Los Angeles County reported another surge in coronavirus cases Friday, prompting health officials to urge residents to curtail large New Year’s celebrations.The county recorded more than 27,000 new cases on the final day of 2021, more than during the last two days combined. Roughly 1 in 4 people who are being tested are positive for a coronavirus infection, officials say.The numbers underscore how the highly transmissible Omicron variant is spreading throughout Southern California with unprecedented speed. California’s reported average daily coronavirus caseload has more than quadrupled in the last two weeks — pushing infection levels significantly higher than at any point during the summer surge.“The days ahead will be extraordinarily challenging for all of us as we face extraordinarily high case numbers reflecting widespread transmission of the virus,” Barbara Ferrer, L.A. County’s public health director, said in a statement Friday. “In order to make sure that people are able to work and attend school, we all need to act responsibly.”Ferrer said residents should avoid large indoor gatherings. The risk of transmission is highest for people who are unvaccinated or have underlying health issues.“Indoor parties, in particular, create significant risk, as this virus can be spread through aerosolized droplets. Older individuals, those with underlying health conditions and those who are immunocompromised should take additional precautions by not attending indoor gatherings with non-household members during this surge,” the county said in a statement.Despite the ballooning infections, far fewer patients are hospitalized with COVID-19 than during the last two surges, and health officials appear increasingly optimistic that symptoms in cases linked to Omicron are less severe than those from other variants of the coronavirus.“With explosive transmission likely to continue for some weeks to come, all efforts now need to focus on protecting our healthcare system from becoming overwhelmed. Since most people in our hospitals with serious illness from COVID are unvaccinated, those not yet vaccinated or boosted need to please stay away from others as much as possible to avoid getting infected or infecting others,” Ferrer said Friday.But the huge increases in cases has health officials worried that hospitals may be slammed by a crush of patients if transmission remains this high.Over the weeklong period to Wednesday, California announced an average of 23,637 new coronavirus cases per day, according to data compiled by The Times. Two weeks earlier, during the week to Dec. 15, the average was 5,792 per day.The summertime spike peaked at a bit more than 15,000 new daily cases. During the worst days of last winter, the daily average case count was closer to 45,000.Officials say the latest wave is likely fueled by factors that include increased travel and gatherings for the winter holidays and the breakneck proliferation of Omicron — which has mushroomed statewide since its presence was first confirmed at the start of the month.There have been growing calls across California to rein in gatherings.In Sonoma County, health officials said Wednesday that holiday gatherings accounted for 40% of new COVID-19 cases where the source of infection was known.“Throughout the pandemic, we have seen cases spike around holidays as people gather to celebrate with loved ones,” Sonoma County Health Officer Dr. Sundari Mase said in a statement. “Let’s start the new year on the right foot by staying safe and avoiding outbreaks.”San Diego County health officials urged residents to keep New Year’s celebrations limited to family and close friends who are vaccinated and boosted if eligible.On Thursday, California issued new recommendations for isolation of people infected with the virus, guidance that is stricter than that made earlier this week by the U.S. Centers for Disease Control and Prevention. California recommends that asymptomatic, coronavirus-infected people can exit isolation after the fifth day following a positive test, but only if they get a negative test result.By contrast, the CDC’s recommendations don’t ask for a follow-up negative test; the agency recommends that those ending isolation continue wearing a mask around others for five additional days.