The cases of COVID-19 have risen dramatically in Europe and China due to highly transmissible variants of the omicron virus. In the U.K., a recent analysis by the Health Security Agency found that the BA.2 subvariant of omicron is growing 80% faster than BA.1, the virus that caused the wave of infections in the U.S. last winter. Both cases and hospitalizations are rising in several European countries, as well as in the U.K., where the BA.2 strain is dominant, says CNN.

But here in the U.S., where the BA.2 is also the prevailing form of COVID-19, new case rates have not begun to rise. Over the past two weeks, they have even fallen by 1%, according to The New York Times. The expected surge that affected Europe and other parts of the world hasn’t happened, puzzling experts.

“It has not taken off,” said epidemiologist Michael Osterholm, at the University of Minnesota. There may be several reasons why this has not happened — yet.

1. More immunity. Even though our overall vaccination rates are not as high as those in Western Europe, our polarized view of COVID-19 may have imparted greater immunity. Some communities scored high vaccination rates, while people who eschewed vaccines in conservative areas may have achieved natural protection through infection. Unfortunately, they also had greater death rates. But some experts like William Hanage, associate professor of epidemiology at Harvard, say that we can still expect caseloads to rise, but not as dramatically as in Europe.
2. Fewer laboratory tests. The increase in home testing means that fewer cases of COVID-19 are appearing in government data that relies on the results of laboratory testing. Since uninsured people have had to pay for some tests, the number of tests has been reduced for low-income people, says The Times. Therefore, the number of cases may be dramatically underreported.
3. It’s too early to tell. Statistics show that about 73% of Americans are now immune to the omicron variant, according to WebMD. The high percentage of immunity from vaccination and previous infection tends to shorten new illnesses and reduce the amount of virus circulating overall. But experts warn that this percentage may not be high enough to stop new waves or reduce the burden in hospitals, as immunity wanes over time. “It may be too early to see a signal,” epidemiologist Jennifer Nuzzo, of Brown University, told the Times.
4. We don’t know enough about how the new variants spread. Even though the BA.2 variant has become the dominant virus in some countries, such as India and South Africa, it has not caused a spike in cases. Epidemiologist Osterholm admits “I don’t know, and I don’t think anybody knows,” when asked why the U.S. has not mirrored the European surge in COVID-19 cases. He points out that only the original omicron variant spread globally in predictable ways, while other versions have risen and fallen without causing increase in cases. Virologist Stephen Goldstein, of the University of Utah, adds: “This virus has certainly surprised us before, and we need to stay vigilant.”