Quantifying transmission of norovirus during an outbreak.
BACKGROUND
Healthcare workers are thought to play a role in nosocomial transmission of norovirus, but the level and direction of norovirus transmission between patients and healthcare workers in sustaining transmission during an outbreak have not been quantified.


METHODS
We developed a method for finding plausible transmission trees of who acquired their infection from whom. We applied the method to data from an outbreak of norovirus in 4 wards of a psychiatric institution in the Netherlands in 2008. The simulated transmission trees were based on serial intervals for time between symptom onsets, weighted for the number of days that healthcare workers were present. The obtained transmission trees were linked to the Barthel Index, a measure of patient reliance on healthcare in their basic daily activities.


RESULTS
The dominant recognized transmission route was from patient to patient (64%), followed by patient to healthcare worker (29%). The overall estimated reproduction number for healthcare workers was low compared with patients (0.25 vs. 1.20; mean difference = 0.95 [95% confidence interval (CI) = 0.60 to 1.30]). The average number of all subsequent cases attributable to the downstream branch of one single infected healthcare worker in the transmission tree was 4.4 compared with 6.5 for cases attributable to one single infected patient (mean difference = 2.1 [95% CI = -4.7 to 8.9]). In the ward with patients requiring the highest level of care from healthcare workers, the attack rate among healthcare workers was highest.


CONCLUSIONS
This approach provides a framework to quantify the magnitude and direction of transmission between healthcare workers and patients during a norovirus outbreak. The utility of this method in outbreaks of other infections and in different settings should be explored.
Division of Clinical Epidemiology & Biostatistics, Institute of Social and Preventive Medicine (ISPM), University of Bern, Bern, Switzerland. jheijne@ispm.unibe.ch
Janneke C M
JC
Heijne
Marc
M
Rondy
Linda
L
Verhoef
Jacco
J
Wallinga
Mirjam
M
Kretzschmar
Nicola
N
Low
Marion
M
Koopmans
Peter F M
PF
Teunis
D017250:Caliciviridae Infections / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D003428:Cross Infection / Q000453:epidemiology / Q000635:transmission* / Q000821:virology; D004196:Disease Outbreaks / Q000706:statistics & numerical data; D005759:Gastroenteritis / Q000453:epidemiology / Q000821:virology*; D006778:Hospitals, Psychiatric / Q000706:statistics & numerical data; D006801:Humans; D017758:Infectious Disease Transmission, Patient-to-Professional / Q000706:statistics & numerical data; D017757:Infectious Disease Transmission, Professional-to-Patient / Q000706:statistics & numerical data; D009426:Netherlands / Q000453:epidemiology; D029322:Norovirus