A South Korean medical worker (L) wearing protective gear handles a sample tube from a man (C) suspected of carrying the MERS virus in Seoul, South Korea.

Three years after the mysterious MERS virus first emerged in humans, and as South Korea reports its 19th death in the current outbreak, scientists and drugmakers say there is no excuse for not having a vaccine that could have protected those now falling sick and dying.

South Korea's health ministry reported four new cases of Middle East Respiratory Syndrome (MERS) on Tuesday, bringing the total to 154 in an outbreak that is the largest outside Saudi Arabia. The ministry also said three patients infected with the MERS virus had died, taking the death toll to 19 in an outbreak that began in May.