|MERS: "A slowly growing epidemic"|
The study estimates that there have been 940 symptomatic cases so far and 62% of infections have been unreported. The World Health Organization (WHO) has officially confirmed only 155 cases to date.
"At the very least there probably have been double the number of infections," said Neil Ferguson of the MRC Centre for Outbreak Analysis at Imperial College. "But it's considerably more likely in my view that we've had maybe five to ten times more human infections than that. And symptomatic human infections."
The study compiled databases of confirmed and probable cases. They looked at epidemiological data including incubation periods and estimated transmission rates. A 1.0 transmission rate means each infected person infects one other person.
When infection control methods are used, the virus does not appear to be self-sustaining. However, when there are no efforts to limit exposure, the virus appears to have a transmissability rate of 0.8 - 1.3. This is significant because an epidemic can become self sustaining when it reaches approximately a 1.0 transmission rate.
How can so many cases be missed by health officials? Surveillance methods tend to focus on the victims who visit hospitals, so only the sickest of the infected are being counted.
According to this report, one of the problems faced by researchers is the scarcity of publicly available data. ”We would certainly be in a better position if there was fuller reporting.” Ferguson said.
In Saudi Arabia, 'ground-zero' for the disease and origin of the majority of cases, only the most seriously ill patients with pneumonia-like symptoms who require ICU treatment are even being tested for MERS. According to Dr. Anthony Mounts, MERS expert for the WHO, “I know that their surveillance strategy is focused on intensive care patients."
Contact testing of confirmed cases has shown the presence of asymptomatic infections, that is with mild or no symptoms and these carriers are capable of spreading the virus to others.
Researchers speculate that the main method of transmission may be person to person at this point, or there could still be an animal source which continues to infect humans, who then spread the virus to other humans.
Ferguson feels that if there was a sustained animal source there would be more sick animals. He believes, "we have a human-to-human epidemic, with limited efficiency of transmission compared to flu, and mild presentation in most or many cases, similar to flu.”