What Is "Herd Immunity"? Not What You Think It Is.

A simple graphic illustration of "flattening the curve," spreading out the incidence of disease in an attempt to not overwhelm a country's medical care facilities. "Protective measures" means social distancing, frequent hand-washing, quarantine of known cases and the like. It does not mean vaccination. (source)

by Thomas Neuburger

The phrase "herd immunity" is being used a lot these days to describe how our species will deal with the ravages of disease caused by COVID-19. Most people, I'm afraid, think of "herd immunity" as a process by which "the herd" — us humans — will catch the disease from others, develop the antibodies needed to ward it off in the future, and go on our way knowing we can't catch it again or transmit it to others. Thus, as this immunity spreads society protects itself.

For people who think this way, the main COVID-19 problem is how to "flatten the curve" — to make the spike in serious cases as low and spread out as possible so that our medical care facilities are not overwhelmed while the rest of us become immune. The graph at the top illustrates this concept.

The assumption in this thinking is that the immunity of the bulk of the population will eventually protect the most vulnerable, the one's most likely to become seriously ill and possibly die. Seen in this way, developing "herd immunity" as quickly as is reasonable is our best shield against COVID-19.

Great Britain, under the leadership of Boris Johnson, is banking on just that. As Umair Haque writes, "According to Robert Peston, one of the UK’s top journalists, Britain’s leaders’ brilliant idea is … 'to allow the virus to pass through the entire population so that we acquire herd immunity.' Other journalists have noted the same thing." Summarizing, he adds that "they think that everyone getting sick ... confers resistance on a nation."

In short, according to this thinking, everyone getting sick is a good thing for the country.

Herd Immunity Depends on Vaccination

Unfortunately, this isn't how "herd immunity" works. It actually works this way:

Herd immunity describes how a population is protected from a disease after vaccination by stopping the germ responsible for the infection being transmitted between people. In this way even people who cannot be vaccinated can be protected. For example, the bacteria meningococcus and pneumococcus can cause blood poisoning (septicaemia) and meningitis. In most people the bacteria live harmlessly in the throat and do not causes disease, but sometimes they get into the bloodstream leading to these severe infections. They can live harmlessly in the throat of one person but if they spread to someone who is particularly susceptible (such as a young baby) they can cause severe disease. By being vaccinated an individual is not only protected from being infected themselves but they then also cannot pass this infection onto other people, where it may cause severe disease. However, for herd immunity to work a large proportion of the population need to be vaccinated. [emphasis added]

The following diagram illustrates how a communicable disease passes through an unvaccinated population versus a vaccinated one:

People are shown as circles. Infectious agents (germs) spread between the people in orange, although they do not get severe disease. When the infection reaches people who are highly susceptible (red) they get the disease and can be very sick or die.

In the lower panel, the people in green have been vaccinated. This now protects those in yellow as well, who had previously got the infection and possibly the disease. Although the figure only shows a few people being vaccinated, in reality many people have to be vaccinated for herd immunity to work.

There's another useful diagram here.

In short, many people getting sick doesn't prevent the rest, including most vulnerable, from also getting sick; only vaccination does that. For some diseases, the percentage of population that needs to be vaccinated is large; for others, the percentage of vaccinations can be smaller:

The more contagious it is then the more people need to be vaccinated for herd immunity to work. For example, measles is very contagious. Before the use of the measles vaccine, every person with measles would infect another 10-15 people and so the disease would spread very quickly. To achieve herd immunity for measles at least 90-95% of the population need to be vaccinated. A disease like polio is less contagious, and 80-85% of the population would need to be vaccinated for herd immunity to work. Although this is lower it is still a very high proportion, especially given that some people cannot be vaccinated for medical reasons. [emphasis added]

For many diseases — tetanus, for example — vaccination protects only the vaccinated individual, since tetanus is harbored in the soil and can be "caught" by anyone. Fortunately, COVID-19 is not this type of infection.

Magical Thinking

The conclusion we should draw from all this is that the "wait it out" strategy — get the disease as late as possible so that, if you get really sick, there will be a hospital bed for you — is not much protection at all.

If you get really sick, health care will obviously help, but there really is no cure for COVID-19 at the moment. Health care will keep you from dying of secondary causes — additional diseases you might come down with if your immune system is compromised, for example — but if COVID-19 wants you, the only prevention is to not get sick with it in the first place.

Placing one's hopes, or a society's hopes, in a misunderstood version of "herd immunity" is an exercise in magical thinking. If you're in charge of a society's response and this version of herd immunity is your answer, it's an exercise in malicious thinking.

Vaccine Availability

The second conclusion applies to society as a whole. Allowing COVID-19 to spread through a population in the absence of a vaccine is a death sentence for many individuals; if a person is likely to die if they get it early, they're just as likely die if they get it late. This puts a premium on the speed at which a vaccine is developed — and made available. (A vaccine is no good if it's not available, for cost reasons, for example.)

Which leads to a corollary: There is no neoliberal solution to this problem. The profit system is exactly the wrong way to deal with the threat posed by COVID-19. The minute testing and vaccination is subject to profit constraints, the solution begins to fail, since people are less likely to be tested and vaccinated. That puts everyone — the whole herd, as it were — at risk.

The best solution to this problem is also the most moral — test everyone for free; vaccinate everyone for free. Do it fast and do it now.

The privatizers will try, of course, to get their profit-seeking hands on both the tests and the vaccine. They may even succeed. But if they do, COVID karma, unlike climate karma, will be instant. Deaths won't just rise in a decade or two, or in their grandchildren's generation; they'll rise in earnest immediately, and the dead will have the names of their killers written on their foreheads for everyone else to see — "Thank you, Biologics.com. I died for you."