Background

Previous research suggests a potential pandemic may be greater for avian influenza H7N9 than for H5N1. This research investigated the difference in human-clustered and sporadic cases of H7N9 virus and estimated the relative risk of clustered infections.

Methods

Comparative epidemiology and virology were performed among 72 sporadic confirmed cases, 17 family clusters (FCs) caused by human to human transmission and eight live bird market clusters (LCs) induced by co-exposure to the poultry environment.

Results

The case fatality of sporadic cases, FCs, and LCs (36% vs. 26% vs. 29%, respectively) did not differ among the three groups (p?>?0.05). The average age (36 vs. 60 vs. 58 years), co-morbidities (31% vs. 60% vs. 54%), avian exposure (72% vs. 100% vs. 83%) and H7N9-positive rate (20% vs. 64% vs. 35%) differed the most (p?<?0.05) in FCs, LCs and sporadic cases, respectively. These higher risks were associated with increased mortality.

There was no difference between the primary and secondary cases of LCs (p?>?0.05); however, exposure to a person with confirmed avian influenza A(H7N9) (primary 12% vs. secondary 95%), history of visits to a live bird market (100% vs. 59%), multiple exposures (live bird exposure and human to human transmission history) (12% vs. 55%) and median days from onset to antivirus treatment (6 vs. 3 days) significantly differed in the index and secondary cases from FCs (p?<?0.05). Mild cases were found in 6% vs. 32% in the primary and secondary cases of FCs (p?<?0.05), respectively. Twenty-five isolates from the three groups showed 99.1–99.9% homology and increased human adaption.

Conclusions

There was no statistical difference in the case fatality rate and limited transmission between FCs and LCs. However, the severity of the primary cases in FCs was much higher than that of the secondary cases due to the older age and greater underlying disease of the patients.