Background on Middle East Respiratory Syndrome Coronavirus (MERS‐CoV) sequence
9 June 2015
Preliminary understanding of viruses in the Korea/China cluster
When sequence data are compared, viruses from the Korea/China cluster are similar to viruses sampled from across the Arabian Peninsula. However, the collection of viruses available for comparison is limited. Many sequences are from older viruses; newer viruses from 2015 come from a very small geographical area in Saudi Arabia.
The analysis suggests that the closest matches are with the 2015 viruses from Saudi Arabia. Recent data from a wider geographical area would be needed to confirm this conclusion.
Human cases have recently been reported in Germany, Oman, Qatar, Saudi Arabia, and the United Arab Emirates. Analysis of viruses from these countries could help answer questions about the place of the Korea/China cluster in the broader geographical picture.
Limited clues from sequence analysis
Viruses have genetic material that changes when the virus copies itself during replication. Different types of changes occur, but the ones most commonly studied are mutations.
How often mutations are introduced depends on the virus type. Like influenza viruses, the MERS coronavirus is an RNA virus. Unlike DNA viruses, RNA viruses have no “proof-reading” mechanism, which means that small mistakes made as the virus copies itself hundreds of thousands of times a day are not corrected. Frequent mutations can be expected. These changes are random events, often meaningless and sometimes difficult to interpret.
Careful analysis of sequence data can yield clues about the source of infection, whether cases are linked, and how viruses are being spread. The presence of mutations does not, of itself, indicate that a virus is becoming more transmissible or more or less virulent.
Preliminary analysis does not suggest that a more transmissible virus is emerging in Korea.
The severity of MERS is governed by many factors, including the patient’s age, the presence of co-morbidities, and a history of smoking. No evidence suggests that the disease pattern in Korea is any different. The severity of a virus cannot be determined from sequence data.
Why more data are needed
When assessing changes in a virus over time, scientists use a phylogenetic tree as a convenient way to illustrate genetic relationships, or “clusters”, among viruses. The “trunk” of the tree shows the ancestor of the viruses that subsequently evolved from it. Branches in the tree are formed by viruses that descended from the common ancestor. Branches located near each other indicate closely-related groups.
The accurate location of viruses in the tree depends on how completely viruses circulating in humans and animals have been documented in the so-called “reference collection.” The more complete and systematic the collection of samples is, the more robust are the conclusions that can be drawn from the tree.
The fact that MERS viruses circulating in animals are not routinely sampled in the Middle East creates gaps that hinder the accurate interpretation of relationships among different virus sequences.
Until more strains of the MERS coronavirus have been isolated from the animal host, most likely dromedary camels, scientists cannot determine whether changes in the virus represent genuine adaptations in humans, or whether these changes derive from separate introductions of the virus to humans from animals.
Strains of the virus that are identical to human strains have been isolated from dromedary camels in several countries, including Egypt, Oman, Qatar, and Saudi Arabia.