By some estimates, about 40 percent of the population of the United States will have been infected with the omicron variant of Covid-19 by the time the current wave fully subsides. The WHO estimates that half of Europe will have been infected as well. And nearly all of those infections will have occurred between mid-December and the beginning of February.

It’s hard to say for sure, but there’s good reason to think that never before have so many people been infected with an emerging virus in such a short timespan. For most of history, diseases traveled much slower, carried by travelers on boats or horses.

It took years for great historical plagues like the Black Death to spread across Europe and Asia. One of the biggest infectious disease-caused mass death events in history, the spread of smallpox and other European-originating diseases through the Americas, is estimated to have taken a few years to a decade.

But now, thanks to our far more interconnected world, an incredibly contagious virus required only about two months to go from when it was first detected — November 11 in Botswana — to when likely more than 2 billion people had been infected.

We are incredibly lucky that omicron seems to be milder than previous strains of Covid-19 and that both vaccinations and previous exposure have built up immune resistance. The massive spike in cases around the world — while badly taxing health care systems — hasn’t been matched by an equal spike in hospitalizations and deaths.

I think it’s hard to appreciate what a massive bullet we dodged: If omicron had been substantially more deadly, there is very little we could have done to stop the death toll.

When defenses are overwhelmed

Earlier strains of the virus were successfully contained in some countries by maintaining strong border controls, aggressively quarantining people, and using traditional epidemiological tools like contact tracing.

China quashed a large initial outbreak with unprecedented measures, including surveillance, sealing off cities, locking people in their homes, and other policies more extreme than those employed even in other countries that successfully suppressed the virus, like New Zealand.

Nothing the world has tried works as effectively against a variant as contagious as omicron. It spread in Australia, which largely kept the first strains of Covid-19 out for two years. Australia now reports 1.5 million infections overall, and about 1.3 million of them in the last three weeks (though cases appear to be trending sharply down now).

China hasn’t yet acknowledged that even its draconian measures may not be a match against omicron, but it seems all but inevitable — especially when travelers for the Olympics start showing up.

Of course, omicron’s spread was aided by the accumulated fatigue of nearly two years of grappling with shifting Covid rules, plus the fact that the vaccinated and boosted are highly protected from hospitalization and death, reducing the risks after contracting an infection. But the high infection numbers in countries that have genuinely tried to keep a tight lid on omicron suggests to me that the world would lose the fight even if we’d been willing to have it.

It’s very hard to guess what would have happened if omicron had come first, and had been met with the social distancing measures, lockdowns, and caution that dominated in the first months of the pandemic — much less what would have happened if omicron had come first and the US had responded with a true closed-borders, forcible-quarantine approach like that of Australia.

One possibility, though, is that no society has an effective way to stop the spread of a virus as contagious as omicron — even if it kills 10 percent of those it infects, like the original SARS virus in 2003, or even if it has devastating long-term consequences for nearly everyone it sickens. We are terrifyingly vulnerable.

The warning shot

We don’t have to be. There have been very few advances in personal protective equipment over the last few decades, but it should be possible to design masks and personal ventilation units that are less unpleasant to wear and do more to prevent infection.

We can get better at altering vaccines to fit a new disease and deploying them fast — but it’d have to be incredibly fast to stay ahead of something that spreads the way omicron spreads.

And we can improve early detection, so we have more time to react to diseases that move as swiftly as omicron.

Let’s not kid ourselves, though: Omicron was a preview of what would happen if an extremely contagious new virus emerged. And what happened was that 40 percent of the US, and what the WHO estimates will be more than 50 percent of Europe, are infected in less than two months. Most of those infections would have been incredibly costly — both economically and socially — to prevent, even if the virus had been deadly enough to warrant the most extreme measures we’re capable of taking.

One note of optimism: Some experts think that a disease as contagious as omicron is nearly guaranteed to be less deadly than SARS-1 was in 2003. There are some evolutionary reasons to expect a trade-off between a disease’s transmissibility and its virulence. But that trade-off doesn’t always show up in real-life diseases; it would not be wise to count on it.

And we can’t even count on omicron being the last curveball this pandemic has to throw at us. All those infections created a perfect opportunity for new variants to develop, such as the BA.2 omicron variant currently spreading in parts of Europe and Asia. It doesn’t appear to be more dangerous yet, but we should be prepared for Covid-19 to throw us more curveballs.

Researchers who study biological risks have been warning us about the dangers a severe pandemic could pose for a long time, and a fast-moving, highly contagious virus has always been the nightmare scenario. But it’s one thing to write, as I did before omicron emerged, that in principle it is possible some variant of Covid-19 would emerge that’s as contagious as measles. (Omicron isn’t even as contagious as measles, by our current R0 estimates.) It’s another thing to see a highly transmittable variant happen, and see exactly what results.

Forty percent of the US in two months. That would have been apocalyptic if the viral variant were a deadly one. If we press the snooze button on this wake-up call, we might not get another one.

A version of this story was initially published in the Future Perfect newsletter. Sign up here to subscribe!