It's unlikely SpaceX or Boeing will transport astronauts to the ISS for a couple more years, according to a report from the Government Accountability Office (GAO). Due to the potential for even greater delays beyond 2019, alternative plans must be established.
Once the Commercial Crew Program is up and running, NASA can finally stop relying on Russia's Soyuz rocket to get astronauts to the ISS.
Since delays are likely, the GAO is recommending the US space agency figure out alternative plans for transporting astronauts beyond 2018. NASA has already purchased more Soyuz seats for next year and it'll figure out a further contingency plan.
Without a viable contingency option for ensuring uninterrupted access to the ISS in the event of further Commercial Crew delays, NASA risks not being able to maximize the return on its multibillion dollar investment in the space station, - GAO
Boeing is going to use a crew capsule called the CST-100 Starliner on top of an Atlas V rocket. SpaceX will be using a modified Dragon capsule on top of a Falcon 9.
Those vehicles must be certified by NASA before they join the program. Certification requires crewed test flights that both companies anticipate will occur in 2018. But the GAO doesn't expect certification will be provided until 2019, assuming other delays don't get in the way.
If the program doesn't start by 2019, NASA will need to extend its contract with Russia.