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by Jerry Oltion

How Vaccines Work

ON THE 14TH of May, 1796, Edward Jenner inoculated an eight-year-old boy named James Phipps with the fluid from a cowpox sore on the hand of a milkmaid who had contracted the disease from a cow. In that era, "inoculation" meant scratching the skin and rubbing the infectious agent into the wound. James did indeed develop cowpox, but a milder case of it than the milkmaid's. And bonus: He was now immune to smallpox, a much more dangerous, often deadly, disease.

Many people consider this the beginning of the concept of vaccination, and indeed the term "vaccination" comes from the Latin word for "cow" (vacca), but in fact the practice had already existed for at least a thousand years, maybe even two, in China and India. They had been using smallpox scabs either powdered and blown up the nose or rubbed on the skin like Jenner's cowpox innoculation.

The theory was that the disease would take longer to develop if the infectious agent was dried and powdered, or if it had to work its way through the skin and muscle tissue, and that extra time would allow the body to build up a defense against it. That theory was correct. Most smallpox cases that were contracted by inoculation were less severe than natural cases, and they did indeed convey immunity for the rest of the inoculated person's life.

So how does exposure to a disease, or to a similar one in the case of cowpox, provide lifetime immunity?




Our bodies contain billions of blood cells, constantly circulating through our veins and arteries and capillaries, supplying oxygen to all the living tissue of our bodies. But scattered among the reddish oxygen-bearing cells are much larger, equally specialized white cells whose job is to look out for foreign agents within the body and eliminate them before they can cause harm.

How does a single cell know what's supposed to be there and what's not? In a word: antigens. Antigens are molecules present on the surface of invading cells but not on normal human body cells. White cells test everything they encounter to determine if it has any of these antigens present, and thus whether it's friend or foe.

There are many different types of white cells. First responders, called macrophages ("big eaters"), have a general ability to tell bacteria and viruses from human body cells. They're not particular about which type of bacteria or viruses they're dealing with; they just launch an all-out attack and eat anything foreign.

But they don't digest everything. Macrophages save the antigens and head back to the lymph nodes, little bean-sized organs scattered throughout the body, where they present those antigens to other white cells called, not surprisingly, "lymphocytes."

There are two major kinds of lymphocytes: B-cells and T-cells. B-cells have much more discriminating antigen detectors than macrophages. When a B-cell that's tuned to a specific invading antigen recognizes that antigen, it begins dividing into large "plasma cells" that replicate its specific antigen detector over and over within themselves, then those plasma cells burst apart, creating little Y-shaped cell fragments called antibodies that carry those duplicated receptors throughout the body. (That's why antigens are called antigens. They're antibody-generators.)

When an antibody finds a matching antigen, it binds to that antigen and prevents it from doing what it was designed to do: infect a human cell. Multiple antibodies will swarm the surface of a bacterium or virus, rendering it inoperative.

An antibody-covered invader attracts T-cells, which engulf the invader and kill it. T-cells can also recognize an infected body cell and kill it before it releases a new load of infectious agents.




Once the body's immune system gets the upper hand, it's just a matter of time before the infection is routed. But since disease-causing agents (bacteria and viruses) can reproduce about as fast as white cells, gaining the upper hand isn't always a given. Disease-causing organisms have evolved to act quickly and evade the immune system until they gain the upper hand, so in many cases a person becomes quite sick and may even die from the infection before the immune system can mount an effective defense.

So our bodies have developed a fast-response system. After an infection, assuming it doesn't kill us, some of those B-cells and T-cells that were activated to fight it become memory cells. They remain on alert, ready to jump into action producing antibodies and wiping out the invaders the moment they're detected again.

That's why someone who has had the measles doesn't get the measles a second time. Their body remembers what a measles virus looks like, and the moment it detects one, the B-cells release antibodies to it and the T-cells wipe it out. The virus has no chance to establish itself.




So imagine if you could trigger the production of memory cells without giving someone the disease itself. You could prime the body's immune system to react quickly to an infection, wiping it out with ready-made soldiers that are just waiting for the call to action.

That's exactly what vaccines do. A vaccine is just an antigen that triggers the B-lymphocytes to produce antibodies and T-lymphocytes to produce soldiers against that specific antigen. Those activated cells become memory cells, ready to leap into action if the complete infectious agent ever invades the body.

The trick is to present the antigen without creating a wholesale infection. In the early days of innoculation through scratches in the skin, people often caught full-blown cases of the disease being innoculated against. Jenner's experiment with cowpox was an attempt to use a similar but less dangerous infectious agent that would convey immunity to the more serious disease, smallpox. That worked because the two diseases are so similar that they share the same antigens.

More modern vaccines use attenuated (partially disabled) or completely killed infectious agents. They still present their specific antigens to the body's immune system, and still trigger the production of antibodies and memory cells, but they don't have the ability to cause disease.

Work progresses on developing vaccines for diseases we haven't conquered yet. It's tough to find the right antigen and find the right way to introduce it to the immune system.




That's how vaccines work in your body. But how do they work in society?

Imagine that just one person gets a vaccination against smallpox. That person could safely encounter another person with smallpox (or a milkmaid with cowpox) and not come down with the disease. But the disease would still infect anyone else it reached. And smallpox is very infectious. Pretty much everybody who encounters it comes down with it. And about one third of the people who contract it die.

So vaccinating one person only helps that person. Vaccinating half the population helps half the population, but as long as there are enough people to keep the virus alive from generation to generation, it does society as a whole no good. Any unvaccinated person will still have ample opportunity to encounter the disease, and to spread it to others.

At some point, however, if we vaccinate enough people, the disease doesn't have a big enough reservoir of unvaccinated people to live in, and the disease dies out, at least within the vaccinated society. Other societies may harbor reservoirs of the disease, but the virus can't sustain itself within the vaccinated society.

We call this level of protection "Herd immunity." The threshold for herd immunity varies with the virulence of the disease. For measles, a highly contagious disease, the vaccination rate must be 92-95% in order to confer herd immunity. For influenza, the rate can be as low as 33-44% depending upon the particular strain.




Of course no vaccine can be 100% effective. Some people's bodies have compromised immune systems and won't generate the antibodies and memory cells necesssary to confer immunity. In the case of flu vaccines, because influenza is such a variable disease, the makers of the vaccine can't tell ahead of time which antigens will be present in an upcoming year's outbreak, so they have to make an educated guess. Sometimes that guess is off the mark, and the vaccine isn't as protective as they hoped.

In rare cases, people react badly to the vaccine. The most common reaction is fainting after the injection, which is a psychosomatic response to anxiety, not a direct response to the vaccine. Some people can be allergic to the egg protein or gelatin present in some vaccines, but this happens in less than one in a million doses. In the early days of vaccination with attenuated viruses, if the virus wasn't sufficiently knocked out, the vaccine could cause the disease it was designed to prevent. This is nearly unheard of nowadays.

But scare stories abound. People who faint say the vaccine knocked them out. People still tell stories about their great uncle who caught polio from the vaccine—back in 1950. And in 1998 a British researcher published a paper connecting the MMR (measles, mumps, and rubella) vaccine to child autism. The paper—and the "science" behind it—was flawed and the supposed connection has been disproven many times over, but the scare story has become an epidemic in itself, to the point where an estimated 1 out of 4 parents believe that the connection is real.

Also, certain religions discourage vaccination, and large segments of the population simply resist anything that the government tells them they should do. And all these factors are becoming worse, not better, as we move backwards into the 21st century.

What this means is that we're losing our herd immunity. Measles is making a comeback in the Pacific Northwest as I write this, and a 6-year-old Oregon boy nearly died of tetanus last year because he wasn't vaccinated. We almost eliminated polio, but it looks like that may make a comeback. And so on. All because of misplaced fear and misunderstanding.

Why am I writing about this topic here, in this venue? Because we're readers and writers of stories. Science fiction (and to some exent fantasy as well) sets the tone for how our society views the world. We're where we are today, for good or for bad, largely due to the hopes and dreams that were instilled in the heads of the people who are now running things. Science fiction and fantasy had a lot to do with those hopes and dreams.

It's my hope that those of us who care about the future will educate ourselves about things like vaccination, and in turn educate others through our stories and our discussion of those stories. Ideas are much like diseases, in that they spread through contact. Let's spread the true science about vaccination, rather than the scare stories that turn out, after all, to be fiction.


Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.

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