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-02-2013Researchers
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.
--
Communicated by:
ProMED-mail
<promed@promedmail.org>
[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]
http://www.promedmail.org/direct.php?id=20131005.1982621
