The main problem is this. As the CDC defines it, the word breakthrough can refer to any presumed infection by SARS-CoV-2 (that is, any positive coronavirus test) if it’s detected more than two weeks after someone receives the final dose of a COVID-19 vaccine. But infections can come with or without symptoms, making the term imprecise. That means breakthroughs writ large aren’t the most relevant metric to use when we’re evaluating vaccines meant primarily to curb symptoms, serious illness, hospitalizations, and death. “Breakthrough disease is what the average person needs to be paying attention to,” Céline Gounder, an infectious-disease physician at Bellevue Hospital Center in New York, told me. Silent, asymptomatic breakthroughs—those that are effectively invisible in the absence of a virus-hunting diagnostic—are simply not in the same league... To be clear, breakthroughs of any severity are an entirely expected part of the vaccination process. No vaccines are 100 percent effective at preventing infection or disease. But our current crop of COVID-19 shots comes pretty damn close with regards to stymieing symptoms, especially the severe ones that can signal a deadly case. The Moderna and Pfizer shots have consistently demonstrated very high COVID-prevention rates, often in the 90s; Johnson & Johnson’s, for the most part, isn’t far behind. Symptomatic breakthroughs are the cases that wedge themselves in the gap between excellent effectiveness and perfect effectiveness; in other words, we saw them coming. Even out in the messiness of the real world, symptomatic breakthrough cases are proving themselves quite rare. The overwhelming majority of the COVID-19 cases we’re seeing are among the unvaccinated. And when the virus does affect the immunized, it seems to accumulate to lower levels, and spread less enthusiastically to new hosts; it’s causing, on average, milder and more transient symptoms.