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Factors Influencing Control of the H7N9 Epidemic

By Eric Toner, MD, April 19, 2013

The ongoing outbreak of a novel H7N9 avian influenza in eastern China poses a significant threat to public health. Although there have not yet been cases outside of China, the experience with SARS taught us that emerging infectious diseases may be only a plane ride away. Clinicians should be aware of and follow this outbreak. To that end, we have been tracking and providing commentary on important events related to the H7N9 outbreak and their significance, as below.

Confirmed Cases

As of 8 am EDT April 19, 2013, Chinese authorities have confirmed 88 human cases of H7N9 infection with 17 deaths (19% case fatality rate).1 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.2

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.”3 The NYT also 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 H7N9.3

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.4 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; 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. Were pigs or wild birds, for instance, 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 this 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. Xi urges efforts to contain H7N9 bird flu. Xinhua. April 19, 2013.

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

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

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