By Kate Kelland and Ben Hirschler LONDON, June 15 (Reuters) - Three years after the mysterious MERS virus first emerged in humans, scientists and drugmakers say there is no excuse for not having a vaccine that could have protected those now falling sick and dying in South Korea. The facts behind the coronavirus that causes Middle East Respiratory Syndrome (MERS) have been slow to emerge, partly due to a secretive response in Saudi Arabia, which has suffered an outbreak stretching back to 2012. But scientists do know that it is similar to the deadly SARS virus, that it probably originated in bats, that it is linked to camels, and can pass from person to person. They also understand its molecular structure. That all yields scientific detail for researchers to begi...