Home Coronavirus How 'Herd Immunity' Affects Contagious Diseases like COVID-19
How 'Herd Immunity' Affects Contagious Diseases like COVID-19

How 'Herd Immunity' Affects Contagious Diseases like COVID-19

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By Deborah Barkin Fromer
Faculty Member, School of Health Sciences, American Military University

The term “herd immunity,” originally coined almost a century ago, has been in common use since the pandemic spread of COVID-19, so it’s important to understand and know more about what the term implies.

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What Is Herd Immunity?

The Association for Practitioners in Infection Control and Epidemiology prefers the term “community immunity.”

According to its description: “Herd immunity (or community immunity) occurs when a high percentage of the community is immune to a disease (through vaccination and/or prior illness), making the spread of this disease from person to person unlikely. Even individuals not vaccinated (such as newborns and the immunocompromised) are offered some protection because the disease has little opportunity to spread within the community.”

Childhood Measles, Mumps, Rubella, Chickenpox and Polio Are Examples of Herd Immunity

Childhood infections like measles, mumps, rubella, chickenpox and polio are classic examples of herd immunity. In the 1930s, A.W. Hedrich noticed that after a significant number of children had measles and subsequently became immune, the number of new infections among susceptible children temporarily decreased. Efforts to control and eliminate measles altogether were unsuccessful until mass vaccinations with the measles vaccine in the 1960s.

The last large outbreak of measles in California was associated with Disneyland and occurred from December 2014 to April 2015. According to the California Department of Public Health, “At least 131 California residents were infected with measles; the outbreak also infected residents of six other states, Mexico, and Canada.”

“This outbreak illustrates the continued importance of ensuring high measles vaccination coverage in the United States,” the Centers for Disease Control and Prevention said.

Considering how many people visit Disneyland on a daily basis, 131 infections over a four-month period was extremely low. The 131 case number came from those infected at Disneyland then infecting others who were not at Disneyland.

It’s Difficult for an Infectious Disease to Be Transmitted When Vaccination Is Widespread

When a high percentage of a population is vaccinated, it is difficult for an infectious disease transmitted person-to-person to spread because there are not as many people who can be infected. It should not be a surprise that when vaccination rates of measles fall, this disease resurges.

Until there is a vaccine for COVID-19, the only opportunity for immunity is from infection, letting your own immune system do all the work after it’s infected with the virus. The SARS-CoV-2 coronavirus that causes COVID-19 is so contagious that experts estimate about 70% of people in a community would need to be immune to establish herd immunity. In other words, more than 200 million Americans would have to get infected before we would reach this threshold.

The statistic used to calculate this 70% threshold is called the “basic reproduction number” of a pathogen, known as R0. It represents the number of people one contagious person can infect in a community of unprotected individuals. The more contagious a disease, the higher R0 will be, and the more people that need to be immune for herd immunity.

As we socially isolate, quarantine or distance, we lower the R0. Cultural behaviors, population age and underlying comorbidity rates also influence transmission forces within a community, so the R0 across populations may differ with the situation.

Studies indicate that as of early April 2020, no more than two to four percent of any country’s population has been infected with SARS-CoV-2. This suggests that we are not close to reaching herd immunity.

Since SARS-CoV-2 is a novel virus, many features are not yet well distinguished. The ability to establish herd immunity against COVID-19 centers on the assumption that infection generates sufficient, protective immunity.

At present, the extent to which humans are able to generate immunity to SARS-CoV-2 is unclear. But a study in the journal Science suggests we may be social distancing for a while.

About the Author

Deborah Barkin Fromer received a B.S. in biology at the Sage Colleges in Albany, New York, and was certified with the American Society of Clinical Pathologists as a medical technologist in 1976. She worked in the clinical laboratory as a medical technologist specializing in microbiology. In the 1990s, Ms. Fromer became interested in public health, returned to graduate school and completed a Master of Public Health (MPH) degree at Wichita State University in Wichita, Kansas.

Ms. Fromer spent several years at the University of Kansas School of Medicine as a researcher in obstetrics and gynecology, and several years as a researcher and teaching associate in preventive medicine and public health. She has taught online epidemiology and public health courses since 2001.

From 2007-2015, Ms. Fromer was an epidemiologist at the Sedgwick County Health Department in Wichita, Kansas. Her work involved electronic surveillance of reportable disease and medical conditions, investigating outbreaks and illness, solving mysteries, and keeping people in the community educated and healthy.

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