In the Marvel Cinematic Universe, the super-soldier serum that gave Steve Rogers his abilities has a secretive origin. Due to its enigmatic beginnings, fans have posited various theories on how Abraham Erskine's serum was perfected. One theory has tied the serum to Wakanda's Heart-Shaped Herb, the same herb that gave the Black Panther his abilities.
In the theory, the serum's ties to Wakanda began when Howard Stark obtained some vibranium to make Captain America's shield. Since Wakandans won't give away something so sacred, Stark potentially stole the herb and gave it to Erskine for research.
Before the herb came into the picture, the formula was far from perfect. Its first test subject, Johann Schmidt, was horribly altered and reborn as the villainous Red Skull. Theoretically, with the Heart-Shaped Herb, Erskine was able to eliminate any further dangerous side effects.
Coupled with Vita Rays, Erskine's serum was brought to life. As the years progressed, others tried to perfect the serum but missed the vital ingredient of the herb. One of the most famous failed attempts was Bruce Banner's Bio-Tech Force Enhancement which created the Incredible Hulk. Without the Heart-Shaped Herb, future attempts to recreate the formula evolved into less of an exact science and more of a risk, as its use killed its subjects more than gave them abilities.
Another theory that built off the Heart-Shaped Herb idea went into the history of the serum and how it failed more often than not without the herb. One of the rare occasions of success was with Isaiah Bradley, a soldier who was experimented on against his will and was the only one to survive the procedure while getting super-soldier abilities. Years later, the Flag Smashers would use a serum derived from his blood since outsiders did not have access to Wakanda's Heart-Shaped Herb.
The theories build a huge case for the Heart-Shaped Herb's involvement in the super-soldier serum. As it was only perfected with Steve Rogers, it's possible that the herb played the most important part. However, that didn't mean that there weren't other successes without the herb. Bucky Barnes, John Walker, Isaiah Bradley and the Flag-Smashers proved that super-soldiers could exist without it. Another factor is that Vita Rays weren't needed to activate the herb in its raw form, though this could've been added because it may lose its potency when mixed with the other chemicals.
Even though super-soldiers can exist without the Heart-Shaped Herb, there was always something that separated Erskine's original formula from the ones that came later. Erskine's was the only perfected version of the serum, and the result was Steve Rogers. Even Zemo said in The Falcon and the Winter Soldier, "There has never been another Steve Rogers." While the current serum is nearly perfect, it still has its flaws, as evidenced by Walker's lack of control when he first took it. However, for Abraham Erskine, it's possible the Heart-Shaped Herb made his original formula and Steve Rogers such a success.