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H7N9 demonstrates greater transmission, pandemic risk than H5N1
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Cases of influenza A(H7N9) may be more susceptible to sporadic transmission than cases of influenza A(H5N1), indicating a greater potential pandemic risk, according to recent data.
“The data suggest that the severity and average age of laboratory-confirmed H7N9 cases is biased upward, compared to all symptomatic H7N9 cases, by the under-ascertainment of young, mild cases, whereas this is not true for H5N1,” Ying Qin, MD, PhD, from the division of infectious disease at the Chinese Center for Disease Control and Prevention in Beijing, and colleagues wrote. “This suggests quite different risk profiles for these two viruses.”
Qin and colleagues analyzed Chinese CDC data from 720 H5N1 cases and 460 H7N9 cases reported in China between 1997 and 2014. Data from clustered cases and household contacts were used the estimate the relative risk of infection to both related and unrelated contacts. This, along with estimated exposure to the animal reservoir, was then extrapolated to determine a viable reproduction number.
Cases of H5N1 had no age or severity differences from index or sporadic cases and secondary infections; however, there was an increase in severity and age for H7N9 index/sporadic cases compared with secondary cases, with 71% of index and sporadic cases requiring intensive care vs. 31% of secondary cases.
The researchers reported significant associations in H7N9 cases for visiting live bird markets (P = .008). For H5N1 cases, there were significant associations with any poultry exposure (P = .015) and visiting live bird markets (P = .005).
Additionally, 20% of H5N1 cases occurred in clusters compared with 8% of H7N9 cases, and relative risk through blood-related contacts was 8.96 with H5N1 (95% CI, 1.30-61.86) and 0.80 for H7N9 cases (95% CI, 0.32-1.97).
Based on these data, Qin and colleagues concluded that H5N1 may have less of a population-based susceptibility to infection compared with H7N9 cases.
“The long-term and widespread circulation of H5N1 with relatively few human cases and without the emergence of a human adapted strain supports the existence of strong, but as yet unidentified, biological barriers to transmission and adaptation,” they wrote. “In contrast, the data indicate there may be a large number of undetected mild cases of H7N9, suggesting more widespread human susceptibility to H7N9 than to H5N1.”
Disclosure: Qin reports no relevant financial disclosures. Please see the full study for a list of all other authors' relevant financial disclosures.
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