There is finally a glimmer of hope in the war against the coronavirus. Daily new infections have declined by more than 30 percent in the past two weeks, with 45 states reporting sustained decreases. Hospitalizations are trending down. Though January has been the deadliest month thus far, some models project that the worst may soon be behind us, with warmer months on the way and vaccinations picking up speed.

This is the best-case scenario. But a much more ominous scenario looms, too.

That scenario is what happens if the emerging covid-19 variants gain traction in the United States. There are at least three variants that appear to have acquired mutations that make them more transmissible than the existing strains. Named B.1.1.7, B.1.351 and P.1, these mutants were first identified in Britain, South Africa and Brazil, respectively. British scientists are finding that B.1.1.7 may be 30 percent more lethal in addition to being up to 70 percent more contagious.

The higher degree of transmissibility is what causes the greatest concern. Let’s take an illustrative example where the virus reproduction rate is 1.1 — meaning that 1.1 new infections stem from each one case. Within 10 cycles, 16 people are infected. If the reproduction rate is now 70 percent higher, an astounding 600 people are afflicted, with resulting exponential increases in hospitalizations and deaths.

Their high transmissibility is how these mutants have rapidly taken hold in other countries and replaced existing variants. B.1.1.7 was first detected in September and became the dominant strain in much of Britain by December. That variant is believed to be increasing by 70 percent each week in Denmark, despite the country being in a strict lockdown. Here in the United States, the Centers for Disease Control and Prevention predicts that B.1.1.7 could overtake all other strains by March.

What would happen then? First, there could be a catastrophic surge far surpassing anything we’ve seen to date. It would occur on top of a high level of infection and could quickly overwhelm a health-care system that has yet to recover from the last surge. This time, new infections could spread with far greater speed and wreak unimaginable devastation.

Second, activities that are relatively safe now would become higher risk. This could derail efforts to reopen schools. Britain’s schools had in-person instruction throughout the fall but then abruptly closed in an attempt to rein in the new variant. In a primary school in the Netherlands, an outbreak of covid-19 afflicted nearly 15 percent of teachers, students and families, with the B.1.1.7 variant responsible for a large fraction. Schools in that country, too, are now closed. With a more transmissible strain, we would need to recalibrate every activity. People might need N95 masks just to go to the grocery store; restaurants might have to shutter indefinitely; and lockdowns could become inevitable.

This worst-case scenario makes the race to vaccinate even more pressing. There is evidence that the existing vaccines might not be as effective against some variants. Initial results released by Moderna show that its vaccine produced a similar level of neutralizing antibody to B.1.1.7 as existing variants, but a sixfold decrease against B.1.351. The company is doing further research into the implications; currently, they are saying that the level of response is still above what’s needed for immune protection, though there could be a decrease in the protective effect. Both Moderna and Pfizer are investigating booster shots to target the mutants.

Some have interpreted this news to mean that they should wait for a reconfigured vaccine instead of getting vaccinated now, but the opposite conclusion is the right one. The more time goes on, the more these variants take hold, and the more harm they will cause. And it’s not only the variants we know of that we should fear. As vaccine expert Peter Hotez said to me, homegrown strains could be developing here in the United States. “In places where there’s been a lot of virus transmission, we’re seeing these very aggressive variants. I have to believe that this is going on in the U.S. as well.”

The race is on — but we know how to win it. Carlos Del Rio, an Emory University infectious diseases physician, explained it to me like this: “In order for viruses to mutate, they have to be transmitted to other people. If we slow down transmission, we will slow down variants.” He urges the public to double down on public health measures such as mask-wearing and physical distancing, and our government to put every effort into expediting mass vaccinations.

America could be passing our darkest hour, but calamity could also still lie ahead. Once again, as it has been throughout the pandemic, what happens next is up to us.

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