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How Does Herd Immunity Keep a Country Safe From Diseases?

teenagers in the Netherlands
Teenagers are seen wearing gloves after Dutch prime minister Mark Rutte announced a herd immunity plan on March 17, 2020 in The Hague, Netherlands. The herd immunity theory consists of having a large part of the population infected by the COVID-19 virus in order to build immunity. Critics have questioned this plan. Pierre Crom/Getty Images

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In the 2000 book "Bowling Alone," political scientist Robert D. Putnam argued that social capital in America was declining. As one way to support that point, Putnam pointed to statistics involving membership in community organizations. People simply saw each other less, according to Putnam. There was no chance to meet the neighbors down the street, socialize with other members of the community or get to know anyone outside of your own house. And that was if you even saw the people in your own house — Putnam believed that technology such as television and the internet had completely negated the need to speak to anybody.

Putnam's point was summed up in the very title of the book: People in the United States were bowling more, but they were heading to the local alley by themselves. The old days of joining a league and fraternizing with the same group of people every week were over. Now, people were shut off from all social connections and bowling alone.

But beyond the societal problems that Putnam believes can arise from declining social capital, an "every man for himself" approach can have tremendous effects on public health. Keeping populations of a community free from viral disease rests in part on the success of herd immunity. Herd immunity rests on the principle of safety in numbers; if more people are immune to a certain virus, either through vaccination or through already having the disease, then more people in the population, even if they themselves aren't immune, are protected from the disease.

To illustrate the point, let's return to that bowling alley where people are bowling by themselves. Let's say the guy on the first lane contracts influenza, and he passes it along to the woman on the second lane. If that woman isn't immune to influenza, then the disease will likely continue its path lane by lane until every person in the bowling alley is suffering. But if that woman is immune, then the disease stops with her, because the virus has nowhere else to go (assuming that the guy in our example didn't have contact with anyone else). By her immunity, she protected all the people on subsequent lanes, even if they didn't get a flu shot that year.

Sounds simple, right? But it may not be that simple.

Vaccination and Herd Immunity

For many of us, chickenpox (known in clinical circles as varicella) was a routine part of childhood – a minor annoyance, but rarely life-threatening. For that reason, a chickenpox vaccine was met with some skepticism when it was introduced in 1995. Still, there was a push in the United States to get children vaccinated, and studies have shown that the effort had a tremendous impact on the disease and the costs associated with it. Chickenpox hospitalizations dropped by a massive 93 percent as of 2012 compared with the pre-vaccine period. During the period of 2006 to 2012 when a second dose became recommended, the hospitalization rate declined by another 38 percent, alleviating costs all around [source: CDC].

Not only does a push for a chickenpox vaccine protect a child and his or her classmates, it can also protect grandparents who may not have had chickenpox. Protecting the elderly is also the idea behind another vaccination that many of us line up for each winter: the flu shot. Between 12,000 and 61,000 Americans have died each year of the flu each year since 2010 [source: CDC]. Of those, 90 percent of flu-related deaths and 50 to 70 percent of hospitalizations are people over the age of 65 [source: CDC]. While the elderly are encouraged to get a flu shot, it's actually more effective if the herd around them is vaccinated, including caretakers and visitors that might include germy grandchildren.

The success of the polio vaccine demonstrated the benefits of using immunization to protect a population, and herd immunity can be achieved for a whole host of diseases beyond chickenpox and the flu, including measles, mumps and smallpox. And while the term herd immunity seems to imply that the whole herd should be vaccinated, the whole herd is merely protected if a certain percentage is immunized. Mathematical models can be used to determine exactly what percentage of the population needs to be vaccinated to prevent a communicable disease. For example, if approximately 80 to 85 percent of a population is vaccinated against polio, then herd immunity is achieved. But measles is more contagious than polio, so 90 to 95 percent of a population needs to be vaccinated for herd immunity to be achieved [source: Oxford Vaccine Group].

However, just because herd immunity is achieved doesn't mean it's completely foolproof. Vaccines themselves aren't 100 percent effective, and diseases can still strike those not immune to them. In some instances, an immunization is only effective for a few years, so the protection may have worn off a majority of the herd at the time of an outbreak. Childhood vaccinations may only delay the age at which outbreaks occur. For example, if a pregnant woman contracts rubella, or German measles, she and the unborn baby face far more complications than a child who endures the disease.

So, if vaccines don't work or just put off a mass outbreak in the herd, do we even need them at all? Some people don't think a shot in the arm is worth the good of the herd.

MMR vaccinations, Samoa
Nurses from Hawaii help out with MMR vaccinations on Dec. 6, 2019, in Apia, Samoa, during a measles outbreak. Chikara Yoshida/Getty Images

If the word "vaccine" strikes fear in your heart, then you likely came of age not in a time dominated by worries about polio, but in a time when vaccines make news because of side effects ranging from mild irritation to death. The most notable example is probably the debate regarding the claim (not supported by any scientific studies thus far) that the thimerosal in certain childhood vaccines may have led to an increase in autism. These days, in spite of the lack of scientific evidence to support a link between vaccines and autism – and the fact that thimerosal is no longer used in most childhood vaccines – some people have serious doubts about whether it's worth protecting herd immunity at the cost of exposing children to these supposed risks.

That means that levels of vaccinations have dropped, particularly in certain parts of the United States where exemption from vaccinations is granted for religious, philosophical or personal reasons. But socioeconomic factors appear to play a bigger role than religious or personal reasons for lack of vaccination. A 2017 CDC summary noted that the percentage of children without any vaccines has risen to 1.3 percent (for children born in 2015), compared with a 2001 survey that found 0.3 percent of children aged 19 to 35 months hadn't been vaccinated at all. The summary found this was significantly more common among children who were uninsured, Medicaid-insured or who live in rural areas – 17.2 percent of unvaccinated children were uninsured, for instance, compared with 2.8 percent of all children [source: Hill].

A study by Health Testing Centers using CDC data found that 27 states had kindergarten populations with vaccination rates below state targets in 2018.

Parents who forgo vaccinations may believe they can rely on herd immunity to protect their children: Children who aren't immunized may be protected by the children that did get vaccinations. However, some parents who did vaccinate their children consider this freeloading. In addition, as parental fears cause vaccination rates to drop, the safety of the herd, particularly its weakest members, can become compromised. Some children are not able to be vaccinated, because of health reasons and truly need to rely on the herd for their immunity.

Public health officials worry that herd immunity may have been too successful for its own good. Because all children were immunized in the past, many parents today haven't seen the effects of a widespread polio epidemic or a measles outbreak, diseases that still prevail in countries where vaccines are unaffordable. But what those parents may have seen, however, are reports that detail the side effects of a vaccination, though reports of extreme reactions to vaccines are usually very rare. For example, the polio vaccine is thought to be responsible for eight deaths a year. That's frightening, but health officials say that the fear can't compare to the peak of the polio epidemic, when fear of the iron lung was endemic.

While the unvaccinated children represent a threat to herd immunity just by walking around, the refusal of some people to get vaccinated obviously affects the system of supply and demand. Certain vaccines may not be manufactured if there's a decline in demand for them, meaning that those that truly do benefit from and need their protection might not have access to them anymore. In other words, those of us who mean to get flu shots but never do may inadvertently cause the level of available vaccines to plummet, making it harder to strengthen the herd.

swedish family covid 19
A family eats dinner during the COVID-19 outbreak on April 18, 2020, in Ostersund, Sweden. Sweden has avoided implementing a full lockdown, with many shops, restaurants and schools remaining open. David Lidstrom/Getty Images

Just because science has come a long way in addressing major illnesses doesn't mean that large-scale breakouts can't happen anymore. The 2020 coronavirus disease (COVID-19) pandemic has left health care experts and world leaders grappling with what the best course of action is to combat the virus for the long term. But while herd immunity is effective for some illnesses, whether it's a viable solution for COVID-19 remains to be seen.

In fact, evidence to date suggests that herd immunity is likely not a realistic option. For example, the country of Sweden opted to shun social restrictions adopted by much of the rest of the world in favor of offering optional guidelines to citizens and relying on herd immunity. Initial reports from April 2020 indicate that it hasn't worked out so well, however, with Sweden experiencing a COVID-19 death rate of 8 percent, compared with neighboring Denmark (4 percent) and Norway (under 2 percent), which enacted greater restrictions from the get-go [source: Gittins]. The British government had originally decided to follow a similar approach until public outcry forced them to adopt stricter measures. The Swedish government health chief claims herd immunity in Sweden may be possible by May.

Although the school of thought around COVID-19 changes practically day by day, it stands to reason that herd immunity might never be the miracle solution that everyone's praying for. This is because the virus that causes COVID-19 (SARS-CoV-2) is probably like other coronaviruses and influenza in that it will mutate over time. When it mutates, any antibodies from a prior infection will provide little, if any protection. So, people who come down with COVID-19 in 2020 might be protected for a few months to a year or two, but lifelong immunity is decidedly unlikely [source: D'Souza and Dowdy]. Even if it turns out that this particular strain of COVID-19 miraculously doesn't mutate, it'll take some serious time (months or years) for herd immunity to be achieved, meaning that most of the population is either vaccinated or has become immune to the disease by prior infection [source: Sanchez].

Don't forget that COVID-19 is about 10 times more deadly than the flu, and even higher among vulnerable populations like the elderly. As a result, many fingers are crossed that a vaccine or effective treatment will be available sooner rather than later.

Last editorial update on Apr 20, 2020 04:14:13 pm.

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Sources

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