If Americans do not limit their holiday gatherings this year, the post-Christmas coronavirus surge could have devastating effects on a country that has already topped 18 million cases of COVID-19 and has experienced the deadliest year in U.S. history largely due to the pandemic. “The virus transmission that took place during the Thanksgiving period has brought many health systems across this country to the brink of failure and we are still not yet finished seeing that impact,” Dr. John Brownstein, chief innovation officer at Boston Children’s Hospital, told ABC News on Wednesday.”Layering in the December holiday travel and gatherings means we will have a third jump in this wave or a surge on surge on surge,” Brownstein continued. “Unfortunately, this increase will likely lead to catastrophic impacts on hospital capacity and the acceleration of terrible mortality milestones through February.” Brownstein’s warnings come as the Transportation Security Administration (TSA) screened over 2 million passengers at airports across the U.S. on Saturday and Sunday, a consecutive record high for the first time since March, despite the Centers for Disease Control and Prevention (CDC) urging Americans to stay home in the fight against COVID-19. ALLERGIC REACTIONS TO VACCINES ARE RARE, SHORT-LIVEDCases and hospitalizations are already surging across the country, with California, in particular, feeling the firsthand effects of what experts said Tuesday is likely a continued impact from Thanksgiving gatherings. Many hospitals in the state are at or on the brink of capacity, and Gov. Gavin Newsom has warned that COVID-19 hospitalizations in California could reach 100,000 by January.In a joint press conference, officials with some of the state’s larger hospital systems, including Kaiser Permanente, Sutter Health and Dignity Health, as well as officials with California Health and Human Services Agency, and others, pleaded with Californians to continue to take precautions against COVID-19, saying to not “share your air with others.” HEAT MAY EFFECTIVELY KILL COVID ON PLANES, BOEING STUDY FINDS”As we’ve heard, California is in a crisis mode – we are breaking records that we do not want to break,” such as the number of hospitalized COVID-19 patients in the state, and the number of patients requiring ventilators or intensive care, said Dr. Thomas McGinn, the executive vice president of the Physician Enterprise at Dignity Health, during the conference. McGinn said medical workers have a “simple prescription” for Californians: “Do not share the air.” “It’s been a long nine months,” he added. “Now is the time to be disciplined, now is the time to stick with it.” Fox News’ Janine Puhak contributed to this report.