The Metroid vaccine that saves Samus's life in Metroid Fusion.

Vaccine "Metroid" was an anti-X Parasite serum that saved Samus Aran from death in Metroid Fusion.


Once the X Parasite had infested Samus's central nervous system, she lost consciousness, causing her Gunship to drift into an asteroid belt. Her ship's emergency systems automatically ejected the Escape Pod before impact, which Biologic Space Laboratories later retrieved and transported to Galactic Federation Headquarters. During the journey, the X multiplied within her, corrupting large areas of her Power Suit. It was learned that the organic components of her Power Suit were so integrated with her nervous system that it could not be removed while she was unconscious. Large portions of her suit had to be surgically removed, greatly altering the physical appearance of her suit. Unfortunately, the X in her system were too embedded to be removed safely; her chances of survival were minimal at best.

However, a cure was soon proposed: using a Metroid cell to make an anti-X vaccine. This cell came from the baby that Samus had found on SR388 and donated to the Ceres Space Colony for research. The vaccine was injected and the X were immediately destroyed, meaning the infant Metroid had effectively saved Samus's life a third time. Because the vaccine worked so well, the doctors (correctly) hypothesized that the Metroids were the natural predators of the X.

Side effects[]

While confronting the X aboard the Biologic Space Laboratories research station, it became evident that the vaccine had done more than just save Samus's life; while her body remained visibly unchanged, the vaccine had given her many strengths and weaknesses shared by Metroids, including their immunity to the X Parasites and their extreme vulnerability to sub-zero temperatures. Later on, Samus realized that she was an easier target for the SA-X; as the SA-X is the result of the X-infected portions of her Power Suit, it has access to all of Samus's weaponry, including the Ice Beam. Its tactics in fighting Samus even resembled the preferred Metroid-killing tactics of Samus herself; freeze the target with the Ice Beam, then hit it with a Missile.

It was later revealed that the vaccine also ran the risk of turning her into a Metroid. She developed an energy draining ability that also extended to machinery. However, her prior infusion with both Thoha and Mawkin DNA allowed some stability[1] until a member of the latter tribe, Raven Beak, destabilized that combination. Her DNA was eventually restored by an X Parasite that had reanimated the corpse of a Thoha, Quiet Robe.

It is unclear whether or not absorbing the Quiet Robe X Parasite completely removed or merely suppressed Samus's Metroid mutation.


  • Although vaccines are generally only effective in providing immunity to a disease before infection can take place, the Metroid vaccine also possesses conventional curative properties. It also makes use of Metroid cells and genetic material as opposed to dead or inactive X-parasites, making it closer to an advanced form of gene therapy or CRISPR than a conventional vaccine.
  • Metroid Fusion Special Edition: Rebirth of Samus is clearer in that the vaccine is made from antibodies of the baby.
  • The Federation calls the vaccination and curing of Samus a breakthrough, but are unaware that the Chozo had specifically engineered the Metroids to prey on the X.
  • There is one other vaccine in the Metroid series, the Galactic Federation vaccine that was developed to cure the Aurora Units of the corruption virus. However, it is digitally uploaded and not physically used.



  1. ^ Metroid Dread, Nintendo (2021)
    ADAM (Raven Beak): It appears your Metroid DNA has caused you to become a Metroid. Ordinarily this would have occurred immediately after the transplant. Your Thoha genes must have slowed down the process. They were the only tribe capable of controlling the Metroids. Though you possess their DNA, you are not a Thoha. Your physical prowess suggests you must also possess Mawkin DNA.