It's "an arithmetic inevitability" that some people vaccinated against the coronavirus will contract so-called "breakthrough" infections—but a vaccinated person's immune system responds to the virus in distinctive ways that dramatically reduce the risk of serious disease, Katherine J. Wu writes for The Atlantic.

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How common are breakthrough infections?

According to the Washington Post, it's difficult to track the absolute rate of breakthrough infections, both because many such infections are asymptomatic and because surveillance testing has declined in areas where vaccines are readily available.

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It's clear, however, that breakthrough infections are uncommon and very rarely cause serious disease. According to CDC director Rochelle Walensky, 97% of all Covid-19 hospitalizations and 99.5% of all Covid-19 deaths are currently among the unvaccinated.

So why are breakthrough infections so much milder? It's because a vaccinated person's immune response looks very different than an unvaccinated person's, Wu writes.

How an unvaccinated person's immune system responds to the coronavirus

When an unvaccinated person is exposed to the coronavirus, Wu writes, their only protection comes from the body's innate defenders, which are "short-lived and woefully imprecise."

"They'll sink their teeth into anything they don't recognize, and are easily duped by stealthier invaders," Wu writes, making them "a pretty flimsy first line of defense."

If left to its own devices, an unvaccinated person's so-called "adaptive" immune system will eventually develop a more powerful, customized response to the coronavirus—but it can take weeks to do so.

By then, Wu writes, "the virus may have run roughshod over everything it can." It may be too late for the immune system to prevent serious long-term damage or even death.

Why a vaccinated person's response is so different

By contrast, a vaccinated person's immune system has already been trained to mount that more powerful, adaptive response. Vaccine shots "act as confidential informants, who pass around intel on the pathogen" before an infection occurs, Wu writes.

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In particular, vaccination prepares the body's adaptive B cells to more quickly produce antibodies when they encounter the coronavirus, and T cells can more quickly target and kill off infected cells. This enables a vaccinated person to mount a powerful immune response much more quickly.

"In the best-case scenario, the virus might even be instantly sniped at by immune cells and antibodies," Wu writes. But even if the virus overwhelms the body's initial defenses, the vaccine has prepared the body to mount a fuller response.

"A breakthrough, despite what it might seem, does not cause our defenses to crumble or even break," Wu writes. "[I]t does not erase the protection that's already been built."

Rather, according to Deepta Bhattacharya, an immunologist at the University of Arizona, the virus must overcome "backup layer after backup layer" of defenses. So even if a virus continues to spread through a person's body, "[e]ach stage it has to get past takes a bigger chunk" out of it, Bhattacharya said.

That's why people who have been vaccinated tend to experience fewer and milder symptoms and recover more quickly, Wu writes.

"People tend to think of this as yes or no—if I got vaccinated, I should not get any symptoms; I should be completely protected," Laura Su, an immunologist at the University of Pennsylvania, said. "But there's way more nuance than that."

"The vaccines were developed to keep us out of those terrible institutions we call hospitals," William Schaffner, an infectious disease expert at Vanderbilt University, said. "We have to keep coming back to that." (Gale, Washington Post, 7/25; AP/Modern Healthcare, 7/21; Cohen, Forbes, 7/22; Wu, The Atlantic, 7/26)