Infect Chemother.  2016 Jun;48(2):108-117. 10.3947/ic.2016.48.2.108.

Epidemiologic Parameters of the Middle East Respiratory Syndrome Outbreak in Korea, 2015

• Park SH
• Kim WJ
• Yoo JH
• Choi JH

• KMID: 2327978
• DOI: http://doi.org/10.3947/ic.2016.48.2.108

Abstract

BACKGROUND
Epidemiologic parameters are important in planning infection control policies during the outbreak of emerging infections. Korea experienced an outbreak of Middle East Respiratory Syndrome coronavirus (MERS-CoV) infection in 2015, which was characterized by superspreading events in healthcare settings. We aimed to estimate the epidemiologic parameters over time during the outbreak to assess the effectiveness of countermeasures.
MATERIALS AND METHODS
Publicly available data pertaining to the MERS outbreak in Korea were collected. We estimated the incubation periods of 162 cases whose sources of exposure were identified and the temporal trend was evaluated. Factors influencing incubation duration were analyzed. The generational reproduction number (R(g)) and case reproduction number (R(c)) were estimated over time.
RESULTS
The estimated median incubation period was 7.4 days (95% CI, 6.9-8.0). Median incubation periods tended to be longer over time as the disease generation progressed: 6.16 days (95% CI, 5.38-6.97), 7.68 days (95% CI, 7.04-8.44), and 7.95 days (95% CI, 6.25-9.88) in the first, second, and third generations, respectively. The number of days of illness in the source cases at the time of exposure inversely correlated with the incubation periods in the receiving cases (HR 0.91 [95% CI, 0.84-0.99] per one illness day increase; P=0.026). This relationship was consistent (HR 0.83 [95% CI, 0.74-0.93] per one illness day increase) in the multivariable analysis incorporating clinical characteristics, the order of generation, and a link to superspreaders. Because the third generation cases were exposed to their source cases in the early stage (median one day) compared to the second generation cases (median 6 days), the temporal trend of incubation periods appears to be influenced by early isolation of symptomatic cases and reduction of potential exposure to source cases in the later stage. R(g) declined rapidly from 28 to 0.23 in two generations. R(c) dropped below the epidemic threshold at one on May 31, 2015, which approximately coincided with the initiation of the stringent countermeasures.
CONCLUSIONS
Despite the initial delay, the stringent countermeasures targeted towards second generation cases appeared to effectively contain the MERS outbreak in Korea as suggested by the decline of R(c) shortly after implementation. Except for superspreading events, the transmission potential for MERS-CoV seems to be low. Further research should be focused on characterizing superspreaders in comparison to non-transmitting cases with regard to environmental, behavioral, and virologic and host genetic factors in order to better prepare for future outbreaks of MERS-CoV.

Keyword

Reproduction number; Incubation period; Middle East Respiratory Syndrome; Korea; Countermeasures

MeSH Terms

Coronavirus Infections*
Delivery of Health Care
Disease Outbreaks
Family Characteristics
Infection Control
Korea*
Middle East Respiratory Syndrome Coronavirus
Middle East*
Reproduction
Sick Leave

Figure

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  Figure 1 The distribution of incubation periods (A) and serial intervals (B) in the Middle East respiratory syndrome outbreak in Korea in 2015. The estimation included 162 cases with a single source of exposure for incubation periods and 153 cases with identified onset of symptoms for serial intervals. The fitted distributions are plotted against the empirical cumulative density function of observed incubation periods (midpoint of exposure to symptom onset) and serial intervals (black line). The 95% confidence intervals for the 5th, 50th, and 95th percentiles of these fitted distributions are also plotted. Bootstrapped estimates are in grey shading.

• 

  Figure 2 The temporal trend of incubation periods (A), serial intervals (B) and time form symptom onset to confirmation (C) during the Middle East respiratory syndrome outbreak in Korea in 2015. In the box plots, the box extends from the 25th to 75th percentile (interquartile range, IQR) of observations with the center line indicating the median. The bars define the upper (75th percentile + 1.5 IQR) and lower values (25th percentile-1.5IQR). In (C), the gray bars indicate the mean days from symptom onset to confirmation with standard errors.

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  Figure 3 Differences in distributions of incubation periods according to the disease generation (A), a link to superspreaders (B), and days of illness in source cases (C).

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  Figure 4 The epidemic curve of the Middle East respiratory syndrome outbreak in Korea in 2015 (A) and daily estimates of the case reproduction number Rc (B) and the instantaneous reproduction number Rt (C). Rc is depicted with 95% confidence intervals (vertical bars) where the grey region indicates Rc below 1 (B). Rt is shown with 95% credible intervals in grey shading and a dotted line indicates Rt at 1(C).

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