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The vaccine (疫苗) news continues to seem very encouraging. Britain started its mass vaccination effort and the U.S. isn’t far behind.

But there is still one dark cloud hanging over the vaccines that many people don’t yet understand.

The vaccines will be much less effective at preventing death and illness in 2021 if they are introduced into a population where the coronavirus is still severe—as is now the case in the U.S.

A vaccine is like a fire hose (消防龙头). A vaccine that’s 95 percent effective, as Moderna’s and Pfizer’s versions appear to be, is a powerful fire hose. But the size of a fire is still a bigger determinant of how much destruction occurs.

At the current level of infection in the U.S. (about 200,000 confirmed new infections per day), a vaccine that is 95 percent effective—distributed at the expected pace—would still leave a terrible toll (伤亡人数) in the six months after it was introduced. Almost 10 million or so Americans would catch the virus, and more than 160,000 would die.

This is far worse than the toll in a different situation where the vaccine was only 50 percent effective but the U.S. had reduced the infection rate to its level in early September (about 35,000 new daily cases). In that case, the death toll in the next six months would be kept to about 60,000.

It’s worth pausing for a moment on this comparison. If the U.S. had maintained its infection rate from September and Moderna and Pfizer had announced this fall that their vaccines were only 50 percent effective, a lot of people would have panicked.

But the reality we have is actually worse.

How could this be? No vaccine can get rid of a pandemic immediately, just as .no fire hose can put out a forest fire. While the vaccine is being distributed, the virus continues to do damage.

There is one positive way to look at this: Measures that reduce the virus’s spread—like mask-wearing, social distancing and rapid-result testing—can still have great consequences. They can save more than 100,000 lives in coming months.