Q: But can you really tell from the genome whether the virus has adapted to humans?
C.D.: It’s almost impossible to tell from the sequence whether there has been a functional change in the virus. But we do know that certain parts of the genome are particularly important, and one of those is the receptor-binding domain of the spike protein.
Q: The part of the virus that binds to human cells.
C.D.: Exactly. And we do not see any changes there. The rest of the virus genome is also really closely related to previously sequenced MERS genomes. You really have to look quite hard to find any changes at all, and when you do find changes, there are other MERS genomes that had those changes, too. This is not like influenza viruses, which have a way higher mutation rate. Coronaviruses have correction enzymes so in general they are more stable genetically.
Q: Still, the number of infections is going up.
C.D.: You cannot compare the new numbers to those from a few months ago. Until the 26th of March, 459 tests had been done in all of Saudi Arabia this year. Then in just 1 month, just in the city of Jeddah, 4629 PCR tests were done. Something dramatic changed, and that is the case definition.
Before, tests were done on patients who had pneumonia and required [intensive care]. But now people are being tested not because they are sick, but because they had contact with a patient. Some of these tested positive, but many of them don’t really get sick.
Q: Could the test results be wrong?
C.D.: No, when I was in Jeddah I really tested the central lab where all the PCR for MERS is done. I made them run almost 200 PCRs with water, interspersed with some real samples. The tests were done on all the machines they use, with two different PCR assays and by two different technicians. To my surprise, there were no false positives at all.
We also reran samples from six health care workers who had cared for a patient in Tabuk; they seemed to be positive at a very low level. I can say unequivocally, these results are real. When you look at the PCR data on all the recent tests, they are often very weak signals. These people probably just have a very low concentration of virus in the throat. It's hard to interpret. It’s possible that these are infections that are quickly controlled by the immune system. That may happen frequently in health care workers dealing with several really sick patients.