Two distinct waves of H7N9 – Credit WHO Risk Assessment
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Since the initial outbreak was announced last spring, the World Health Organization has released total of five Rapid Risk Assessments on the emerging H7N9 virus in China (all available at this link). Today, the WHO updated their assessment with a short announcement (below), which references their latest in-depth analysis of the H7N9 virus, dated February 28th.
WHO Risk Assessment of human infection with avian influenza A(H7N9) virus
On 11 March 2014 WHO conducted a risk assessment in accordance with the WHO recommendations for rapid risk assessment of acute public health events.
Taking into consideration all information available to date to WHO, it is concluded that the public health risk from avian influenza A(H7N9) virus has not changed since the previous assessment of 28 February 2014.
Below you’ll find some excerpts from this most recent analysis, and a link to the entire 4-page document.
WHO RISK ASSESSMENT
Human infections with avian influenza A(H7N9) virus28 February 2014
Summary of surveillance and investigation findings
Human cases of avian influenza A(H7N9) virus infection to date
A total of 375 laboratory-confirmed cases of human infection with avian influenza A(H7N9) virus, including 115 deaths, have been reported to WHO: 367 cases by China National Health and Family Planning Commission, two cases by the Taipei Centers for Disease Control (Taipei CDC), five cases by the Centre for Health Protection, China, Hong Kong SAR, and one case in a Chinese traveller, reported from Malaysia. The cases occurred in a first wave (n=133) from February to May 2013; then two cases were reported in July and August; and from October 2013 a second wave of human cases has been occurring (n= 240 to date) (fig 1)1.Cases have been reported in both men and women, and across a wide age range. The age distribution in the second wave is very similar to the first wave, with most cases occurring in middle-aged and older men, few in children and even fewer in teenagers and young adults. The mean age is slightly lower in the second wave (53 versus 58 years) compared with the first wave. Infections in men are still more frequently reported than those in women. The case fatality rate among reported cases in the second wave is currently similar to that of the 30% reported in the first wave, though it might increase as some patients are still hospitalized
in critical condition. Similar to the first wave, most of the cases in the second wave were considered severe, with the exception of children, who have been primarily presenting with a milder clinical picture.Virus characteristics
Comparison of avian influenza A(H7N9) viruses isolated from humans and environmental samples using haemagglutination inhibition assays shows that limited antigenic diversity exists and they remain antigenically similar to the candidate vaccine viruses derived from A/Anhui/1/2013-like viruses. Unlike the surface genes, the internal genes were more diverse through reassortment with avian influenza A(H9N2) viruses. All recent avian influenza A(H7N9) viruses that have been tested remain susceptible to the neuraminidase inhibitor class of antiviral drugs. See alsohttp://www.who.int/influenza/vaccines/virus/201402_h5h7h9h10_vaccinevirusupdate.pdf.
Source of human infectionAlthough much remains unknown about this virus, such as (1) the animal reservoir(s) in which it is circulating, (2) the main exposures and routes of transmission to humans, and (3) the distribution and prevalence of this virus among people and animals (including the distribution in wild birds), human infection appears to be associated with exposure to infected live poultry or contaminated environments, including markets where live poultry are sold, given the following:
- Around 80% of human cases report a history of exposure to birds or live poultry markets.
- The viruses isolated from humans are avian influenza viruses and genetically similar to those
isolated from birds and the environment.- Targeted testing of poultry and environment in live poultry markets that are
epidemiologically linked with human cases of H7N9 infection have revealed more positive
results than testing in areas not linked with human cases.
Current evidence suggests that these avian influenza A(H7N9) viruses do not transmit easily from poultry or environments to humans, although their transmissibility may be greater compared with highly pathogenic avian influenza A(H5N1) viruses.Evidence regarding human-to-human transmission
Risk assessment
Information to date suggests that this virus does not transmit easily from human to human, and does not support sustained human-to-human transmission. The number of clusters of human cases remains comparable to the first wave. Since July, six small family clusters (of 2 to 3 family members) with possible household transmission were reported. With the exception of the family clusters, enhanced surveillance has not revealed additional human infections among contacts of confirmed cases so far in the second wave. Considering that a few cases of H7N9 infection since October were detected through influenza-like illness (ILI) surveillance, continued vigilance is warranted.
This 25 February 2014 risk assessment has been prepared in accordance with WHO’s published recommendations for rapid risk assessment of acute public health events and will be updated as more information becomes available.
Overall, the public health risk from avian influenza A(H7N9) virus has not changed since the previous assessment published on 21 January 20142
.
What is the likelihood that additional sporadic human cases of infection with avian influenza A(H7N9) viruses will occur?The understanding of the epidemiology associated with this virus, including the main reservoirs of the virus and the extent of its geographic spread among animals, remains limited. However, it is likely that most human cases were exposed to the H7N9 virus through contact with infected poultry or contaminated environments, including markets (official or illegal) that sell live poultry. As the virus source has not been identified nor controlled, and the virus continues to be detected in animals and environments, further sporadic human cases are expected in affected and possibly neighbouring areas.
Other avian influenza viruses such as highly pathogenic avian influenza A(H5N1) have demonstrated a seasonal pattern in which animal outbreaks and human cases have been less frequent in summer months and more frequent in winter months in temperate zones. An increase in avian influenza A(H7N9) virus infections in humans has been noted since October 2013, after a period of relatively few human cases over the summer, indicating that exposure to or infection with these avian influenza A(H7N9) viruses may follow a similar seasonal pattern.
