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Important Questions about H7N9 in China

Image of rooster in cage
Eric Toner, MD
Date posted:
April 12, 2013
Publication type:

Our Perspective on H7N9 Influenza in China: April 12, 2013

More in this series:

April 15 • April 18 • April 24 • May 6


The epidemic of novel H7N9 avian influenza in eastern China that was first reported on March 31, 2013, continues with no sign of abating. As of 9:00 AM (EDT) today, there have been 38 confirmed cases, with 10 deaths. A synthesis of information that has emerged in the past week follows.

Source of the Outbreak

The mostly likely source of the human infections is infected domestic birds. To date, the H7N9 virus has been detected in pigeons and chickens in live markets in and around Shanghai; it does not appear to cause illness in the birds that carry it. So far, the virus has not been found in pigs or other mammals.2Many, but not all, of the human cases have had some contact with poultry or live animal markets.

Human Cases

Of the 38 confirmed cases, 19 have been clinically severe, 9 have been mild, and 10 have died. Most of the cases reported in the last week occurred in elderly people, which is consistent with the earlier cases.1So far, there has been no confirmed evidence of sustained person-to-person spread.1 However, limited spread between close contacts cannot yet be excluded. A related virus, H7N7, has been known to spread between people. Currently, more than 760 contacts of known cases are under observation.


Genomic analysis of sequence data indicates that this novel H7N9 virus resulted from the reassortment of gene segments from 2 distinct sources. The hemagglutinin and neuraminidase genes are both from H7N9 viruses found in wild birds in Korea and other parts of east Asia; the other 6 gene segments originated from other avian influenza viruses found in ducks and chickens in the Yangtze River delta near Shanghai.3Reassortment is the shuffling of viral genes that can happen when an animal is infected with 2 different influenza viruses simultaneously. It appears that this is what has happened with the novel H7N9 virus in birds.


Researchers in a number of countries are gearing up to produce vaccines once they receive samples of the virus from China. As Uyeki and Cox from the CDC note, however, H7 vaccines in the past have not been very immunogenic in humans.4

Significance and Important Questions

The lack of apparent sustained human transmission is important. Whether the disease is spreading from person to person or not determines whether we are watching the birth of a pandemic or a cluster of zoonotic infections. Continued careful observation of the contacts of confirmed cases and testing of people with respiratory illness will be critical.

The incubation period of disease caused by the H7N9 virus has not been determined; therefore, recent travelers to the affected area should be monitored for signs of illness.

Serological testing to detect evidence of prior infection will be important, as well, to identify previously unrecognized mild infections, but these studies will take considerable time.

The case fatality rate (CFR) of confirmed cases is now quite high—over 25%. This figure is likely to drop some as more mild cases are recognized, and as severe cases are treated early with antiviral drugs. It is, however, disturbingly high.

The fact that the virus does not make birds sick is a problem for control of the epidemic. The key to control, at least in the short term, is culling infected birds, but it is difficult to identify infected birds if they show no signs of illness. This experience stands in sharp contrast to that of H5N1, a virus that causes severe illness in poultry and makes infected birds easy to identify.

While it is not unusual for birds to be asymptomatic carriers of influenza viruses, we have not seen a low pathogenic avian virus5 in the past that causes such severe illness in people. Other provinces in China and other countries in the region should be conducting disease surveillance (testing) on their birds to determine the spread of this virus.

The development, testing, manufacture, distribution, and administration of effective animal and human vaccines is important to controlling the infection in birds and protecting people at risk. However, experience indicates that this is not a simple task or one that will be accomplished rapidly. Even if seed strains are made available quickly, it will likely take many months for a safe and effective human vaccine to be made widely available.


  1. Human infection with influenza A(H7N9) virus in China – update April 11, 2013. World Health Organization. Accessed April 12, 2013.

  2. The OIE, World Organisation for Animal Health, highlights the exceptional nature of the influenza A(H7N9) event notified by China [news release]. Paris, France: OIE, World Organization for Animal Health. April 11, 2013. Accessed April 12, 2013.

  3. Top Chinese lab reveals H7N9 source. Xinhua. April 10, 2013. Accessed April 12, 2013.

  4. Uyeki T and Cox N. Global concerns regarding novel influenza A (H7N9) virus infections. NEJM. April 11,2013. DOI: 10.1056/NEJMp1304661. Accessed April 12, 2013.

  5. Influenza viruses. US Centers for Disease Control and Prevention website. Accessed April 12, 2013.



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