Discussion
Acute viral respiratory tract infections cause considerable
morbidity and mortality and pose a risk
of outbreaks in health care settings.25-27 We describe
a cluster of MERS-CoV infections and report
health care–associated human-to-human
transmission of MERS-CoV. The 65% case fatality
rate in this outbreak is of concern.
We and others have found that the severity of
illness associated with MERS-CoV infection
ranges from mild to fulminant.7,9-17 The clinical
syndrome is similar to SARS, with an initial
phase of nonspecific fever and mild, nonproductive
cough, which may last for several days before
progressing to pneumonia.28 Some patients
with MERS-CoV infection also had gastrointestinal
symptoms, a finding similar to that with
SARS.29 MERS-CoV is known to infect cell lines
of the intestinal tract,30 but it is not yet known
what proportion of ill patients shed virus in
their stool. In the majority of patients in this
cluster, fever was high and persistent, but the
pattern of pulmonary involvement on chest radiography
was variable. It is noteworthy that the
survival rate was higher among patients whose
cases were identified by means of active surveillance
during the outbreak than among those
whose cases were identified clinically. Although
a possible explanation is that the patients whose
cases were identified by means of active surveillance
were younger and healthier than the patients
with primary cases, it is more likely that
enhanced surveillance was more effective at detecting
less severe disease than was identification
of clinical features.
Our estimates of the distribution of the incubation
period are similar to those for SARS-CoV
infection, which was estimated to have a median
incubation period of 4.0 days, with 5% of cases
developing within 1.8 days and 95% within 10.6
days.31 Our estimates of the serial interval of
MERS-CoV infection are somewhat shorter than
those for SARS-CoV (median, 7.6 days vs. 8.4
days), perhaps because transmission of MERSCoV
infection appears to occur earlier in the
course of the illness.32 Our small sample led to
wide confidence intervals; however, bootstrapped
sampling of our data showed the robustness of
our estimates with the inclusion and exclusion
of particular cases.
The rapid transmission and high attack rate
in the dialysis unit raises substantial concernsabout the risk of health care–associated transmission
of this virus. The apparent heterogeneity
in transmission, with many infected patients
not transmitting disease at all and one patient
transmitting disease to seven others, is reminiscent
of SARS.33,34