Fast-growing COVID-19 variants are contributing to an alarming rise in new cases around the nation, yet California remains resistant — for now.

This week, California had the third-lowest new case rate of any state in the union, sporting just 45 new cases per 100,000 residents, bested only by Arkansas and Alabama.  But variant-fueled outbreaks have pushed case rates up 13 percent nationwide and far higher in the worst states, contributing to fears of a fourth wave.

As the Golden State plans to fully reopen in June, the contrasting numbers have sparked an unsettling question: Could California’s luck disappear, or has the worst of its pandemic passed?

“We find ourselves at a confusing juncture in the pandemic, with a lot of cross-cutting forces — we’re toggling between some days that feel very optimistic and quote ‘impending doom’,” said Dr. Bob Wachter, chair of the Department of Medicine at UC San Francisco, in an update to colleagues Thursday. “All of these numbers are just fantastic, and yet you hear about other parts of the country and other parts of the world and the variants, and it’s easy to be pessimistic.”

Over the past six weeks, Michigan has emerged as the country’s worst hotspot for the virus. The state’s latest case rate is more than 10 times worse than California’s, with 492 new cases per 100,000 residents in the past 7 days.

Other states showing concerning trends are mostly clustered in the north and east of the country. New Jersey, New York, Minnesota and Pennsylvania round out the list of the five states with the highest rates, followed by Connecticut, Delaware and New Hampshire, according to the latest data from the U.S. Centers for Disease Control and Prevention.

Several factors may explain why California cases have stayed low while other states spike, said Dr. John Swartzberg, professor emeritus at UC Berkeley’s School of Public Health. Many residents of the Golden State could still be protected by antibodies hanging on from the breathtaking winter surge, and the population may be masking and social distancing more vigilantly than residents elsewhere.

But perhaps more important, California has so far mostly escaped the grip of the B.1.1.7 strain first identified in the U.K. and now ripping through Michigan. The more contagious coronavirus variant has grown from less than 5% of sequenced samples in the U.S. in mid-January to over a quarter by mid-March, according to the CDC. Florida leads the nation with nearly 3,500 known B.1.1.7 cases, Michigan has identified more than 2,000 cases, and Minnesota more than 1,500.

It’s impossible to know how and why B.1.1.7 took hold in Michigan and is far less prevalent elsewhere, Swartzberg said. The strain is about 50% more contagious than the classic coronavirus.

“The virus is like a forest fire that blows an ember out, and an area randomly lights up, and it’s like ‘Why there?’ ” Swartzberg said. “It may be because the wind blew it there — or because we just don’t know — but we don’t see the virus spread uniformly through the U.S. We see it flaring somewhere, and then flaring somewhere else.”

Still, California B.1.1.7 cases are on the rise, with 980 statewide as of Thursday. This week, the Stanford Clinical Virology Laboratory also confirmed five more cases of the so-called “double-mutant” strain first identified out of India and a case of the P.1 “Brazil” variant.

The B.1.427/B.1.429 “West Coast” strains remain the most common variant in California; together they are responsible for more than 12,000 cases. But they’re less contagious than the U.K. variant and have yet to prove as serious, Wachter said.

“So we are pretty lucky that’s the dominant variant here, whereas if we had the U.K. variant, we’d be in worse shape,” Wachter said.

California’s hope to maintain its gains depends on vaccinating as many people as possible before variants take greater hold and the June 15th reopening date occurs, said Dr. Peter Chin-Hong, an epidemiologist with UCSF. After a slow start, California has vaccinated about 35.1% of its population compared to 34.7% of Americans nationwide, according to the Kaiser Family Foundation vaccine tracker.

Although the hardest-hit states have reached similar thresholds — about 32% of Michiganders have been vaccinated, for instance, and 35% of Minnesotans — they didn’t have as much of a head start before B.1.1.7 cases took off.

“We have ammunition from vaccines, and we also have ammunition from natural immunity — that’s keeping us in the forcefield,” Chin-Hong said. “I’m pretty optimistic.”

But others are less certain that the state will be able to avoid a fresh swell of cases in the coming months. No future uptick is likely to be as bad as what the state experienced in the winter, Swartzberg said, but he’d still bet that “we’re going to have a problem.”

“If history is prologue, I can make a good argument that we’re in a very precarious position right now,” Swartzberg said. “We’re still in a good position, but it’s precarious.”