COVID vaccines might stop a ‘4th wave’ in the U.S. But the rest of the world isn’t so lucky.

You wouldn’t know it by following the news from the United States, where daily COVID-19 cases have fallen by 80 percent over the last 10 weeks — and where they continue to fall.

You wouldn’t know it by following the news from the United Kingdom, either. There, daily cases have plummeted 90 percent over the same period.

And you certainly wouldn’t know it by following the news from Israel, where normal life has all but resumed.

But despite the rosy view from a few select countries, the total number of COVID-19 cases worldwide is actually rising right now. In fact, after peaking on Jan. 11 and dropping by half over the next month — the first overall decline of the entire pandemic — global cases reversed course in mid-February and began to rebound.

Since then, new daily cases have climbed nearly 20 percent overall. In France, they’re up more than 30 percent. In Brazil, they’re up more than 50 percent. In Italy, they’re up more than 80 percent. In India, they’re up more than 110 percent.

The dreaded “fourth wave,” in other words, has arrived — even if it hasn’t arrived in the U.S.

These divergent trajectories offer up a troubling preview of the next phase of the pandemic, and they should trouble everyone, including residents of countries like the U.S., the U.K. and Israel, where the virus finally seems to be retreating.

Why? Because the single biggest difference between these recovering countries and the rest of the world is immunity — both the kind acquired from prior infection and, going forward, the kind acquired from vaccination. As more dangerous variants take over, hard-hit countries with higher rates of vaccination are (mostly) withstanding the onslaught. Hard-hit countries with lower rates of vaccination are (mostly) not. This suggests that until vaccination has ramped up everywhere, the virus will keep spreading, evolving and threatening to dodge our defenses.

And that’s not just someone else’s problem. The more vaccines that rich countries buy up — and the slower that some of them vaccinate their own populations — the more likely it is that poorer, less protected countries will serve as breeding grounds for variants, prolonging the risk for everyone.