Ellie is the cure to the Cordyceps virus because she is immune to it. She is the first, and perhaps only immune survivor of the virus and, therefore, can be studied.
Because the virus doesn’t turn her into an Infected, doctors can extract her blood and examine the benign Cordyceps inside her that allows her to reject the virus.
Ellie can be turned into a cure because she was born with Cordyceps already inside of her. Her mother was bitten moments before giving birth, which led to the infection getting inside Ellie.
Fortunately, it didn’t turn her into a Clicker. Instead, the Cordyceps in her system tricks any future Cordyceps into believing she is already infected.
The Fireflies think they can take the benign Cordyceps inside of Ellie and turn it into a vaccine that will prevent anyone from becoming infected.
While the 2013 video game didn’t go into extraneous detail about why Ellie is the cure, the TV series examines this element more, as well as the more scientific aspects of the Cordyceps fungal infection.
After all, in the early episodes of The Last of Us TV show, there were more references to the cause of the Outbreak and how the fungus was able to spread so rapidly in the first place than the game ever had.
Will the Vaccine Actually Cure People in The Last of Us TV Show?
It’s unclear if the vaccine will work in The Last of Us TV show. While the Fireflies are hopeful it will, there is no guarantee.
Ellie is the only immune survivor they have found and is, therefore, their first and last attempt at making a successful vaccine. If that fails, there’s no alternative way of making one.
Before testing on Ellie, the Fireflies attempted to test a vaccine on monkeys at the University of Eastern Colorado. This, unfortunately, failed, causing them to abandon that research and move to a new base of operations.
Additionally, it is also unclear how they would effectively roll out a vaccine to the world, as it is in such an apocalyptic state that it seems unlikely everyone would be receptive to Fireflies injecting them with a vaccine.
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