Doomsday

In 2008, an unknown killer virus, dubbed "The Reaper Virus" infected Scotland. Unable to contain the outbreak or cure the infected, The UK government built a massive 60-foot wall that isolated Scotland from the rest of Britain. The quarantine was deemed a success; however, the extreme method employed by the government destroyed diplomatic and economic relations between the UK and the rest of the world. This led to massive unemployment, civil unrest and extreme economic turmoil, causing the UK to collapse into a dystopia.
In 2035, 27 years after the quarantine, during a raid, police discover several people infected with Reaper Virus. The Prime Minister John Hatcher shares with domestic security chief Captain Nelson satellite footage of survivors in Scotland. Believing a cure may exist, Hatcher orders Nelson to send a team into Scotland to find medical researcher Dr. Kane, who was working on a cure when Scotland was quarantined. Nelson chooses Major Eden Sinclair to lead the team.
Sinclair's team crosses the wall and goes to Glasgow, Dr. Kane's last known location. In Glasgow, while searching the local hospital for survivors, Sinclair and her team are ambushed by a group of marauders. Sgt. Norton and Dr. Stirling manage to escape, while the team suffers heavy casualties. Sinclair and Dr. Talbot are captured by what turns out to be a huge gang of brutal cannibals. Sinclair is interrogated by the gang's leader, Sol, who plans to use her as leverage to cross the wall and leave Scotland. He leaves and joins the rest of the gang, who grill Dr. Talbot and eat him. With the help of Cally, another prisoner, Sinclair escapes and kills Viper, Sol's dedicated partner. After meeting with Norton and Stirling, they escape on a train. Cally reveals that she and Sol are Kane's children. Unsatisfied with Kane's rule, Sol left to create his own army.
Meanwhile, Hatcher plans secretly to isolate the central London area because of the infection. However, an infected man breaks into his office and tries to kill him. Nelson shoots and kills the man, but his blood splatters on Hatcher. Michael Canaris, Hatcher's right-hand man, quarantines him and takes his place as de facto Prime Minister. Hatcher later commits suicide.
After leaving the train, Sinclair's group are arrested by soldiers armed with archaic weapons and armor. They are taken to a medieval castle and imprisoned. Their leader, Marcus Kane, tells Sinclair the truth: there is no cure, only people with a natural immunity. Originally a medical researcher, he was devastated when his family was left behind during the quarantine. After losing his wife, Kane became a twisted, sadistic leader. He sentences Sinclair and her group to death, pitting Sinclair against Telamon, his executioner, in a small fighting ground to entertain his followers. Sinclair subdues and kills Telamon. During the duel, the rest of the group escapes, retrieves their equipment and rescue Sinclair.
Sinclair, Norton, Stirling and Cally escape to a fallout shelter entrance on horseback. They locate an underground facility in the forest and find an intact 2007 Bentley Continental GT, which they fuel up. Kane's medieval knights arrive and kill Norton. Sinclair and the others drive the Bentley back to the quarantine wall. On the way, they are intercepted by Sol's gang, who seeks to avenge Viper's death. After a high-speed chase, Sol and many of his men are killed and Sinclair's group escape.
The car is tracked down by a government gunship. Canaris arrives and reveals he will let the virus continue as a form of population control and profit. Cally's blood is to be used as a vaccine for the virus. Calley and Stirling board the gunship, while Sinclair chooses to stay behind. Nelson flies into the quarantine zone to speak with Sinclair and she gives him a recording of the conversation, the evidence to bring down Canaris. The recording is later broadcast to the rest of the country. Sinclair retrieves Sol's head and returns to his gang. Looking at the head, they cheer, accepting Sinclair as their new leader.

Question: Canaris wants to use the virus to control what?
Answer: the population