A study of the recent hospital outbreak of MERS-CoV (Middle East respiratory syndrome coronavirus) in Saudi Arabia reveals, among other things, that the virus spread in three hospitals and that some patients transmit it much more than others do.
The report, published today in the New England Journal of Medicine, shows that 21 of 23 cases involved person-to-person transmission in healthcare facilities, and that 9 cases were in hemodialysis patients.
Testing of more than 400 healthcare workers and household contacts of MERS patients turned up only 7 additional cases, the report says, which supports previous findings that the virus doesn’t spread very readily. Investigators found that some patients didn't spread the virus to anyone else, but one of them infected seven others.
The report was prepared by a large international team with members from Saudi Arabia, Canada, the United Kingdom, and the United States.
All the hospitals involved are in Al-Hufuf (also spelled Al-Hofuf) in Al-Ahsa governorate of eastern Saudi Arabia. The report covers 23 confirmed cases that were identified between from Apr 1 to May 23; 11 probable cases also are considered part of the outbreak.
As of Jun 12, 15 of the 23 patients (65%) had died, 6 had recovered, and 2 remained hospitalized, the report says. Most of the patients were men, and the median age was 56. Disease manifestations included fever in 20 patients, cough in 20, shortness of breath in 11, and gastrointestinal symptoms in 8.
Most of the cases occurred at one general hospital, called hospital A, which has 150 beds, plus a dialysis unit. The event began on Apr 5 with admission of a patient with dizziness and sweating, followed by a fever 3 days later. He was not tested for MERS-CoV, but his son later had a confirmed case.
Another patient, who was on dialysis, was admitted Apr 6 and put in a room next to the first patient. By Apr 11 he had a fever, and he underwent dialysis in the hospital on Apr 11 and 13. Between Apr 14 and 30, MERS-Cov was confirmed in nine more patients who were receiving dialysis in hospital A. Eight of these cases developed before or within 1 day after infection control steps were taken in the dialysis unit.
One dialysis patient who had a confirmed MERS-CoV infection was admitted to a medical ward on Apr 21. In the following week, two other patients, located two and three rooms away from the dialysis patient, fell ill with the infection.
The virus spread to "hospital C" when a patient who was infected at hospital A underwent dialysis at hospital C while sick. Two other patients at the latter hospital subsequently were infected.
In addition, eight MERS-CoV patients were transferred to "hospital D," a regional referral hospital. One of those patients passed the virus to two others at hospital D, and another passed it to a physician there, the authors concluded.
The team monitored 217 household contacts of patients with confirmed cases. They found only five cases—three confirmed and two probable—in adult relatives of three of the patients. One of them was treated at another hospital, "hospital B," where the report lists no other cases.
Only two confirmed cases were detected among more than 200 healthcare workers who were monitored after exposure, according to the report.
In mapping transmission chains, the team found that one patient passed the infection to seven other people, one passed it to three others, and four transmitted it to two persons each. The authors say this variability in transmission is "reminiscent of SARS" (severe acute respiratory syndrome), which is caused by another coronavirus. Some patients in the SARS epidemic in 2003 were described as "super spreaders."
The median incubation period in the outbreak was 5.2 days (95% confidence interval, 1.9 to 14.7 days). On the basis of recent MERS cases, the World Health Organization has said the incubation time may run as long as 10 to 14 days.
The investigators obtained full genome sequences from isolates from four patients. From a phylogenetic analysis of these sequences and from other data, they estimated that the date of the most recent common ancestor of MERS-CoV was Aug 18, 2011. This broadly agrees with the conclusion of a German team that, in a Lancet Infectious Diseasesreport this week, estimated the date of the most recent common ancestor as mid-2011.
The authors were unable to determine if the hospital outbreak involved just one, or more than one, transmission of the virus from the community.
Also, they couldn't answer another key question about the virus: whether person-to-person transmission occurred through respiratory droplets or direct or indirect contact and whether aerosol transmission occurred over a distance of more than 1 meter.
The report says the pattern of the outbreak is consistent with the assumption that patients were infectious only when they had symptoms, but this doesn't rule out transmission during the incubation period or during asymptomatic infection.
In other findings, the authors note that the survival rate was higher for patients who were identified through active surveillance than for those who were identified clinically. They say the likely reason was that active surveillance was better at picking up less-severe disease.
Assiri A, McGeer A, Perl RM, et al. Hospital outbreak of Middle East respiratory syndrome coronavirus. N Engl J Med 2013 (Early online publication). [Abstract]