Project Rebirth was a project assigned to the US Army by the US Government. This was so the Army could have a line of Supersoldiers to assist them in World War I. The project was part of a large operation called Weapon Plus. The operation was a collaboration between US, British and German scientists and it was lead by Dr. Abraham Erskine and Koch. When World War II started, the Geman part split from the original experiment and Koch took over that portion. Dr. Erksine became the primary leader of the USA program and created the chemical formula and the specific experiment which could create Supersoldiers. This formula has not been reproduced by any other scientist. Many people were intent on recovering Erksine's work since it was believed that the serum would work on any type of person although, the chemical and Vita Ray process was shown to work in 1 out of 4 patients.
Two other subjects were tested before this, Queen and Clinton McIntyre. McIntyre a.k.a. Protocide was a failed experiment which left him suspended in animation until he was revived by AIM in the modern day. Queen was the very first testing of the experiment and was tested on before Steve Rogers.
Before testing on Rogers, Erksine had to refine the process multiple times but eventually converted the frail Rogers into Captain America. Since the process had to be refined multiple times, it is suggested that other failed participants may be found. Original history makes the claim that Captain America was the only test subject.
All of the changes Erskine made to the process which made the experiment successful with Rogers, including the Vita Ray portion, were lost when he died as he never documented his research. The German spy who killed Dr. Erskine found some of his research, and that was the basis of Rogers II using this process to become Captain America again in the 1950's.
After Erksine's tragic murder, a man named Wilfred Nagel took over the project. Nagel's attempt to recreate the project on mass amounts of people did not succeed. But he did succeed in creating African-American Supersoldiers (the most widely known one was Isaiah Bradley). Three hundred soldiers were sacrificed and taken from Camp Cathcart to recreate Erksine's original experiment. Of those three hundred African-American soldiers, only five men survived and the rest either died or were executed. Nagel claimed to the families that they died in battle.
Nagel did make a few successful samples like Colonel Walker Price. Price was kept young and muscular into his 90's.
Weapon Plus believes Steven Rogers I is its most successful creation.