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What Is Herd Immunity?

18 May 2021

Whether on social media, in your local newspaper or at the Sunday dinner table, talk of herd immunity has intensified as COVID-19 vaccine production and administration has ramped up. But, what does herd immunity look like, how do we get there and how long will it take to achieve? We sought out answers to these questions for you to separate fact from fiction.

How does herd immunity work?

Herd immunity occurs when a large number of people in a community, city, state or country become immune to a disease. In more simplistic terms, you can break the chain of transmission once enough people become resistant and can’t spread the disease throughout a community. Without a host, the virus dies off.

A common myth going around is that you can only reach herd immunity if enough people become infected. This is a dangerously false assumption. 

We can as a society achieve herd immunity two ways: through prior infection or vaccines. So, when you see or hear herd immunity referenced with COVID-19, all that means is when enough people are resistant to COVID-19 to stop it from spreading.

Although herd immunity can come from prior exposure, the World Health Organization (WHO) notes reaching that threshold by exposing people to a virus is “scientifically problematic and unethical,” adding that allowing COVID-19 to spread through populations “will lead to unnecessary infections, suffering and death.” Put simply, vaccinations are pivotal to slowing the spread of COVID-19 and reaching herd immunity.

Vaccines mimic the immune system response your body creates when actively fighting a disease, creating antibodies to recognize and fight off viral and bacterial infections that cause illnesses. Once vaccinated, your body is immediately able to destroy pathogens if you become infected again.

Herd immunity carries a colloquial meaning of sorts, as no preventive measures or vaccinations will provide you with 100 percent immunity from COVID-19. If we all had 100 percent immunity, the disease wouldn’t exist anymore. Even if you’re vaccinated, you’re still at risk — albeit a minuscule risk — of contracting COVID-19, something the CDC calls breakthrough infections. To date, these breakthrough infections have amounted to less than 1 percent of people who have been fully vaccinated, and nearly a third of the patients were symptom-free. That’s why wearing masks and respecting social distancing protocols are still important.

How many people need to be vaccinated for herd immunity to work?

This is a popular question few have answers for. Unfortunately, there isn’t a magic number because herd immunity rates vary by disease. Measles, mumps, polio and chickenpox are examples of common infectious diseases that were mostly eradicated in the U.S. thanks to vaccines and herd immunity.

Prior herd immunity thresholds

Measles requires almost 95 percent of a population to be vaccinated, while the polio threshold is about 80 percent, according to WHO. Why is measles higher than polio? Measles is highly contagious and spreads through the air, thus more people need to be vaccinated to reach the immunity threshold.

Here are some more herd immunity thresholds of common global diseases during the last century, according to the Journal of the American Medical Association:

  • Smallpox: 84 percent
  • H1N1 influenza (1918): 74 percent
  • SARS epidemic: 72 percent
  • Zika: 51 percent
  • H2N2 pandemic (1957): 40 percent
  • Ebola: 33 percent
  • H1N1 influenza (2009): 25 percent

COVID-19 herd immunity threshold

As the pandemic has evolved, so have possible herd immunity figures. Midway through 2020, the WHO projected that number at 60 percent to 70 percent between vaccinations and prior infection.

Before the pandemic even started, at least 70 percent of Americans would have needed immunity to halt COVID-19 from spreading, according to Johns Hopkins University. As of April 2021, some experts think that number could be between 70 percent and 90 percent.

It’s important to remember these figures are predictions for a reason. They are simply forecasts of what may happen.

Herd immunity varies and evolves over time. That’s why many researchers and epidemiologists use R0, or a virus’ reproduction number, to measure how contagious or transmissible it is. The higher the number, the harder it is to control the disease. The R0 of measles is 12 to 18, meaning for each person infected, 12 to 18 more people will become infected. This high rate of transmission is what caused the herd immunity threshold to be so high at 95 percent. In 2020, the WHO estimated COVID-19’s R0 to be between two and four.

Where do we stand now?

The U.S. still has a ways to go to reach herd immunity. By the end of April, more than 96 million Americans had been fully vaccinated, or 29 percent of the population. More than 141 million Americans (43 percent of the population) had at least received their first dose.

It’s also unclear how many more Americans are immune due to prior exposure. How long COVID-19 antibodies last in your body has been a hot topic for months. As recently as January 2021, a study of 200 people revealed 98 percent had antibodies a month after developing symptoms. The antibodies remained steady for months before slightly declining six to eight months after infection.

While more than 43 percent of the population receiving at least one vaccination, the trends also indicate a subset of Americans either have no plans to get the vaccine altogether or won’t receive their second shot for various reasons, according to the New York Times. More than five million people (8 percent) of people who received their first dose of Pfizer or Moderna skipped their second dose.

Among the reported barriers, some people chose not to receive the second shot because they feared side effects. Other people felt one shot provided them with enough protection. In some cases, people attempted to receive their second dose, but their vaccine provider either didn’t have any shots available or didn’t have the correct brand.

However, the reported figures don’t account for the possibility of someone getting vaccinated at two different locations and the records not syncing up. For example, tribal nations in Oklahoma opened COVID-19 vaccinations to the entire state in March. A person could have received one shot from the Oklahoma State Department of Health and another from a tribal nation out of convenience. If two agencies don’t sync the records, the health department could classify that second dose as skipped.

While the 8 percent rate for skipping the second dose seems high, it still means 92 percent followed through with both shots. That number is higher than the 70 percent and 80 percent of people who completed the two-dose shingles vaccine in recent years.

The reality is, many Americans may simply opt to not receive the vaccination for personal reasons. Take, for instance, annual influenza vaccines. Each year, between 45 and 50 percent of Americans 18 and older receive the flu vaccine. Additionally, about 1 percent of parents don’t vaccinate their children from diseases, such as measles, polio, or typhoid, after birth.

When will we reach herd immunity?

The farther we get into 2021, the more the focus has shifted away from herd immunity. Instead, the goal is to first control the disease.

Once vaccine rollout is complete and those who want to be vaccinated receive shots, then herd immunity forecasts will be more accurate. Two professors at Johns Hopkins hope for 50 percent vaccination rates by the end of the summer with the goal of herd immunity by winter 2021 when the majority of the population is vaccinated.

In between now and then, COVID-19 cases and deaths may move on a rollercoaster-like trajectory, decreasing for periods then increasing again when restrictions become more relaxed and more unvaccinated people begin to gather in large groups.

While vaccine hesitancy is a large barrier to herd immunity, this is just part of the uphill climb. The longer the virus remains in our communities, the greater the chance it has to mutate into other variants that could make it tougher to defend against.

The same can be said for yearly influenza outbreaks. While you can receive an annual flu vaccine, the virus mutates with such frequency that antibodies last months instead of years. Coronaviruses aren’t quite like the flu, but any chance of mutation will decrease a vaccine’s effectiveness and make it that much more difficult to return to normal.

How long vaccines provide protection is another variable to when we’ll reach herd immunity. The Pfizer vaccine is 90 percent effective at providing at least six months of immunity against COVID-19 and other variants, the company announced in March. The words “at least” are key here — immunity could be longer, but drug manufacturers only have six months of data to go off. Then, in April, Pfizer said vaccine recipients would likely need an additional third booster shot within 12 months of their original dose. A booster shot trains your immune system to recognize viral or bacterial infections.


To get your COVID-19 vaccine, schedule an appointment through the Oklahoma State Department of Health. Vaccines are currently available to anyone over the age of 16. 


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