When Kennedy was first shot, at 12.30 pm on November 22, 1963, the bullet went through his neck but did not kill him. It was another shot, five seconds later, that damaged his brain and skull and killed him. Between that first shot and the fatal blow were five seconds in which the president’s life might have been saved had his Secret Service agents not failed to take evasive action, suggests Vanity Fair, in an article adapted from Susan Cheever’s book "Drinking in America: Our Secret History.”
The article describes how JFK’s Secret Service agents responded during the shooting: “Roy Kellerman, the leader of the security detail, did not seem to know what was happening. He thought a firecracker had gone off. William Greer, (an Irish-born immigrant from Tyrone) at the wheel of the president’s car, did not immediately speed up or swerve away from the shots. Paul Landis, in the vehicle trailing Kennedy’s, did not jump forward to protect the president with his body; neither did Jack Ready.
According to the Vanity Fair report, nine of the 28 Secret Service men who were in Dallas with Kennedy that day had been out until the early hours of the morning. A few were sleep-deprived and had been drinking while traveling with the president. However, this type of partying by Secret Service members was not unusual.
In his 2008 book, "The Echo from Dealey Plaza," Abraham Bolden, one of the only African American agents in Kennedy’s detail, writes that he was later framed and sent to prison as retaliation for speaking out about the lax behavior tolerated within the department.
The day of the assassination, Bolden says, he was standing in the Secret Service office in Chicago, just after the president was shot, when he heard a fellow agent say, “I knew it would happen. I told those playboys that someone was going to get the president killed if they kept acting like they did. Now it’s happened.”
Bolden told VF: “The biggest problem I ran into with the Secret Service when I was an agent was their constant drinking.
http://www.irishcentral.com/roots/history/jfk-secret-service-agents-too-hungover-to-react-in-dallas-says-author