Via Popular Science, Jason Tetro has a very useful article: MERS May Be A Concern But It’s Not SARS. Excerpt:
Over the nearly two years since the causative virus, the MERS coronavirus (MERS-CoV) was identified, researchers have gained significant insight into the ongoing epidemic. They have also seen just how this virus compares with SARS and the answer always appears to be the same. While MERS is a bad actor, it is no SARS and most likely will not cause a pandemic.
The first comparison is epidemiological. MERS appears to be a respiratory disease with the potential for other complications including diarrhea, muscle and joint pain, kidney injury and organ failure. There are quite a few similarities with SARS however a closer look at the progression of infection shows these two are not at all alike.
SARS is a two-stage infection requiring up to several weeks of incubation before symptoms begin. When they hit, the person remains ambulatory for upwards of ten days to three weeks. Then, when the second stage occurs, the patient inevitably requires hospitalization and in some 10% of cases, death occurred.
MERS, on the other hand, requires only about 5 days for onset of symptoms and there appears to be only one stage. When symptoms begin, especially in severe cases, the person is all but debilitated and requires rapid medical attention. Death occurs in about a quarter of the population although those numbers continue to drop.
The reason for this reduction in mortality stems from the rise of either mild or asymptomatic MERS infection in healthy individuals. The median age of death is about 52 years of age and most severe cases occur in people with underlying conditions such as diabetes, renal failure, heart disease and immunosuppression.
This suggests MERS may not be a perfect pathogen and normal healthy individuals may be able to fight off infection. In comparison, SARS infected people uniformly and equally as deadly; the median age of death was closer 43 years. Some did exhibit lower levels of infection, possibly due to prior infection and there were some who simply did not exhibit any symptoms.
The next comparison lies in the spread of the two viruses and how best to control it. SARS was easily transmitted in the environment while MERS continues to require close contact with an infected person or animal. Some may feel this might change allowing MERS to be spread just as rapidly. Yet the biological differences between the two means of infection suggest this may be unlikely.