"It is hard for many people to intuitively appreciate exponential growth," Yale science professor and physician Nicholas Christakis tweeted Wednesday. "Nothing seems to be happening for a long time, until a lot happens at once." A lot happened on Wednesday night.

In short order, President Trump gave an Oval Office address, since clarified, that included a partial ban on travel to and from Europe, the NBA indefinitely suspended its season, Tom Hanks announced that he and his wife have tested positive for the COVID-19 coronavirus, and Sen. Maria Cantwelll's (D-Wash.) office said one of its staffers has also tested positive, putting the new virus inside the U.S. Capitol.

But right before any of that happened, former Alaska Gov. Sarah Palin was unmasked as the bear on The Masked Singer, and she rapped the Sir Mix-A-Lot hit "Baby Got Back."

Maybe that should have been a sign.

"When something dangerous is growing exponentially, everything looks fine until it doesn’t," Megan McArdle writes in The Washington Post. "The crisis in northern Italy is what happens when a fast doubling rate meets a 'threshold effect,' where the character of an event can massively change once its size hits a certain threshold. In this case, the threshold is things such as ICU beds. If the epidemic is small enough, doctors can provide respiratory support to the significant fraction of patients who develop complications, and relatively few will die. But once the number of critical patients exceeds the number of ventilators and ICU beds and other critical-care facilities, mortality rates spike."

In any case, you can watched Palin's masked performance below. Peter Weber

For two weeks, COVID-19 cases and deaths have dropped steadily from the alarmingly high peak of America's third wave of the pandemic. Vaccination rates are rising, new vaccines and millions more doses of approved ones are coming online soon, and deaths have dropped dramatically in places where significant numbers of people have been inoculated, especially nursing homes. But "this is not a time to relax," President Biden said Thursday as he celebrated the country's 50 million's vaccination. "We must keep washing our hands, stay socially distanced, and for God's sake — for God's sake — wear a mask."

It turns out, a fourth wave of the pandemic already appears to be building.

Cases have leveled out worldwide, too. "The most likely explanation is the more contagious variants of the virus, like the B.1.1.7 variant, which was first detected in Britain," David Leonhardt writes at The New York Times. Business Insider's Jim Edwards points out that the uptick in cases and deaths could be statistical noise, but notes that worrisome new variants have also popped up in California and New York.

"Taking into account the counterbalancing rises in both vaccinations and variants, along with the high likelihood that people will stop taking precautions, a fourth wave is highly likely this spring," Apoorva Mandavilli reports at the Times, citing a majority of 21 experts interviewed on the pandemic. "But they stressed that it is not an inevitable surge, if government officials and individuals maintain precautions for a few more weeks," and "COVID-19 deaths will most likely never rise quite as precipitously as in the past."

"The good news," Mandavilli writes, is that "despite the uncertainties, the experts predict that the last surge will subside in the United States sometime in the early summer. If the Biden administration can keep its promise to immunize every American adult by the end of the summer, the variants should be no match for the vaccines. ... For now, every one of us can help by continuing to be careful for just a few more months, until the curve permanently flattens." Peter Weber

The Democrats' push, most prominently by Sen. Bernie Sanders (I-Vt.), to raise the federal minimum wage to $15 an hour hit a significant snag on Thursday. But Costco, the No. 2 U.S. bricks-and-mortar retailer, raised the ante anyway, announcing Thursday — at a Senate hearing chaired by Sanders — that it is raising its own minimum wage to $16 an hour, starting next week. Costco set its lowest hourly wage at $15 in 2019, a year after raising it to $14. The federal minimum wage has been $7.25 an hour since 2009.

"I want to note: this isn't altruism," Costco CEO Craig Jelinek said at the Senate Budget Committee hearing. "At Costco, we know that paying employees good wages and providing affordable benefits makes sense for our business and constitutes a significant competitive advantage for us." About 90 percent of Costco's 180,000 U.S. workers are hourly employees, and 20 percent of them earn its minimum wage. The average hourly wage is $24, and Jelinek said the company has been paying a $2 hourly hazard bonus since March. That will end next month but be converted to wage increases company-wide, he added.

Costco's raise could pressure its large competitors to follow suit, CNN says. Target and Best Buy raised their minimum wage to $15 last year, while Walmart's minimum wage is $11, rising soon to $13 an hour for about a quarter of its workforce. Amazon's minimum wage has been $15 an hour since 2018. Peter Weber

Prince Harry evidently thought he was going to be on "Carpool Karaoke." "This is subtle — where's the Range Rover?" he asked James Corden when Corden arrived in an open-air bus for a tour of Los Angeles on Thursday's Late Late Show. The non-working-royal prince did rap the theme to The Fresh Prince of Bel-Air, but there was no singing on the tour. There was, however, high tea, an ill-advised real estate push invoking Meghan Markle, and a military-style obstacle course.

Prince Harry seemed to enjoy parts of the tour: He said he's never been allowed to ride on an open-top bus and has always wanted to go sight-seeing, and since he and his wife arrived in the U.S. during coronavirus lockdown, "this is the first time I've had a chance to see L.A." He described a typical day in his quasi-royal Santa Barbara household, starting with breakfast from a waffle-maker gifted to son Archie by his great-grandmother, the queen, and ending with him and the duchess in bed watching Jeopardy! or "a little bit of Netflix."

"And how do you feel about The Crown?" Corden asked. Unlike his brother, Harry has watched the show, and he had thoughts about how his family is portrayed: "They don't pretend to be news, it's fictional, but it's loosely based on the truth. Of course it's not strictly accurate," but "loosely, it gives you a rough idea about what that lifestyle, what the pressures of putting duty and service above family and everything else, what can come from that. I'm way more comfortable with The Crown than I am seeing the stories written about my family or my wife or myself," because one is "obviously fiction" and the latter is "being reported on as fact."

When asked, Harry said he wants Damian Lewis to play him when his storyline starts in The Crown. Corden nominated himself to play Prince William, earning a dubious stare from Harry. "It's not great casting, but it is casting," he said. You can watch him get the last laugh below. Peter Weber

The Senate on Thursday confirmed former Michigan Gov. Jennifer Granholm, 64-35, to lead the Energy Department, with 14 Republicans joining all 50 members of the Democratic caucus to give President Biden his 10th Cabinet-level appointee (plus one deputy secretary). After her confirmation, Granholm tweeted that she's "obsessed with creating good-paying clean energy jobs in all corners of America in service of addressing our climate crisis" and "impatient for results."

Granholm repeated her priorities on MSNBC Thursday night. "I am all about bringing clean-energy jobs" to communities, especially those, like Michigan, reliant on fossil fuels, she told host Chris Hayes. "I am totally obsessed about how to create good-paying jobs in America," and the clean-energy sector "is the biggest opportunity for us."

The market is shifting toward green energy, regardless of what politicians prefer, and the Energy Department's 17 national labs are creating ways to not only expand renewable energy but also "decarbonize fossil fuels," Granholm said. "And honestly, if we can bring the supply chains for all of these clean-energy products to the United States, instead of letting our economic competitors eat us for lunch, the jobs that could be created for us in the U.S. — good-paying jobs — are boundless."

Biden has sent the Senate more nominations, and gotten fewer of them confirmed, than any recent president, Axios reports, citing a count by the Partnership for Public Service and The Washington Post.

"The new president is facing a pandemic without a surgeon general or head of the Department of Health and Human Services, he confronts an economic crisis without his leaders at Labor or Commerce, and domestic terrorism is on the rise with no attorney general," Axios notes. You can track Biden's nominations at The Washington Post. Peter Weber

Illinois state Rep. Chris Miller (R), the husband of freshman U.S. Rep. Mary Miller (R-Ill.), acknowledged Thursday that his pickup truck was parked in a restricted area outside the U.S. Capitol during the Jan. 6 riot, but he said the "Three Percenter" militia sticker on the back window doesn't mean anything.

"Army friend gave me decal," Miller told The Daily Beast in an email late Thursday. "Thought it was a cool decal. Took it off because of negative pub." He said he "never was member" of the militia and "didn't know anything about 3% till fake news started this fake story and read about them." Online sleuths had linked him to the truck visible in footage from a CBS News report, earlier Thursday.

The Three Percenters, founded in 2008, are a "radical militia group" implicated in leading the Jan. 6 siege along with the Proud Boys, the Oath Keepers, and other far-right extremist groups, the FBI said in an affidavit filed in the case against alleged rioter Robert Gieswein. Their name comes from the apocryphal claim that only 3 percent of U.S. colonists fought in the Revolutionary War, and they fashion themselves as the same kind of tyranny-stomping "patriots."

Miller's wife, Mary Miller, is most famous for favorably quoting Nazi leader Adolf Hitler at a "Moms for America" rally outside the Capitol on Jan. 5. "Hitler was right on one thing: whoever has the youth has the future," she told the rally, apologizing later when video of her comments went viral but insisting that "some are trying to intentionally twist my words to mean something antithetical to my beliefs." Peter Weber

In Brazil, where the coronavirus is spreading uncontrollably, the COVID-19 death toll surpassed 250,000 on Thursday evening.

There have been 251,498 confirmed COVID-19 deaths in Brazil since the pandemic began a year ago, the country's Health Ministry said. This is the world's second-highest death toll, after the United States. Jair Bolsonaro, Brazil's right-wing president, has criticized efforts to curb the spread of COVID-19, slamming lockdowns, ignoring social distancing measures, and taunting people who wear masks.

Every day over the last two weeks, Brazil has averaged more than 1,000 daily deaths — the highest numbers since July. Christovam Barcellos of the Fiocruz research institution told Reuters Brazil is experiencing a "second plateau. It's not a second wave, because we've been over five weeks with 1,000 deaths per day."

The new coronavirus variant first found in Amazonas state that is believed to be more contagious has now been detected in at least 17 Brazilian states. Public health experts have been calling on Brazilian mayors and governors to impose lockdowns to keep the variant from spreading far and wide, but the measures that are being implemented, like nighttime curfews, won't do nearly enough.

"Right now, Brazil is the largest open-air laboratory, where it is possible to observe the natural dynamics of the coronavirus without any effective containment measure," neuroscientist Miguel Nicolelis tweeted on Tuesday. "Everyone will witness the epic devastation." Last month, he warned that if Brazil did not enter a lockdown to limit travel and gatherings, "we won't be able to bury our dead in 2021."

Vaccination efforts are now underway, but are off to a slow start. More than 210 million people live in Brazil, and the country has only secured 16 million doses and distributed about half. Brazil is using the vaccine developed by China's Sinovac Biotech Ltd., which researchers say is effective against COVID-19 variants first identified in the United Kingdom and South Africa. Tests are now underway to see if it works on the Amazonas variant. Catherine Garcia