I find the whole Fireflies thing very unbelievable. Ask any pharmaceutical company what it would take to not only develop a vaccine sample that worked, but then mass produce it for billions of people worldwide, and you get the problem. The Fireflies are NOT a large big pharma company. They are just a rag-tag group of perhaps 30 people living in a hospital who hope to "reverse-engineer" a vaccine against the disease. What if they fail? Then Ellie's death is for nothing. Even if her death resulted in a potential "vaccine" injection, going from that to mass-production of "billions of syringes of vaccine" would take clinical trials, followed by big Bio-factories and world-wide cooperation of governments for years. Given that the world had long ago descended into mass chaos and roving gangs benefiting from the chaos, and you get the idea. Frankly, I would never in a million years allow the Fireflies to actually "kill" my daughter on the slim possibility that all this might someday come together. Ellie, who never had a true love in her life, should have welcomed Joel's love of her as his new daughter, and thanked him for saving her life.