With a solid post-apocalyptic storyline, a fantastic performance by everyone’s favourite Will Smith and some terrifying monsters, I Am Legend is a classic sci-fi/horror movie loved by millions. However, there is one glaring mistake when it comes to the vaccine solution offered in the film.

The premise is simple – a man-made disease has wiped out most of humanity and turned many remaining into vampire-like monsters.

Dr Robert Neville (Smith) spends his days exploring the ruins of New York City with his dog while trying to develop a cure for the disease.

When two survivors show up on his doorstep, Neville has to choose whether to doom them to certain death in the outside world or give them sanctuary but risk monsters finding his home.

The thrilling conclusion sees monsters break through Neville’s home defences just as he develops a cure. In the original ending to I Am Legend, Neville sacrifices his life to protect the survivors and his world-saving serum. The alternative ending sees Neville released by the monsters after he cures one of them.

Our science problem rests with the cure itself, which nullifies the whole basis of Neville working on a vaccine. Here is a rundown of the conundrum, first published on sister website The Focus.

Krippin pandemic explained

In 2009 Dr Alice Krippin, a British scientist played by Emma Thompson, genetically re-engineers the measles virus to cure cancer. Initially the virus is considered a “miracle cure” when all 10,009 patients survive clinical trials and are deemed free of the disease.

Unfortunately, they then start to experience rabies-like symptoms, going berserk and biting anyone within reach, spreading the virus through blood-saliva contact.

Despite the military’s best efforts to contain the virus, it breaks out of the Manhattan-based laboratory and mutates into a lethal, airborne strain that spreads across the planet.

Immunity and Darkseekers

In a few months the Krippin Virus has killed 90% of humanity, about 5.4 billion people, and turned another 9.8% of the population, circa 558 million, into killing machines called Darkseekers.

The remaining 0.2% of the human population are said to have a genetic mutation that prevents the virus affecting their body. Crucially, the movie describes non-infected people as “naturally immune” rather than patients who developed immunity through recovery.

Once the virus infects someone the disease overwhelms their body in 48 hours, during which time the host suffers extreme high fever and bleeding from the eyes, while also vomiting blood.

According to I Am Legend lore, the host then develops “primal behaviour, a vicious bloodlust” and views the uninfected as “monsters that must be destroyed”. They also “embrace the new-found power” the Krippin Virus has given them.

Krippin Virus also causes albinism, total hair loss, dilated pupils and extreme intolerance of ultraviolet light. At this point, the host is considered a Darkseeker.

Interestingly, Darkseekers’ terrifying features are rooted in basic biology. For example, the virus causes a defection in the adrenal gland, meaning the body constantly produces excessive adrenalin.

This extreme adrenalin rush not only explains Darkseekers’ enhanced strength and agility but also explains their increased metabolism, heart rate, blood flow and body temperature.

The cure conundrum

Neville builds a virology lab in his basement and uses his own blood as the base ingredient for what he hopes will be a cure for Krippin Virus.

This is where the problem lies. You can’t develop a cure using the blood of someone who’s naturally immune to that disease because of a genetic mutation. That’s not how vaccine development works and, as a virologist, Neville should know this.

A vaccine works by using a weakened or inactive form of a virus/bacterium to trigger an immune response by the host. The body reacts as if this was a real infection, producing antibodies to fight the disease and cells that will remember how to fight that disease in the future.

To produce a vaccine from someone’s blood a person needs to have the correct antibodies in their system, which know how to fight the infection.

Because Neville is naturally immune to the virus thanks to his genetic mutation, he will never have had an immune response to the virus. This means he won’t have Krippin antibodies in his blood.

I Am Legend vs Reality

In reality, if Neville wants to create a cure for Krippin Virus he should start with Darkseeker blood and work to develop a weakened form of the virus that could be used as an immunological trigger.

Working to find a cure may keep Neville busy enough to remain sane in his post-apocalyptic world. However, as an “expert virologist” he should understand the basics of vaccine development and realise he has wasted three years of resources on something medically impossible.

I Am Legend? More like I Am Forgetting Basic Immunology.

In other news, Fans want to know After We Collided's release date - has the launch finally been revealed?