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Why do we develop lifelong immunity to some diseases, but not others?

Childhood vaccines can protect against some diseases for a lifetime.
Childhood vaccines can protect against some diseases for a lifetime. (Image credit: Shutterstock)

Some diseases, like the measles, infect us once and usually grant us immunity for life. For others, like the flu, we have to get vaccinated year after year. 

So why do we develop lifelong immunity to some diseases but not others? And where does the novel coronavirus fit into all this?

Whether or not we develop immunity to a disease often depends on our antibodies, which are proteins we produce in response to infection. Antibodies are one of the body’s most well-known defenses: They coat invading cells and, in the best case, prevent those invaders from hijacking our cells and replicating. After we clear an infection, antibody levels often wane, but at least a few stick around, ready to ramp up production again if that same disease attacks again. That's why an antibody test can tell you if you were infected in the past. It's also what keeps us from getting sick a second time — usually.

Related: Can you get 2 colds at once?

"The body doesn't really forget," said Marc Jenkins, an immunologist at the University of Minnesota Medical School. Usually, when we get reinfected with a disease, it's not because our body has lost immunity. We get reinfected either because the pathogen mutated and our immune system no longer recognizes it, or because our bodies tend to mount a much lower immune response, he said.

Take the flu. This is a virus that can change its genes easily, Jenkins said. Just as our immune systems kill off one version of the virus, another emerges that our immune systems don't recognize. Not all viruses mutate so readily. For example, the polio virus can't easily change its genome, Jenkins said. That's why we've been so successful at (almost) eradicating it. 

The common cold, and other viruses that don’t typically get past our upper respiratory tract, reinfect us not necessarily because they mutate rapidly, but because our body doesn't usually produce many antibodies against these pathogens in the first place, said Mark Slifka, an immunologist at the Oregon National Primate Research Center. "Our bodies are not worried about the upper respiratory tract," he said. That's what we're seeing with mild cases of COVID-19. The virus sticks to the upper respiratory tract, where the body does not treat it like a threat. In a 2020 preprint study (meaning it hasn't been peer reviewed yet) published in the database MedRxiv, 10 out of 175 patients who had mild symptoms recovered from COVID-19 without developing detectable antibodies.

For diseases that don't fall into either of these categories — meaning they don’t mutate rapidly and they generally prompt a strong immune response — immunity tends to last much longer. A 2007 study published in the New England Journal of Medicine found that it would take more than 200 years for even half of your antibodies to disappear after a measles or a mumps infection. The same study found similar results for Epstein-Barr virus, which causes mono. Still, antibody responses don't always last a lifetime. That same study found that it takes around 50 years to lose half of our chickenpox antibodies, and 11 years to lose half of our tetanus antibodies. That means that without a booster shot, you could theoretically become infected with one of these diseases as an adult. 

Scientists still aren't sure why we maintain our antibody responses longer for some diseases compared with others. It's possible that some of these more common diseases, such as chickenpox and mono, actually are reinfecting us more frequently than we realize, but that the antibodies we do have crush the infection before we notice, Jenkins said. And in those cases, the immune system would be at full capacity again and again because of the reinfections. "It keeps our immunity vigilant," he noted. In contrast, "with tetanus, we're probably very rarely getting exposed, we're not stepping on a [dirty] nail very often."

Related: Do rusty nails really give you tetanus?

Other scientists point out that the human immune system is trained to target pathogens that "look" a certain way, Slifka said. Bacteria and viruses tend to be symmetrical with a repetitive pattern of proteins across their surfaces. (Think about COVID-19 — it's a ball with evenly spaced spikes all over it.) One theory suggests that we mount a larger and longer-lasting immune response to more repetitive-looking pathogens. For example, the antibodies we produce against variola, the highly repetitively-structured smallpox virus, last a lifetime. Tetanus, however, isn't repetitive at all. It's the toxin produced by tetanus bacteria, not the bacteria itself, that makes us sick. Based on this theory, it's possible that our bodies aren't as well-trained to target this single, asymmetrical protein, Slifka said.

So, will immunity to the new coronavirus — whether that comes from infection or a vaccine — be as long-lived as our immunity to smallpox, or will we need a new vaccine every year? While it’s true that some people aren’t mounting large antibody responses, Jenkins is still hopeful for the former. All the evidence both from natural infections and from vaccine trials suggest that most people are making neutralizing antibodies, the variety which prevents viruses from entering our cells, Jenkins said. And unlike the flu, SARS-CoV-2, the virus that causes COVID-19, isn't mutating quickly, Jenkins noted. 

"This virus has the features of viruses that we've been very successful in vaccinating against," Jenkins said.

Originally published on Live Science.

Isobel Whitcomb

Isobel Whitcomb, a contributing writer for Live Science, covers the environment, animals and health. Her work has appeared in Scholastic, Fatherly, Atlas Obscura, and Hakai Magazine. Isobel's roots are in science. She studied biology at Scripps College in Claremont, California while working in two different labs, and completing a fellowship at Crater Lake National Park. She completed her master's degree in journalism at NYU's Science, Health, and Environmental Reporting Program. She lives in Brooklyn, where you can find her riding her bike or running in Prospect Park.

  • JoeMerica
    I have read elsewhere that our bodies' immune system seems to remember viruses better if they were a serious threat - if they see it replicating rapidly or doing damage. With the mRNA approach, arent we teaching our immune system that this is NOT much of a threat? We create a few of them from our own cells but its pretty mild. Wouldnt it follow that the immunity would not last long?
    Reply
  • PaulyRoc
    The article "why do some diseases infect us again and again" seems well intended, but uses a terrible example of annual flu shots as if the annual flu shot was made up of the same flu virus - not the case. Tetanus is a bacterial infection and antibodies are created for viral infection, mostly. Then uses chicken pox as another example, which is a form of herpes. Herpes generally goes "dormant" and does not leave the body EVER, but as we age and our immune system weakens we are prone to viral re-emergence as shingles, a lessened version of chickenpox. The antibodies of any virus your body develops is lifelong. Tcels are not the same, but related. Some viruses are knocked out prior to the immune system creating antibodies as the Tcels handled the virus. Everyone is different in that regard. You can catch things more than once before your body actually develops antibodies, some folks just don't have strong immune systems. This is stuff that I learned in primary school. There is nothing out there that says it has changed. The info that COVID antibodies last 4 mon or 9 mon are misstatements as there have not been studies longer than that and can only represent the 4 or 5 or 6 month time frames. I don't know much about mRNA, but I understand that it works like Tcels and (hopefully) teaches the immune system to develop antibodies so it essentially is using how the immune system works to artificially generate something that ordinarily would happen naturally. Your logic, JoeMerica, stands to reason. Only time will tell. If the vaccines are ever FDA approved, that would be a good indicator that the vaccine creates lifelong antibodies as opposed to being artificial Tcels with limited duration. IMO
    Reply
  • Bril2424
    43rd
    admin said:
    Why do some diseases infect us again and again?

    Why do we develop lifelong immunity to some diseases, but not others? : Read more
    Depending on how efficient they are, viruses mutate so they can continue to infect their host species. Some mutate rapidly while some don't.
    Reply