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* some vaccinated people will and make sure low-income countries get vaccines and the other tools they need to fight the pathogen. Just as variants weren't a surprise, neither were so-called breakthrough cases, in which people who have been vaccinated end up getting infected anyway. Until we have the 100 percent effective vaccines or drugs I mentioned in the previous section,* some vaccinated people will still become infected. As more people get vaccinated in a given population, the total number of cases will go down, and a growing percentage of the cases that do occur will be breakthroughs. Here's one way to think about it. Imagine that COVID starts spreading through a town with a fairly low vaccination rate. A thousand people get so sick that they end up in the hospital. Out of those 1,000 severe cases, 10 are breakthroughs. Then the virus spreads to the next town over, which has a high vaccination rate. That town sees only 100 severe cases, of which 8 are breakthroughs. In the first town, breakthroughs represented 10 out of 1,000 severe cases, or 1 percent. In the second, they made up 8 out of 100, or 8 percent of the total. Eight percent sounds like bad news for town #2, right? <~?~Insert art 012_Gate_9780593534489_fpo_r1> <~?~NO CAPTION FOR THIS ILLUSTRATION (Illustration comparing a town with low vaccination rate to a town with high vaccination rate.)> * In the medical field, "effectiveness" and "efficacy" mean different things. Efficacy is a measure of how well a vaccine works in a clinical trial. Effectiveness is a measure of how well it works in the real world. For the sake of simplicity, and because "efficacious" is an eyesore of a word, I'll use "effectiveness" to mean both.