The Responsibility to Protect

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Vaccinations protect more people than just those who are vaccinated – they also protect the unvaccinated. Health authorities call this protection “herd immunity” (people around us are referred to as our “herd”). But there’s a catch: there has to be a threshold number of people who get vaccinated before this collective immunity takes effect. Conversely, low levels of herd immunity are often associated with epidemics, such as the measles outbreak in 2014 – 2015 that was traced to exposures at Disneyland in California.

Tara C Smith, PhD, of Kent State, wrote a popular essay this month explaining how vaccination and herd immunity go together. Note that the necessary level of (herd) immunity in the population isn’t the same for every disease:

For measles, a very high level of immunity needs to be maintained to prevent its transmission because the measles virus is possibly the most contagious known organism. If people infected with measles enter a population with no existing immunity to it, they will on average each infect 12 to 18 others. Each of those infections will in turn cause 12 to 18 more, and so on until the number of individuals who are susceptible to the virus but haven’t caught it yet is down to almost zero. The number of people infected by each contagious individual is known as the “basic reproduction number” of a particular microbe (abbreviated R0), and it varies widely among germs.”

For instance, the R0 of pertussis (whooping cough) is 12-17; polio and smallpox 5-7; mumps 4-7; HIV 2-5; influenza, including the 1918 influenza pandemic 2-3; and Ebola 1.5-2.5.

Here’s the thing. If you know how many secondary cases to expect from each infected person, you can figure out the level of herd immunity needed in the population to keep the microbe from spreading. Tara Smith:

This is calculated by taking the reciprocal of R0 and subtracting it from 1. For measles, with an R0 of 12 to 18, you need somewhere between 92 percent (1 – 1/12) and 95 percent (1 – 1/18) of the population to have effective immunity to keep the virus from spreading. For flu, it’s much lower — only around 50 percent. And yet we rarely attain even that level of immunity with vaccination.

Based on that arithmetic, the following table shows what percentage of the population needs to be vaccinated by disease to prevent its outbreak:

R values

Notice that the higher the R0 value, the higher the percentage of people in the community that need to be vaccinated.

Which brings us to the question of who it is that we need to protect – who are the unvaccinated? In general it’s people who are immune-compromised. For example, children who cannot be vaccinated because their immune system is too immature to develop the adaptive immune response that the vaccine is supposed to illicit. Infants who have not yet been vaccinated or have just received a vaccination. The elderly who, because of their age, are often immune-compromised. The sick, whose immune systems can’t withstand the dose of a weakened virus in a vaccine. Those for whom the vaccine didn’t take. And here’s a detailed eye-opening list provided by the CDC that pairs a particular vaccine with health status and warns against vaccination in such cases. All told, we’re talking about a huge swath of people that need protection from infectious disease through herd immunity.

In other words, vaccination campaigns for the flu and other diseases are about much more than individual health. They’re about achieving a collective resistance to disease that involves the whole community.

In the field of international relations there’s a UN doctrine called the Responsibility to Protect. It says that if a nation can’t or won’t protect its own people from harm, then other nations have a right and an obligation to step in and do so. Similarly, on the level of community relations, we know that the very young, the old, and the sick, can’t protect themselves from harm – disease – through vaccination. And so it falls on each one of us to do so: to vaccinate, thereby protecting not just the vulnerable but ourselves and our families at the same time.

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