Update Report: WHO RISK ASSESSMENT of H7N9 on 10 May 2013

WHO has issued a risk assessment about H7N9 on 10th May 2013.


Summary

A total of 131 confirmed cases of humaninfection with avian influenza A(H7N9) virus have been reported to WHO by ChinaNational Health and Family Planning Commission and one case by the TaipeiCenters for Disease Control (Taipei CDC). Although cases have been reported inboth sexes and across a wide range of ages, most cases have occurred amongmiddle-aged and older men. Thirty-two people have died, and most of the othercases are considered severe. In addition to the case reported by Taipei CDC(with a history of recent travel from Jiangsu), cases have been reported fromAnhui, Fujian, Henan, Hunan, Jiangsu, Jiangxi, Shandong and Zhejiang, and themunicipalities of Beijing and Shanghai.

 H7N9 virus

Much remains unknown about this virus,including the animal reservoir(s) in which it is circulating, the mainexposures and routes of transmission, and the scope of the spread of this virusamong people and animals. Investigations are ongoing and evidence isinconclusive. Nevertheless, human infection appears to be related to exposureto live poultry or contaminated environments because:

. The virus in humans is geneticallysimilar to that found in animals and the environment (live bird markets).

. Most human cases (approximately three outof four patients) report a history of exposure to animals, mostly chickens.

. The virus has been detected in poultry inlive bird markets.

. The number of human cases appears to havedecreased after closure of live animal markets.

Whether other potential reservoirs of thisvirus may exist, including in other domestic and wild bird species, andmammalian species, has not yet been determined. And although two familyclusters have been reported, there is no evidence of sustained human-to-humantransmission:

. Monitoring and testing of contacts(>2000 people) of confirmed cases has detected few infections.

. Testing of more than 20,000 people withinfluenza-like illness (ILI) in March and April has confirmed only sixinfections with H7N9. This finding suggests that milder cases of H7N9 infectionare not occurring in large numbers.

This is the first time human infection withthe avian influenza A (H7N9) subtype has been detected. Previously, sporadiccases of human infection with other influenza A (H7) viruses have been reportedwhich were associated with outbreaks of infection in poultry. The few A (H7) humaninfections that have occurred generally resulted in mild illness andconjunctivitis, with the exception of one death.

Genetic and laboratory characterization ofavian influenza A(H7N9) viruses isolated from humans indicates that:

. The virus contains a group of influenzavirus genes from multiple origins.

. Some genetic changes, including aminoacid substitutions associated with increased affinity to alpha 2-6 receptors,suggest that H7N9 may have greater ability to infect mammals, including humans,than other avian influenza viruses.

. Sequence variations among the genes ofthe isolates suggest that there has been more than one introduction of thisvirus from animal into humans.

. Genetically, these viruses are in generalexpected to be sensitive to the neuraminidase inhibitors oseltamivir andzanamivir, but resistant to the antiviral drugs amantadine and rimantadine. Theisolates have a haemagglutinin structure that is associated with lowpathogenicity in birds.

The virus has not been reported to causesevere disease in poultry. The absence of this signal limits the ability toeasily detect the virus in birds, in contrast to avian influenza A(H5N1).

Riskassessment

This risk assessment supersedes thedocument posted on 13 April 2013. It has been prepared in accordance with WHO’spublished recommendations for rapid risk assessment of acute public healthevents and will be updated as more information becomes available. The risk hasnot changed since the previous assessment.

What is the risk that more human cases willoccur in the affected areas?

The understanding of the epidemiology ofthis outbreak and virus remains limited, including the main reservoirs ofinfection and the extent of geographic spread among animals. However, it islikely that most human H7N9 infections have been associated with contacts withanimals or live bird markets.

Further human cases should be expected.Other avian influenza viruses such as H5N1 have demonstrated a seasonal patternin which human cases have been less frequent in summer months and more frequentin winter months. It remains to be seen whether H7N9 infections will follow thesame seasonal pattern. Most human cases have resulted in clinically severeillness.

What is the risk of human-to-humantransmission?

There is no evidence of sustainedhuman-to-human transmission. However, two family clusters suggest that limitedhuman-to-human transmission may occur where there is close contact betweencases and other people, as occurs in families and potentially in healthcaresettings. Moreover, the genetic changes seen among these viruses that suggestadaptation to mammals is of concern, and further adaptation may occur. Shouldsustained human-to-human transmission occur with an increased number ofclinically severe cases, health systems are likely to be strained.

What is the risk of international spread ofH7N9 by travelers?

There is no indication that internationalspread has occurred. An infected person, whether symptomatic or not, whotravels to another country, could spread the infection. However, as the virusdoes not appear to cause sustained human-to-human transmission, extensivecommunity spread is unlikely. If transmissibility were to increase, then thepossibility of spread would likewise increase.

Does WHO recommend any travel precautionsrelated to H7N9?

WHO does not advise special screening atpoints of entry with regard to this event, nor does it currently recommend anytravel or trade restrictions.