When breakthrough cases do arise, it’s not always clear why. The trio of vaccines now circulating in the United States were all designed around the original coronavirus variant, and seem to be a bit less effective against some newer versions of the virus. These troublesome variants have yet to render any of our current vaccines obsolete. But “the more variants there are, the more concern you have for breakthrough cases,” Saad Omer, a vaccine expert at Yale, told me. The circumstances of exposure to any version of the coronavirus will also make a difference. If vaccinated people are spending time with groups of unvaccinated people in places where the virus is running rampant, that still raises their chance of getting sick. Large doses of the virus can overwhelm the sturdiest of immune defenses, if given the chance.
The human side of the equation matters, too. Immunity is not a monolith, and the degree of defense roused by an infection or a vaccine will differ from person to person, even identical twins. Some people might have underlying conditions that hamstring their immune system’s response to vaccination; others might simply, by chance, churn out fewer or less potent antibodies and T cells that can nip a coronavirus infection in the bud.
Read: You’re not fully vaccinated on the day of your last dose
The effects of vaccination are best considered along a spectrum, says Ali Ellebedy, an immunologist at Washington University in St. Louis. An ideal response to vaccination might create an arsenal of immune molecules and cells that can instantaneously squelch the virus, leaving no time for symptoms to appear. But sometimes that front line of fighters is relatively sparse. Should the virus make it through, “it becomes a race [against] time,” Ellebedy told me. The pathogen rushes to copy itself, and the immune system recruits more defenders. The longer the tussle drags on, the more likely the disease is to manifest.
The range of vaccine responses “isn’t a variation of two- to threefold; it’s thousands,” Ellebedy told me. “Being vaccinated doesn’t mean you are immune. It means you have a better chance of protection.”
For these reasons and more, Viviana Simon, a virologist at the Icahn School of Medicine at Mount Sinai, in New York, dislikes the term breakthrough case, which evokes a barrier walling humans off from disease. “It’s very misleading,” she told me. “It’s like the virus ‘punches’ through our defenses.”
Vaccination is actually more like a single variable in a dynamic playing field—a layer of protection, like an umbrella, that might guard better in some situations than others. It could keep a lucky traveler relatively dry in a light drizzle, but in a windy maelstrom that’s whipping heavy droplets every which way, another person might be overwhelmed. And under many circumstances, vaccines are still best paired with safeguards such as masks and distancing—just as rain boots and jackets would help buffer someone in a storm.
