WHO RISK ASSESSMENT
Human infections with influenza A(H7N9) virus
13April 2013
Summary of available information
As of 13April 2013, a total of 49 confirmed cases of human infection with avian influenza A(H7N9)
virus have been reported to WHO by the China National Health and Family Planning Commission.
Among these cases, the ages range from 4 to 87; 15 are female. Eleven persons have died, and the
majority of the additional cases are considered severe. Of the 49 cases, 6 have been reported today
and further investigations are taking place. The cases have been reported from three provinces:
Anhui, Jiangsu and Zhejiang, and two municipalities, Beijing and Shanghai. All locations are in Eastern
and Northern China.
Two confirmed cases have been associated with possible family clusters, in which one and two
additional family members,respectively, developed severe pneumonia. Close contacts of confirmed
cases and health care workers caring for cases have been monitored for infection. So far, among the
contacts who have been tested by polymerase chain reaction, none has been shown to have
infection.
This is the first time human infection with this influenza subtype, avian influenza A(H7N9) virus, has
been detected. Previously,sporadic cases of human infection with otherinfluenza A(H7) viruses
have been reported. Those cases were associated with outbreaks of infection in poultry in other
countries. These earlierinfluenza A(H7) human infections generally resulted in mild influenza illness
with some conjunctivitis.
Genetic and laboratory characterization of the first three of these H7N9 virusesisolated from
humans indicatesthat:
• the virus contains a group of avian influenza virus genes from three different avian influenza
viruses;
• to date, genetic analyses of the isolates have shown certain changes, including amino acid
substitutions associated with increased affinity to alpha 2-6 receptors, which suggests that
the H7N9 virus may have greater ability to infect mammalian species, including humans,
than most other avian influenza viruses;
• there are sequence variations among the genes of three isolates that suggestthere has been
more than one introduction of this virus from animal into humans;
• these viruses are expected to be sensitive to the neuraminidase inhibitor drugs oseltamivir
and zanamivir, but resistant to the antiviral drugs amantadine and rimantadine;
• the isolates have a haemagglutinin structure thatis associated with low pathogenicity in
birds.
There are several gaps in critical information at this time, including the animal reservoir(s)in which
this virus is circulating, the main exposures and routes of transmission for how human infections
have been acquired, and the current scope of the spread of this virus among animal and human
populations. Avian influenza A(H7N9) viruses have now been isolated from poultry (including duck)
and pigeon in the live birdmarketsin some areas of China, but whether other potential reservoirs of
this virus may exist, including in other domestic and wild bird species, and mammalian species such
as pigs, has not yet been determined clearly.
So far, this virus has not been associated with reports of severe disease in poultry.
Risk assessment
This initial risk assessment, which has been prepared in accordance withWHO’s published
recommendations for rapid risk assessment of acute public health events1
will be updated as further
information becomes available.
What is the risk of the occurrence of further cases in the affected areas of China and other areas?
The epidemiology of this virus among animals, including the main reservoirs of infection among
animals and the extent of geographic spread, is not yet established. However, it is likely that most
human H7N9 infections so far are associated with infection among as-of-yet undetermined animals
and that further human cases of infection should be expected.
What is the risk of human-to-human transmission?
There is no evidence of sustained human-to-human transmission. However the two possible family
clusters suggestthat limited human-to-human transmissionmay occur where there is close contact
between cases and other individuals, as occurs in families and, potentially, healthcare settings.
Moreover, the genetic changes seen among these viruses suggesting adaptation to mammals is of
concern, and further adaptation may occur.
What isthe risk of international spread?
At this time, there is no information to indicate international spread of this virus. However, it is
possible that an infected person, who may or may not have symptoms, could travel to another
country. However, if the virus cannot sustain human-to-human transmission, as appears to be the
current situation, then extensive community spread is unlikely.
WHO does not advise special screening at points of entry with regard to this event, nor does it
recommend that any travel or trade restrictions be applied.
References
Most recent disease outbreak news can be found at:
http://www.who.int/csr/don/en/index.html
Background and summary of human infection with influenza A(H7N9) virus (as of 5 April 2013):
http://www.who.int/influenza/human_animal_interface/update_20130405/en/index.html
Frequently Asked Questions on human infection with influenza A(H7N9) virus, China:
http://www.who.int/influenza/human_animal_interface/faq_H7N9/en/index.html
