Science

Why Does the U.S. Always Seem to Get Coronavirus Variants After Europe Does?

The latest twist on omicron is following a familiar pattern.

A crystal ball held up to a view of an escalator, inverting it.
Photo by Nigel Tadyanehondo on Unsplash

COVID cases have recently spiked in Europe, fueled by dropping temperatures, indoor socializing, and an unwelcome brood of antibody-dodging omicron subvariants. European health officials believe this may only be a taste of what’s to come this winter. Experts are warning these upticks overseas portend a wave in America, and not just because we’re more likely to get sick when we stay inside close to others. The variants B.Q.1. and B.Q.1.1, which first showed up overseas, could specifically help drive spread. “In the past, what’s happened in Europe often has been a harbinger for what’s about to happen in the United States,” Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, tells NPR. “So I think the bottom line message for us in this country is: We have to be prepared for what they are beginning to see in Europe.”

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For much of the pandemic, events in Europe have indeed seemed to be a divination of what will happen next in the U.S. At the pandemic’s onset, the virus ripped through an unprepared Italy just a few weeks before it devasted New York City. During the delta wave, cases began to rise in the UK in June; they rose in the U.S. in July. Omicron caused cases to rise sharply again in the UK in December, then in the U.S. spiking about a week later. Earlier this month, cases peaked in France, Germany, and other countries—turning Europe into a global hotspot. If history is any guide, the European surge suggests the U.S. should begin to experience its own surge soon. But why do cases seem to follow this pattern? Is Europe really a crystal ball for COVID in the states?

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Why the Europe-to-U.S. trend persists is complex, say experts I spoke with, and they emphasized that there’s no definitive answer. “There are a lot of hypotheses out there that could explain it,” says Lauren Ancel Meyers, director of the COVID-19 Modeling Consortium at the University of Texas in Austin. One major factor is global travel patterns. Europe is a linchpin of global travel, meaning waves that begin elsewhere in the world may tend to be channeled through Europe before reaching the United States. Based on common airline routes, it makes sense that COVID would have initially spread from Asia to Europe first. It also makes sense that delta, which originated in India, would have spread to the UK before reaching the United States, as India and the UK have closer travel ties.

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In the first few weeks of the omicron outbreak in South Africa, Meyers and her team analyzed Facebook mobility data to correctly predict that cases would peak in certain European countries before the United States. Travel connectivity could also partly explain why the gamma variant, first documented in Brazil in December 2020, failed to sweep the globe. Travel-wise, South America is the least-connected continent. In other words, travel routes are like well-worn riverbeds that channel the flow of COVID waves, and Europe is mostly upstream of the U.S.

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If travel routes lay down the path for global waves, population immunity could help reinforce the rhythm. We now know that antibody immunity against infection wanes after a few months. This means that all else being equal, people in places that first experienced a wave will be the first ones susceptible to the next wave. This piece of the theory is a little complicated, though, because immunity isn’t just about infections, it’s about vaccines too. Vaccines were not rolled out simultaneously around the world, with wealthier countries hoarding them. Only 23 percent of people in low-income countries have received a first dose. And boosters have proven variably popular around the world. For example, 83 percent of Italians have enthusiastically embraced multiple shots, compared to 38 percent of eligible Americans. And only 7 percent of eligible Americans have shown interest in the latest omicron-specific booster. So: it’s possible that immunity is playing a role in sustaining the Europe/U.S. pattern, but the caveat is the variable of vaccination here. You could imagine a future in which disparities in vaccination—and booster uptake—tip the trend in another direction.

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The final wildcard is that each country has by now been exposed to its own unique mix of variants. This could explain why some more recent waves have bucked the trend seen earlier in the pandemic. Take the omicron subvariant soup. Last winter, both the UK and the U.S. faced BA.1, the original omicron. But the wave in the U.S. was more widespread than Britain’s. When 2nd-gen omicron BA.2 came around, cases surged again in the U.K., but didn’t reach the same peaks in America. America’s wall of immunity was so freshly built that the BA.2 wave crashed against it and receded.

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Cécile Viboud, an infectious disease epidemiologist at the National Institute of Health’s Fogarty International Center and lead on the COVID-19 Scenario Modeling Hub, says this complex immunological landscape makes it increasingly difficult to predict exactly how COVID waves will propagate. People have had “one or more natural infections plus a bunch of vaccinations. And as new waves arise, they boost immunity, so we have really shifting patterns of susceptibility.” At this point, “I think you could almost find any country that at one point has more immunity than another country, and then it flips a few weeks later,” she says.

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Subtler factors could also be at play, too. “There could be sociological or even climatic reasons,” Meyers speculates. “I don’t actually know how the school schedules or the patterns of being indoors or outdoors differ between the United States and European countries, but there could be something about the cadences of life and climate that lead to earlier surges in Europe.”

Both Viboud and Meyers emphasize that these are all hypotheses, and data has yet to establish that the Europe-to-U.S. trend will continue. “It’s not true that 100 percent of the time, what happens in Europe is going to happen here,” says Meyer. “It has often happened, but not always.”

So what does this all mean for the U.S. in the fall of 2022? Cases have peaked in Europe, but the data in the U.S. right now is messy. The number of positive tests each day is holding steady, but fewer people are testing overall, and who calls up their local health department after a positive home test? Levels of the virus in wastewater are creeping upward, according to the CDC, but data from the wastewater surveillance company Biobot Analytics, fails to register this uptick.   It’s possible our population has been so infected in past waves that we have a wall of immunity. It’s also possible that because of dismal booster uptake, we’re more susceptible to the new omicron subvariants—which were scant in the U.S. at the beginning of the month but today account for 25 percent of new cases.

More certain than the Europe/U.S. trend, though, is the fact that it will be winter here soon, driving people indoors, and to large gatherings, likely causing cases to rise somewhat regardless of the variant driving them. How this will affect you specifically may depend on where you live and how behavior mixes with the complex dynamics of the virus and our defenses against it.   “Things happen at different times in different parts of the country,” says Meyers. “Risks can change at a very local level, so pay attention.”

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