As more and more people receive COVID-19 vaccinations, the term "herd immunity" is being tossed around once again as desirable in relation to the devastating virus ravaging countries all over the globe since December 2019.
The term means a situation in which a high percentage of people in a given community (a city, state or entire country) are immune to a disease, either through vaccination or because those people have been exposed to the virus and their immune systems have built antibodies to protect them from it.
People who tout it as the best defense against SARS-CoV-2, the virus that causes COVID-19, are correct -- herd immunity slows down or eliminates the spread of the virus from person to person, preventing contagious diseases from circulating.
The problem with herd immunity and the novel coronavirus is that, even as vaccine rollouts continue, the world is still a long way from having widespread resistance to COVID-19. Experts estimate that from 60% to upward of 90% of a population must have immunity to a disease for it to stop spreading.
For reference, the US population was about 330 million in 2019 -- 90% of that is 297 million. Thankfully, three vaccines have received emergency use authorization from the FDA to fight COVID-19, so herd immunity may be a reality sooner than experts initially thought.
Let's explore what herd immunity looks like, what it means for COVID-19 and how the world can get there, explained by Dr. Jane Orient, executive director of the American Association of Physicians and Surgeons; Dr. Joseph Vinetz, a Yale Medicine infectious disease specialist; and the Centers for Disease Control and Prevention.
What is herd immunity?
Orient tells CNET that herd immunity is what slows and eventually stops outbreaks and epidemics -- this occurs only "when the pathogenic organism runs out of susceptible victims because of isolation, quarantine or immunity of exposed persons."
Orient -- who says she really prefers the term "population immunity" because "we are not livestock" -- emphatically points out that "If it weren't for population immunity … we'd all die like the Martians in The War of the Worlds." (In the movie she's referring to, the Martians who travel to Earth all die from an Earth-borne bacteria that the Martians' immune systems couldn't handle.)
Vinetz explains that a good example of herd immunity is the measles.
"The idea is that if we have herd immunity, it means that we reduce the transmission of a disease within a population," Vinetz says. "The best example to think about is measles. If 95% of everyone are vaccinated against measles, that means 95% are immune to measles, which means that the risk of measles spreading from person to person is very low."
The idea, Vinetz concludes, is to remove the possibility of sustained transmission.
While "active immunity," or immunity a person builds in response to actually contracting a disease, is most effective and long-lasting, experts beg people not to intentionally infect themselves with SARS-CoV-2 in an attempt to build immunity. It simply isn't worth the risks.
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How vaccines promote herd immunity
The purpose of vaccines, Orient says, is "to expose people to something that induces immunity without making them very sick," explaining that vaccines are either tiny doses of a pathogen, a dead microorganism or one that is similar but weaker.
"You need upward of 70% of protective immunity in a population to prevent large-scale propagation," Vinetz explains, and that often requires immunization, particularly in regard to diseases that have been almost eradicated in the US.
Vinetz again points to measles as the best example. It's easily preventable with the highly effective vaccination, most commonly known as the MMR (measles, mumps and rubella) shot. But, in 2019, the US saw the highest number of measles cases since 1992, and the majority of cases were among people who did not get vaccinated against measles.
The 1,282 measles cases in 2019 still pale in comparison to the estimated 3 million to 4 million yearly cases before the vaccine was introduced, a demonstration that vaccines can eliminate or greatly reduce diseases within a region.
"Disease rates are low in the United States today," the CDC explains on its website. "But if we let ourselves become vulnerable by not vaccinating, a case that could touch off an outbreak of some disease that is currently under control is just a plane ride away."
Only one disease -- variola virus, better known as smallpox -- has been completely eradicated. No cases of naturally occurring smallpox have happened since the declaration of eradication in 1980, and babies no longer need the vaccine for this virus, because the global vaccination program was so successful.
Vaccination development and approval requires careful analysis of the risk-to-benefit ratio, Orient says. "For extremely contagious lethal diseases like smallpox, the risk-to-benefit ratio for vaccines is very favorable," she says. "For milder diseases, not necessarily so."
Vaccines can have adverse effects for some people, too, and immunity wanes over time, so the importance of vaccination depends on a number of factors, including severity and prevalence of the disease, vaccine safety and individual patient factors, Orient explains. "Everything is a trade-off," she reminds us.
Orient urges people not to forget that vaccines, though crucial for some diseases, are not the only way to prevent disease -- other things are also important: "Diseases are also prevented by hygiene, sanitation, vector control (e.g. mosquito control) or cure of the sick before disease can be transmitted."
Why we need herd immunity
If there is no herd immunity, people will continue to contract and spread diseases, and, unfortunately, people will continue to die from these diseases. Again, the CDC defines herd immunity, or community immunity, as "a situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely."
"Even individuals not vaccinated (such as newborns and those with chronic illnesses)," the CDC continues in its definition of herd immunity, "are offered some protection because the disease has little opportunity to spread within the community."
SARS-CoV-2, the virus that causes COVID-19, is new to humans, so no one had immunity, naturally or by vaccination. Because the virus is contagious, it spreads quickly and easily.
What will it take to reach coronavirus herd immunity?
There are two ways to produce herd immunity against a disease: Develop and administer a safe and effective vaccination, or wait for the disease to make it around through a population.
Now that three vaccines have been approved in the US for emergency prevention of COVID-19, we're getting closer to a reality that includes herd immunity. However, it will still take months or years for enough people to get vaccinated to achieve true herd immunity. While vaccine rollouts go on, social distancing, mask-wearing, ventilation of indoor spaces and good personal hygiene are still COVID-19 prevention best practices.
If you take Vinetz's estimation that at least 70% of a population must be immune to a disease to produce herd immunity, that means at least 231 million people in the US must be vaccinated or contract and recover from the disease.
As of May 3, around 31.6 million people have had COVID-19 and survived it (thus gaining some degree of immunity to it) 2021, which is about 9.6% of the population. (You can track the cases in the US on the CDC website, which is updated continuously). A total of 32% adults in the US are fully vaccinated, according to data from the CDC, while a little more than 44% of US adults have had at least one dose of a COVID-19 vaccine. Looking only at those who've have been fully vaccinated and those who gained immunity from having COVID-19, we are still only at 41.6% herd immunity. We still have a ways to go to reach that 70% mark, and that might not even be the magic number.
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When will we have coronavirus herd immunity?
Right now, it's a numbers game. Again, about 41% of the population has immunity to COVID-19 and we are aiming for a higher percentage than that. The exact percentage we need for COVID-19 is still not known, but we know that it's higher than it is presently.
Some experts are beginning to question whether we'll ever see herd immunity. In addition to a mobile community, mutating virus strains and a stagnant vaccine hesitancy rate among some citizens contribute to doubt in some scientists that we'll ever reach that magic number -- whether it's 60% or 90%. Instead, some say we must learn to live with COVID-19 as a virus that reappears and, hopefully, does minimal damage as we continue to vaccinate and protect the most vulnerable populations.
The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.