New York the week ending December 21, up from 8,500 confirmed cases the week before. In some areas of New York, less than 30 percent of the population had received the flu vaccine. According to the CDC, there have been at least 24 million illnesses, 310,000 hospitalizations and 13,000 deaths from the flu so far this season.

Recently, 10 pediatric deaths associated with seasonal influenza were reported, bringing the 2024-2025 season total to 57 pediatric deaths, the CDC said. Recent CDC data also shows that influenza levels in the U.S. have risen to the highest levels they have been in 15 years.

Those who are not immunized against influenza and who have underlying health conditions have the greatest risk of getting really sick, or even dying. The influenza vaccine is at every pharmacy, and for most, it’s free. What are you waiting for? Once you receive the vaccine, it will take about three weeks for the body to process it and protect you.  It will not give you a 100-percent guarantee that you wouldn’t get sick, but you will not be as sick as you would have been without it.

Some say they had a bad reaction to the influenza vaccine and are afraid to take it again. That reaction tells you that your body had no previous protection to these influenza strains, and that you needed it. Influenza strains change annually.

Let’s get back to herd immunity. Recall the early days of COVID-19. Remember how afraid you were to go outside, to grocery shop. We wore masks everywhere and bought hand sanitizers in bulk. Look at your life today and think about just how different it looks from 2020. This is because, thanks to vaccines, we’re getting near the herd immunity threshold for COVID-19.

The influenza numbers represent those who were not immunized against influenza. Those contracting the COVID-19 virus today were not vaccinated against COVID-19 and its variants.

The recent measles outbreak in rural West Texas is a classic example. None of the patients were vaccinated. Did you know that measles is more than just a rash and cold symptoms? It could cause subacute sclerosing panencephalitis, a progressive, disabling and deadly brain disorder.

Vaccination rates have declined nationwide. More people are now claiming religious or personal waivers. It’s only a vaccine. It could save your life.

Herd immunity only works as well as the mentality of the herd. It only works when large numbers of the population are on board with the plan. According to the Association for Professions in Infection Control and Epidemiology (APIC), this is the one main drawback because it depends on the condition that people in the same community  share the same views on vaccination.

Travel between states and countries with different vaccination policies and varied vaccination rates can compromise herd immunity and allow the disease to spread.

No person should suffer, or die, from a disease when there is a vaccine to protect them from it.

Michael Kossove is professor emeritus and adjunct professor of microbiology and infection control at Touro University, School of Health Sciences.