Self & Other · The Memory Library
Why Chickenpox Once and the Flu Forever
This is the trick. This is why you only get chickenpox once.
Why Chickenpox Once and the Flu Forever
The next obvious question, if you’ve been reading carefully, is: why doesn’t this work for everything?
If your body remembers chickenpox forever, why don’t you become immune to the flu after one bout? Why don’t colds get fewer with age? Why does your grandmother still get the same upper respiratory infection from your cousin’s kid every Thanksgiving?
The answer is one of the most important ideas in immunology, and it explains an enormous amount about how diseases work in the real world. Memory cells are tuned to specific molecular shapes. If a virus comes back wearing the same shapes, the memory cells recognize it instantly. If a virus comes back wearing different shapes, the memory cells are useless — the body has to start over from scratch, finding new B cells and T cells with the right random receptors, the way it did the first time.
Some viruses can’t change their shape much. The smallpox virus, before we wiped it out, was almost identical from one century to the next. The chickenpox virus is the same. The measles virus is the same. If you encountered measles as a child, the memory cells you made then would still recognize it today. One infection, lifetime protection.
Other viruses change constantly. The influenza virus has surface proteins that mutate so quickly that within a year or two, the version going around looks chemically different from the version that went around last winter. Your memory cells from last year’s flu look at this year’s flu and see a different molecule. They do not respond. The whole adaptive process has to begin again. This is why there is a new flu vaccine every year — it’s not the same vaccine. It’s a fresh attempt to predict, six months in advance, what shape this winter’s flu will be wearing.
The common cold is something else again. There isn’t really one virus that causes the common cold. There are about two hundred different viruses, all of them produce roughly similar symptoms, and your immune system has to remember each one separately. You probably encounter twenty or thirty of them in your life. Each one, the first time, makes you sick for a week. Each one, after that, your memory cells handle quietly. By the time you’re forty, most colds you would have caught at twenty, you don’t even notice.
HIV is the worst case. The HIV virus mutates inside your own body, faster than your B cells can produce antibodies. By the time the antibodies arrive, the version of the virus they were designed to fight is already gone, replaced by mutants the antibodies don’t recognize. Your immune system is, in a strange way, always fighting yesterday’s HIV. We’ll come back to this.
The general rule: the more stable a pathogen, the better your memory works against it. Stable pathogens are the ones we can vaccinate against effectively. Unstable pathogens are the ones we have to keep chasing.
What Vaccines Actually Are
We can now talk about vaccines properly.
A vaccine is, in essence, a way of triggering the memory-formation process without making you sick. That is the entire concept. Everything else is engineering.
Jenner’s original vaccine worked because cowpox and smallpox are similar enough at the molecular level that the antibodies and memory cells you make against cowpox happen to also recognize smallpox. The shapes overlap. He had no idea why; he just had the empirical observation. Modern vaccinology has more precise tools, but the goal is the same: present the immune system with something that looks enough like the dangerous pathogen to trigger the full memory-cell response, without the dangerous pathogen actually being present.
There are several ways to do this, and it’s worth knowing them, because the news will keep using these words and most people don’t know the difference.
Live attenuated vaccines use a real, replicating virus that has been weakened so much that it can barely cause an infection. Your body responds to it the way it would to a real infection, but the disease never really takes hold. The measles vaccine works this way. So does the chickenpox vaccine. Live attenuated vaccines tend to give the strongest, longest-lasting immunity — sometimes lifetime — because the immune response really does see the whole pathogen, doing what pathogens do, just barely. The downside is they can occasionally cause problems in people whose immune systems are very weak (a topic we’ll come back to in Chapter 11).
Inactivated vaccines use a real virus or bacterium that has been killed. The dead pathogen still has all its surface proteins, so your immune system can recognize and remember it, but it can’t infect anything. The flu shot most people get each year is this kind. So is the polio shot used in most countries. Inactivated vaccines are very safe, but the immunity is generally weaker than from live vaccines, which is why you sometimes need boosters.
Subunit vaccines go further: they don’t use the whole pathogen at all, dead or alive. They use just a single piece — one specific protein from the surface of the pathogen, made by other means. Your immune system makes memory cells against that one protein. The hepatitis B vaccine works this way. So does the HPV vaccine, the one that prevents most cervical cancers and several other cancers, and which is one of the few vaccines we have that prevents cancer.