ProMED: CHINA, H7N9 LOW TRANSMISSION POTENTIAL

Published Date: 2013-10-05 13:28:47
Subject: PRO/AH> Avian influenza, human (118): China, H7N9 low transmission potential
Archive Number: 20131005.1982621

Date: Wed 2 Oct 2013
Source: University of Minnesota Center for Infectious Disease Research and Policy (CIDRAP) [edited]
http://www.cidrap.umn.edu/news-perspective/2013/10/flu-scan-oct-02-2013


Researchers who used a sophisticated modeling approach incorporating H7N9 case data from China found hints that the virus has low transmission potential and that the pace of infection slowed in April [2013] after officials closed live-bird markets. The study, published in BMC Medicine today [2 Oct 2013], was conducted by a team from the National Institutes of Health, Arizona State University, and George Washington University.

The investigators used a Bayesian modeling technique [see http://en.wikipedia.org/wiki/Bayesian_statistics] to assess if the outbreak had a reproduction (R) number consistent with unsustained human transmission and if interventions reduced transmission. They compared their estimates with other zoonotic pathogens, including H5N1 avian influenza, variant H3N2 (H3N2v) influenza flu, and Nipah virus. Based on 130 lab-confirmed cases reported in China from March through 20 May 2013, their analysis found that transmission was low in Shanghai and Zhejiang province, and at 0.6 the R was well below the 1.0 level needed to sustain transmission. Researchers also found that the growth rate slowed in mid April [2013], which coincided with the closure of live-bird markets in large Chinese cities in early April. Compared with other zoonotic threats, the transmission threat from H7N9 was lower.

The authors said that, although the findings were based on a small number of cases and need to be confirmed, the modeling technique could be useful for measuring outbreak progression and the impact of control measures in the months ahead and provides a tool for monitoring pandemic potential in near real-time.

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[The reference for the BMC Medicine paper is: Transmission potential of influenza A/H7N9, February to May 2013, China; by Gerardo Chowell, Lone Simonsen, Sherry Towers, Mark A Miller and Cecile Viboud; in BMC Medicine 2013, 11:214 doi:10.1186/1741-7015-11-214; http://www.biomedcentral.com/1741-7015/11/214/abstract. The Abstract follows.

"Background: On 31 Mar 2013, the 1st human infections with the novel influenza A/H7N9 virus were reported in Eastern China. The outbreak expanded rapidly in geographic scope and size, with a total of 132 laboratory-confirmed cases reported by 3 Jun 2013, in 10 Chinese provinces and Taiwan [imported from mainland China]. The incidence of A/H7N9 cases has stalled in recent weeks, presumably as a consequence of live bird market closures in the most heavily affected areas. Here we compare the transmission potential of influenza A/H7N9 with that of other emerging pathogens and evaluate the impact of intervention measures in an effort to guide pandemic preparedness.

Methods: We used a Bayesian approach combined with a SEIR (Susceptible-Exposed-Infectious-Removed) transmission model fitted to daily case data to assess the reproduction number (R) of A/H7N9 by province and to evaluate the impact of live bird market closures in April and May 2013. Simulation studies helped quantify the performance of our approach in the context of an emerging pathogen, where human-to-human transmission is limited and most cases arises from spillover events. We also used alternative approaches to estimate R based on individual-level information on prior exposure and compared the transmission potential of influenza A/H7N9 with that of other recent zoonoses.

Results: Estimates of R for the A/H7N9 outbreak were below the epidemic threshold required for sustained human-to-human transmission and remained near 0.1 throughout the study period, with broad 95 percent credible intervals by the Bayesian method (0.01 to 0.49). The Bayesian estimation approach was dominated by the prior distribution, however, due to relatively little information contained in the case data. We observe a statistically significant deceleration in growth rate after 6 Apr 2013, which is consistent with a reduction in A/H7N9 transmission associated with the preemptive closure of live bird markets. Although confidence intervals are broad, the estimated transmission potential of A/H7N9 appears lower than that of recent zoonotic threats, including avian influenza A/H5N1, swine influenza H3N2sw and Nipah virus.

Conclusion: Although uncertainty remains high in R estimates for H7N9 due to limited epidemiological information, all available evidence points to a low transmission potential. Continued monitoring of the transmission potential of A/H7N9 is critical in the coming months as intervention measures may be relaxed and seasonal factors could promote disease transmission in colder months."

This analysis is consistent with independent studies, and a resurgence of disease should not occur unless there is some significant modification of the genetic properties of the H7N9 virus, or relaxation of the restrictions introduced to control access to poultry markets. - Mod.CP]

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