Making sure unaccompanied youth have stable housing is a signature initiative in “Opening Doors,” the Obama Administration’s plan to end homelessness for all. In practice, the administration must help currently homeless youth exit the shelter system and, at the same time, prevent homelessness among those in precarious housing. Prevention is the toughest nut to crack.  It requires developing an intervention that prevents homelessness and identifying the high-risk kids who need to be “inoculated.”

We already have the immunization.  Research conclusively demonstrates that housing subsidies protect against homelessness.  To best serve unaccompanied youth between the ages of 18 and 24—a critical state in development—policymakers will need more information on the types of housing and service models that help young people transition to independent living and economic self-sufficiency.  It needs fine-tuning, but we know the basics of what works.

After that, it gets harder. To identify the risk group, prediction models can help us isolate characteristics, histories, or circumstances that lead to homelessness. But these models have high rates of false positives, even among highly vulnerable households—many who seem likely to become homeless never end up in shelters. That means it’s not cost-effective to provide services to all the at-risk youth who are poor and unemployed, battling family troubles, struggling to care for young children, or resorting to doubling up and couch surfing—even though many in these groups will become homeless. And we don’t  have the data to identify who in these vulnerable groups truly teeters on homelessness’ brink.

Luckily, we do know who is exiting foster care, one of homelessness’ so-called feeder systems. In 2009, for instance, some 30,000 young people aged out of foster care. For these youth, the statistics are grim.  They are at high risk for joblessness, teenage pregnancy, incarceration, and homelessness. Even more grim, according to HUD data, over the course of a year roughly 5,000 youth aging out leave the state’s care and, with no place else to turn, their next stop is a homeless shelter.

Even though young people aging out of foster care represent only a small fraction of all youth who end up homeless, helping them will put a dent in the problem, and it’s an obvious first step toward the larger challenge of ending homelessness for all youth.