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Factors Influencing Control of the Epidemic

Image of rooster in cage
Eric Toner, MD
Date posted:
April 18, 2013
Publication type:

Our Perspective on H7N9 Influenza in China: April 18, 2013

More in this series:

April 12 • April 15 • April 24 • May 6


The Staff of the Center for Biosecurity of UPMC is providing periodic updates on important news about the ongoing epidemic of H7N9 in China, along with commentary on their significance.

More Cases Confirmed

As of 8 am EDT April 18, 2013, Chinese authorities have confirmed 83 human cases of H7N9 infection with 17 deaths (20% case fatality rate). Many of the newly reported cases are retrospective confirmations of previously suspected cases in the Shanghai area. One new case has been reported in Henan, a province 500 miles west of Shanghai where 2 other cases were reported earlier this week.1

News sources have quoted Dr. Feng Zijian, director of the Public Health Emergency Center of China CDC, as saying that about 40% of those with the H7N9 virus have had no contact with birds. WHO spokesman Gregory Hartl concurred, telling the New York Times that “there are people who have no history of contact with poultry.”2 The NYTalso reported that 47,801 samples were collected from more than  1,000 live poultry markets in China, but only 39 of those samples have tested positive for H7N9.2

The 7-year-old girl in Beijing who was the first confirmed case outside of the Shanghai region has been discharged from the hospital, but public health experts continue to express concern about the 4-year-old boy in Beijing who tested positive for H7N9 but never had symptoms of infection. If human-to-human transmission does prove possible with this virus, then people who are infected but asymptomatic (ie, are carriers) could spread the disease unknowingly.3 Fortunately, there is no confirmation of person-to-person transmission to date. 

Are Birds the Only Source of Infection?

The rising number of cases is not surprising, given retrospective case identification through expanded testing, and as testing continues, the number will continue to go up. It will take time to understand the true extent of this outbreak.

The biggest concern at this point is that a significant percentage of the confirmed cases have not had contact with birds. One possible explanation for this could simply be recall bias—people often forget what they have done. Another explanation could be that human-to-human spread of the disease is occurring but has not yet been detected, even though close observation of more than 1,000 known contacts of confirmed cases has yet to show evidence of transmission.

Alternatively, cases in people who have not had contact with birds could signal that people are becoming infected in places beyond farms and live poultry markets, such as in restaurants, where poultry is slaughtered on site. Another possible explanation could be transmission from other animal reservoir(s) that have not yet been discovered. If pigs or wild birds, for instance, were found to be reservoirs of the virus, that finding would be consistent with the relatively small number of positive tests from the poultry markets.

Factors Influencing Control of the Epidemic

As we have noted previously, the key factor in how this epidemic will unfold is that of sustained person-to-person transmission, which has not yet been documented. While small clusters have been reported and are being investigated, they may not prove significant. Even low levels of transmission among people who live very close together do not signal sustained community transmission. However, if sustained human-to-human transmission does develop, the anticipated size and impact of and the necessary response to this outbreak could all change dramatically.

The other key factor is identification of all virus reservoirs through expanded agricultural testing. Right now, live poultry markets can explain some but not all of the reported human cases. Control will depend on identifying all sources of infection. 


  1. Henan reports 1 new H7N9 case. Xinhua. April 18, 2013.

  2. Perlez, J. Investigators look beyond birds for origin of H7N9 flu strain. New York Times. April 18, 2013.

  3. Shadbolt,P. 4-year-old bird flu 'carrier' worries China. CNN. April 17, 2013. Accessed April 17, 2013.



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