Data: DRC Ministry of Health; Chart: Andrew Witherspoon, Harry Stevens/AxiosThe long-simmering Ebola outbreak in the Democratic Republic of the Congo entered a new and more ominous phase this week, with the cross-border spread of a handful of cases into western Uganda.Why it matters: The cross-border spread puts more pressure on the World Health Organization to declare the nearly yearlong outbreak a public health emergency, which it has proven reluctant to do. A committee will meet Friday to determine whether to take that step.But, but, but: The international spread of Ebola was expected by health experts given the proximity of the outbreak areas in the DRC to the heavily trafficked border with Uganda.Health authorities in Uganda had been vaccinating health workers in anticipation of seeing cases.The overall trajectory of the outbreak remains unchanged, as health workers have been unable to break through community distrust and a violent insurgency inside the DRC.The high percentage of cases that are community cases, ones in which health workers were not aware of infected people or the individuals they have come in contact with, is especially concerning.Details: This Ebola outbreak is already the second largest on record, and even the deployment of a successful vaccine has proven insufficient at arresting its spread. A key reason for this is the challenging security environment in which responders are operating.Health workers have been attacked and killed, and levels of community mistrust remain high in affected areas.“Insecurity and community mistrust are a huge barrier, and we don’t have ready-made answers for dealing with either of those,” says Stephen Morrison, director of the Global Health Policy Center at the Center for Strategic and International Studies.Health workers in the DRC and Uganda are conducting ring vaccination campaigns using a vaccine supplied by Merck known as the rVSV-ZEBOV vaccine.But the vaccine is not sufficient for preventing all new cases and fatalities with so many community cases.“The vaccine is not a silver bullet for this outbreak,” says Morrison.The impact: Given the continued increase in cases in the DRC and the potential for additional spread into neighboring countries, health officials have expressed concern that the vaccine supply may run out in the next few months.In May, a WHO advisory group recommended that the DRC offer lower doses of vaccines and to consider using other experimental vaccines to stretch supplies.What we’re watching: Unlike the 2014–2016 West Africa epidemic, this one has skirted below the radar of world leaders. The appointment of a high-level UN coordinator with extensive security experience has raised hopes of a more robust response.So far, the WHO has taken the lead in responding, sending hundreds of personnel to the region.The CDC has just 15 experts in the DRC, but the security situation has kept them confined to cities away from the outbreak zones, an agency spokesman tells Axios.Morrison and Jennifer Nuzzo, a senior scholar at Johns Hopkins Center for Health Security, say the U.S. and other nations need to reassess their contributions to stopping this outbreak.The bottom line: On its present course, Morrison thinks the outbreak could persist for another 1–2 years. The longer it lasts, the greater the chance it has to reach major population centers.Go deeper: Axios' complete Ebola outbreak coverage