About three weeks ago, COVID case rates in the United Kingdom made an abrupt about-face, spurred on by a more transmissible Omicron subvariant called BA.2. (So far, there is no reason to believe the new subvariant causes more severe disease.) Case rates are rising, too, in Switzerland and Greece and Monaco and Italy and France. Given that BA.2 is already present in the United States, The Washington Post reports that epidemiologists and public-health leaders suspect that North America will be next. After all, the paper said, “in the past two years, a widespread outbreak like the one in Europe has been followed by a similar surge in the United States some weeks later.”
It’s true that watching the Delta and Omicron waves in Europe last year was like peering into a crystal ball of America’s pandemic future. Cases in the U.K. started to rise in early June, peaked roughly a month later, and bottomed out in early August. In the U.S., the surge began in July, peaked in September, and reached a low point in October. Cases shot up again in the U.K. starting around December 10 and peaked on January 4; the U.S. followed on December 18 and January 10, respectively. Britain hit its post-Omicron trough at the end of February. If the pattern continues, we should be hitting ours … right about now.
But this correlation hasn’t always held. If, over the past two years, some surges in European countries have been trailed by ones in the U.S., others simply haven’t been. And the wave we’re now seeing overseas may well end up among the latter. Differences between the U.S. and European nations in variant levels, previous infections, and pandemic policy could keep our case rates on a different track. “There are a number of things which tug the U.S. experience away from the European one,” Bill Hanage, an epidemiologist at Harvard, said in a press call yesterday.
The fact that Europe has, at times, been two steps ahead of us might come down to chance. The most influential variants so far—Alpha, Delta, and Omicron—were each first identified in places—the U.K., India, and South Africa—that are more connected by travel to Europe than to the U.S. These variants simply arrived in Europe earlier than they arrived in the U.S., but that trend could easily reverse. “If the next variant starts in Brazil, then it’s far more likely to go to the U.S. before it comes to Europe,” Graham Medley, an infectious-disease modeler at the London School of Hygiene and Tropical Medicine, told me. “We’re all following each other.”
Vaccination rates, the type of vaccine used, and previous infection patterns could also have influenced the Europe-then-America trend, says Shaun Truelove, an epidemiologist at Johns Hopkins. For example, AstraZeneca’s vaccine, which was commonly used during the early phases of the U.K.’s vaccine rollout, does not ward off infection as well as Pfizer’s and Moderna’s jabs, which were the most popular shots in the U.S. from the get-go. “It’s a very complex system, so it’s hard to say exactly,” Truelove told me.
Differences in policy and behavior could also drive the timing trends, and right now, policies and behaviors in Europe are, shall we say, all over the place. In England, people who test positive for the virus are no longer asked to self-isolate; meanwhile, Spain and Italy only recently dropped outdoor mask mandates. “The restrictions that have been withdrawn in a lot of European countries include restrictions which have never been in place in much of the United States,” Hanage said, which might mean that Europeans’ lives have changed more rapidly than Americans’ over the past several weeks. The U.S., on the whole, hasn’t had many COVID restrictions in place since last summer.
Oddly enough, America’s recent laissez-faire approach to the pandemic has made case rates easier to predict here. Throughout the pandemic, the most difficult part of modelers’ jobs has been accounting for how policy and Americans’ behavior would change, says Lauren Ancel Meyers, who directs the COVID-19 Modeling Consortium at the University of Texas at Austin. But during the winter, schools largely stayed open and Americans largely went about their lives. Suddenly, the projections Meyers and her team made were spot-on. “We’re not used to being that accurate,” she told me.
But that doesn’t mean modelers are ready to say exactly what’s next for America. “What we found in the past two years is that the models have struggled at these critical change points,” Truelove told me. We’ll know if we’ve entered a trough, he said, only after it’s over and case rates climb up again. Meyers said she expects to have better predictions in a week or so. She wants more time to see whether cases start to plateau or increase in parts of the U.S. and to get more information about how long people are protected from infection or disease after a bout with Omicron. She also wants to know more about how easily BA.2 can infect people who have survived either of the two subvariants, BA.1 and BA.1.1, that have been responsible for the bulk of American cases since December.
BA.2 is thought to be slightly more transmissible than BA.1, and it’s already in the U.S. That might sound ominous, considering what’s happening in Europe, and it might also suggest that a U.S. wave is coming soon, according to the pattern set by Delta and Omicron. Hanage assured me that BA.2 will almost certainly beat out other variants here, too, but that doesn’t mean that the U.S. is doomed to suffer an identical surge. When BA.2 entered Europe, it took off almost immediately. In the U.S., Hanage said, its rise has been much slower, possibly because it’s competing with both BA.1 and BA.1.1. Even if BA.2 were to start taking over in earnest tomorrow, it would be doing so during a much lower trough, and probably less virus-friendly weather conditions, than it encountered when it made its bid for dominance in the U.K.
All of that could mean that BA.2 will have less of an effect here than in Europe. It’s happened before: In the last weeks of 2020, the Alpha variant started driving the majority of cases in the U.K. and contributed to a devastating surge. (The U.S. also experienced a devastating surge around the same time, with the highest death tolls of the pandemic, but Alpha wasn’t a major player; if it had been, the winter surge would likely have been even worse.) Alpha didn’t become dominant in the U.S. until the spring, by which point the weather was warmer and Americans were getting vaccinated. That could help explain why the U.S. didn’t experience much of an Alpha bump; if anything, the curve from that period looks more like a plateau. “We rather dodged a bullet with that one,” as Hanage put it.
If we’re lucky, we could dodge another with BA.2. Hanage said his best guess for our next few months is that some parts of the U.S. will continue on their downward trend, but at a slower pace. Other areas will probably experience a bump in cases—wastewater data suggest that may be coming soon. But for now, the size of those bumps is anyone’s guess.