Avian Influenza H5N1 Transmission in Households, Indonesia

BACKGROUND: Disease transmission patterns are needed to inform public health interventions, but remain largely unknown for avian influenza H5N1 virus infections. A recent study on the 139 outbreaks detected in Indonesia between 2005 and 2009 found that the type of exposure to sources of H5N1 virus for both the index case and their household members impacted the risk of additional cases in the household. This study describes the disease transmission patterns in those outbreak households. METHODOLOGY/PRINCIPAL FINDINGS: We compared cases (n = 177) and contacts (n = 496) in the 113 sporadic and 26 cluster outbreaks detected between July 2005 and July 2009 to estimate attack rates and disease intervals. We used final size household models to fit transmission parameters to data on household size, cases and blood-related household contacts to assess the relative contribution of zoonotic and human-to-human transmission of the virus, as well as the reproduction number for human virus transmission. The overall household attack rate was 18.3% and secondary attack rate was 5.5%. Secondary attack rate remained stable as household size increased. The mean interval between onset of subsequent cases in outbreaks was 5.6 days. The transmission model found that human transmission was very rare, with a reproduction number between 0.1 and 0.25, and the upper confidence bounds below 0.4. Transmission model fit was best when the denominator population was restricted to blood-related household contacts of index cases. CONCLUSIONS/SIGNIFICANCE: The study only found strong support for human transmission of the virus when a single large cluster was included in the transmission model. The reproduction number was well below the threshold for sustained transmission. This study provides baseline information on the transmission dynamics for the current zoonotic virus and can be used to detect and define signatures of a virus with increasing capacity for human-to-human transmission

Interval between onset of illness for cases (n = 34) in outbreaks of avian influenza H5N1 infection.

<p>Panel A shows the interval between onsets of illness of index and secondary cases in outbreaks. Panel B shows the interval between onsets of illness of serial cases in outbreaks. Black denotes cases not exposed to zoonotic sources of virus and white denotes cases exposed to zoonotic sources of virus.</p

Transmission parameters for outbreaks of avian influenza H5N1 infection.

a<p>Mean number of secondary cases infected by a single index case in an exposed population of size 5, CI denotes confidence interval.</p>b<p>Mean number of zoonotic cases in an exposed population of size 5.</p>c<p>AIC_C denotes Akaike Information Criterion adjusted for small sample size. This indicates the percent probability that the model is the best amongst those considered.</p

Household size and secondary attack rate for outbreaks of avian influenza H5N1 infection.

a<p>SAR declines to 0.047 when outlier cluster is excluded.</p>b<p>SAR declines to 0.044 when outlier cluster is excluded.</p

Comparison of cases (n = 177) and healthy contacts (n = 496) in outbreaks of avian influenza H5N1 infection.

a<p>Observations  = 561, Goodness-of-fit test: P = 0.17, OR denotes odds ratio, CI denotes confidence interval. OR were adjusted for the inclusion of the three variables in the final multivariate model.</p>b<p>Data missing for two cases and 46 healthy contacts.</p>c<p>Data missing for 66 cases from the 59 outbreaks for which household data were not available.</p