This is a moment of terrible tension. We are reaching an unspeakable milestone: the deaths of half a million Americans from covid-19. At the same time, there is unambiguous good news in the fight against the virus. It is possible, finally, to imagine a day when this devastating pandemic is brought to an end. The progress we’ve made toward defeating covid-19 should sharpen our grief, making it clear how many lives we might have saved had we been unified in our response. But even as we mourn, we cannot despair: There are people who will live if we keep up the hard, lonely work still before us.

Since peaking in early January, the daily tally of new cases in this country has plummeted by more than two-thirds. Hospitalizations, an even more reliable measure of the pandemic since they reflect the number of people suffering from serious disease, are falling, too: Fewer than 60,000 people are hospitalized today with covid-19, as opposed to more than 130,000 for several days last month.

Deaths are a lagging indicator, but those, too, have fallen sharply. On Saturday, according to The Post’s tally, the seven-day daily average of deaths was 1,932 — the first time that figure had fallen below 2,000 since Dec. 4.

Why such dramatic improvement? For a definitive answer, you’d have to ask the virus, but there are lots of possible contributing factors. We’re past the string of holidays and occasions — Thanksgiving, Christmas, New Year’s, the Super Bowl — that encouraged risky behavior such as large gatherings. Maybe the virus is having a hard time finding new targets in some hard-hit communities. It could be that the coronavirus follows some cyclical rhythm that scientists do not yet fully understand.

But however exhausting it might be, we all have dreary, routine work left to do to combat the virus and to protect ourselves and our fellow citizens. It helps that the federal government now sends a consistent message on the need for the simple measures that are known to prevent transmission of the coronavirus: mask-wearing, hand-washing and social distancing.

President Biden and his team not only preach these practices but also model them almost religiously — as opposed to the way Donald Trump and his minions equated masklessness with “freedom.” Misguided souls who are convinced that covid-19 is some kind of hoax are probably still unmoved. But others who might be tempted to go without masks at least now cannot use the behavior of the president of the United States as an excuse.

Another possible factor in the apparent waning of the epidemic is the relative success of the rollout of the Pfizer and Moderna vaccines, which are both cause for hope and a reminder of the need for persistence.

True, demand so outpaces supply — at the point of delivery, at least — that vaccine-seekers loiter outside vaccination clinics at the end of the day on the off chance that there may be leftover doses. Where I live, the official sign-up list for getting vaccinated was first maintained by the county, then by a local hospital, then once more by the county; now it has been taken over by the state health department. Securing an appointment online involved patience and persistence, and quite a bit of swearing at my laptop and my phone. But I did finally get one. It was worth the effort, both for the relief I felt for myself and my family, and for the small contribution I can make to our collective herd immunity.

As of Monday, according to The Post, 44.1 million Americans had received at least one shot of the two-shot vaccines. That represents more than one-third of the initially targeted population — medical personnel, those over 65, essential workers — and about 13 percent of the total U.S. population. We can and must do better. But according to figures compiled daily by the BBC, only Israel, the United Arab Emirates and Britain have vaccinated more of their citizens per capita than the United States.

In other words, we did much worse than other wealthy countries, and even some poor countries, in controlling the spread of the coronavirus and preventing serious illness and death. But we have done much better than most countries in getting vaccines into people’s arms.

The coronavirus variants are troubling, especially the one first identified in South Africa. It is possible that the virus could come roaring back as quickly as it has ebbed. This is a new disease, and anyone who tells you we fully understand it is lying.

But we now have serious, competent leadership that believes in science, not conspiracy theories. And whatever the reasons, the tragic covid-19 toll — still too high — has fallen dramatically from its horrific peak. We have lost an unimaginable 500,000 lives. But we can keep from losing 500,000 more.

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