Operation: Rebirth, also called Project: Rebirth, is a US government project originally administered by the US Army to create a new line of Supersoldiers for its World War II effort. It was part of a larger operation known as Weapon Plus.
Operation Rebirth began as a collaboration between US, British and German eugenicists led by Doctors Reinstein and Koch. When World War II began, Koch took over the German program, and Dr. Abraham Erskine (under the code name of "Joseph Reinstein") became the primary source of the USA program with him independently creating both the chemical portion of the project that has been recreated to certain degrees and the Vita Ray treatment that has NOT been recreated since.
Original text states that Erskine's process would work reliably on any person which was the reason so many were dedicated to recovering Erskine's lost process.
Later text such as "The Adventure of Captain America-Sentinel of Liberty" series state that the chemical/Vita Ray process would only be occasionally successful as "in 1 out of 4" participants.
Two other subjects, Clinton McIntyre, a.k.a. Protocide, a failed experiment who was placed in suspended animation and was revived in the modern era by AIM. And the first mutant experiment designated Queen seem to have occurred prior to the serum being tested on Steve Rogers.
According to these later retcons, Erskine continues to refine the process and eventually succeeds in being able to convert frail Steven Rogers into the first Super Soldier who took the name: Captain America suggesting other failed participants may be found. (The original history states Rogers was the only test subject with no one else involved before him.  But Erskine is murdered moments after Rogers is successfully empowered. All of the refinements Erskine made to the program which made the process successful with Rogers, especially the Vita Ray portion, were lost with his death as he never fully documented all of his research to recorded notes. Though the German spy who murdered Erskine did, which was the basis of Rogers II finding and using the chemical portion of this process to become the Captain America of the 1950s.
With Erskine's murder, Wilfred Nagel took over the American program under the close code name of "JoseF Reinstein". Nagel's attempt to recreate Erskine's successful process, with Rogers, did not succeed in mass production. Nagel's early attempts to recreate Erskine's successful chemical formula resulted in African-American super soldiers (most prominently Isaiah Bradley). Three hundred African-American soldiers were taken from Camp Cathcart and subjected to potentially fatal experiments at an undisclosed location  in an attempt to recreate Erskine's Rogers successful Super-Soldier Formula version. Only five men survived the original trials; hundreds of test subjects left behind at Camp Cathcart and the camp's commander were executed by US soldiers in the name of secrecy, the families of the three hundred were told that they had died in battle.
While Nagel failed to create a working mass production version of Erskine's formulae, Nagel did make relatively successful samples, such as the one that kept then Colonel Walker Price younger than his chronological age and still muscular fit into his 90s.
Although there were many later attempts to recreate or reverse-engineer Operation: Rebirth's Super-Soldier Serum in detail, none are known to have been involved with Weapon Plus except for the attempt that resulted in the creation of Isaiah Bradley's son Josiah X nor have been successful in replicating the missing Vita Ray portion as with Rogers II, the Cap of the 1950s who was to have accurately recreated the chemical portion of the successful sample. Weapon Plus considers Steven Rogers as its most successful creation.