The variant has upended Broadway, ballet and concerts. But the Met has yet to miss a performance, thanks to strict rules, fill-in artists and luck.The Metropolitan Opera had to scramble to find a replacement for its “Magic Flute” conductor after she tested positive for the coronavirus last month. When a wicked stepsister in “Cinderella” tested positive shortly before a performance in late December, the Met enlisted a soprano from another production to sing the role from the wings while a dancer acted it onstage.And earlier this week, when the star of its new production of “Rigoletto,” the baritone Quinn Kelsey, exhibited cold symptoms, the Met insisted on using an understudy, even though Kelsey had not yet tested positive for the virus and had just received some of the best reviews of his career.The Met’s prudence paid off. Kelsey later tested positive, and the rest of the cast had been spared a close contact.The Omicron variant has toppled a slew of Broadway shows, disrupted dance productions, postponed festivals, forced the cancellation of dozens of concerts, and closed the mighty Vienna State Opera for almost a week. But it has yet to stymie the Metropolitan Opera, the largest American performing arts organization, which has not missed a performance this season.Undaunted by the sharp rise in coronavirus cases, the Met has staged more than three dozen performances since late November, including productions of “Tosca,” “The Magic Flute,” “Cinderella” and “Rigoletto.” More than 3,000 people, who wore masks and showed proof of vaccination, filled the auditorium on New Year’s Eve. Rehearsals are in full swing for another two dozen performances this month, each involving hundreds of people: solo singers, orchestra players, chorus members, dancers, actors, stagehands, follow-spot operators, dressers and makeup artists, among many others.More than 3,000 people, who wore masks and showed proof of vaccination, filled the Met for the premiere of “Rigoletto” on New Year’s Eve.Richard Termine for The New York Times“We’re doing everything we possibly can to keep the Met open,” Peter Gelb, the company’s general manager, said in an interview. “I’m determined not to cancel a performance.”The Met’s success so far in managing the surge can be attributed to a number of factors: strict health protocols, a robust system of understudies, the advantages that come from its structure as a large repertory company that mounts a different opera each day — and, to be sure, a dose of luck.“There’s a sense of, ‘We can do this!’” said Sarah Ina Meyers, who directed the revival of “The Magic Flute,” which completed a nine-performance run on Wednesday with the help of far more cover artists than usual. “We’re trying to lift each other up.”Still, Meyers added, after weeks of grappling with last-minute cast changes, drafting and then tearing up plans, “there is profound hope that we can go back to the normal level of crazy.”The Met’s health protocols are among the strictest in the performing arts. The company now gives all employees P.C.R. tests three times a week, recently began having singers wear face masks even at dress rehearsals, and soon will require employees and audiences to have received booster shots to enter its building.The company had a robust system of fallbacks even before the pandemic struck, since its singers must be at their physical best to fill its cavernous opera house without the aid of amplification, and illnesses, whether hay fever or flu, have always required last-minute substitutions. Unlike Broadway, where shows often assign one actor to serve as an understudy for multiple roles, the Met appoints at least one cover for every role, greatly reducing its chances of having to cancel.Being a huge repertory company helps, too. Since it stages a different opera each night, with several titles in rotation onstage and others in rehearsal at any given time, the Met has a large pool of singers and crew members to draw on when a crisis erupts.And since the company performs a great deal of standard repertory, often in productions that remain the same for years, when a singer falls ill it is usually possible to find another who already knows the part (and even the staging) well. There tend to be several days between performances of each title — so a mild illness might only require missing a couple of shows.By pushing forward, the Met’s leaders hope to signal that the opera house can get through the turmoil of the pandemic and beyond. “The fact that we are performing provides a beacon of hope to our audiences and to our donors,” said Gelb, who tested positive for the virus late last month and had to watch live feeds of several key rehearsals from home. “We just have to make sure we survive the pandemic.”Omicron came just as the company was beginning to feel more confident after losing over $150 million in anticipated revenues because of the pandemic. While ticket sales in the fall were overall about 10 to 20 percent below prepandemic levels, there were several successes: a popular new production, “Fire Shut Up in My Bones,” the company’s first work by a Black composer; the staging of a six-hour work, Wagner’s “Die Meistersinger von Nürnberg,” the longest in the Met’s repertory; and a revival of Puccini’s “La Bohème” that was a hit with audiences and critics.As Omicron began to spread, the Met moved to strengthen its virus-control measures. Since the beginning of the 2021-22 season, it has required employees and audience members to be fully vaccinated and to wear masks inside the opera house.Quinn Kelsey, standing, and Craig Colclough wore masks while rehearsing for “Rigoletto” last month.Julieta Cervantes for The New York TimesThen P.C.R. testing of employees and artists increased to three times a week, from twice. The Met began to more strictly enforce a policy prohibiting employees with cold-like symptoms from entering the opera house, even if they have tested negative for the virus. It has also discouraged its employees from attending indoor social gatherings.The rules have been burdensome, especially for singers, many of whom find wearing masks while rehearsing awkward. But after going without stable work for much of the pandemic, as the Met and other institutions were closed, they have complied.“It’s uncomfortable, it’s something that we wish we didn’t have to do,” Kelsey, the Rigoletto, said of the masking requirement. “But at the end of the day it just means we’re that much closer, we hope, to putting all this mess behind us.”Even with the health protocols, the coronavirus has wrought havoc, sidelining singers, orchestra players, dancers, actors and stage hands. Since Thanksgiving, 124 people have tested positive for the virus among the Met’s stage crew, construction, wardrobe, wig and makeup, and costume departments, though most are now back at work.In the orchestra, eight people have tested positive; they, too, are largely working again. The Met has a pool of extra musicians who play regularly even when there are no illnesses, making substitutions relatively easy. (New York City Ballet, which halted its jam-packed “Nutcracker” schedule on Dec. 21, had instituted a rule that three connected virus cases within the company would spur a shutdown, to prevent further spread.)When Kelsey came down with cold-like symptoms this week, his cover, Michael Chioldi, jumped into action, getting fitted for costumes and going over technical cues just a few hours before the performance.“It’s been very stressful,” Chioldi said in a telephone interview from his dressing room shortly before his debut on Tuesday. “We’re just really, really hoping and praying that the Met stays open and that we can fill in when people go out, because inevitably people are going to get the virus.”Linda Gelinas (right, with Maya Lahyani in green and Stephanie Blythe) jumped in to act one of the stepsisters in “Cinderella” while Vanessa Becerra sang the role from the wings.Met OperaWhen the singer playing the stepsister in “Cinderella” became ill, the Met brought in a soprano, Vanessa Becerra, who happened to be taking part in “The Magic Flute.” She sang the role from the wings while Linda Gelinas, a former Met principal dancer who had not performed with the company in six years, acted it.With only a few hours to prepare, Gelinas studied videos and raced to memorize stage directions.“I thought it was a joke, but then I very soon realized, ‘Oh my gosh, they’re serious,’” Gelinas said. “Once the decision was made, we just went full speed ahead.”With Omicron infections still rising, it is unclear whether the Met can maintain its streak — and whether audiences will continue to turn out in large numbers. Attendance has been uneven in recent weeks. While it was 87 percent at the New Year’s Eve opening of “Rigoletto,” “Tosca” is expected to end its run this month at just 55 percent.But opera fans have celebrated the Met’s ability to remain a bastion of live music even as other venues have taken a pause.JunHyeok Lee, 27, a student at Baruch College from South Korea who attended the “Rigoletto” opening, said he felt privileged to be there at a time of uncertainty about the virus.“It’s a great blessing,” Lee said. “I’ll go every time unless the Met stops.” More