IOWA CITY —
Like H5N1, H7N9 appears to be an avian virus. First identified in humans last month near Shanghai, it's now infected at least 24 people in four of China's provinces; seven of them have died, according to official World Health Organization numbers. (Other sites are reporting as many as 33 cases and 9 fatalities). So far, we haven't seen any evidence of sharing among humans, which is a good sign, though some possible family clusters are being investigated. And while there's some concern that Chinese hospitals have been covering up possible cases of the infection (as happened during the early weeks of the SARS outbreak in 2002), Chinese authorities do appear to be proactive now that the outbreak is international news. The Ministry of Agriculture released findings of H7N9-positive pigeons from a marketplace in Shanghai, which led to a cull of poultry from that market in an attempt to stem new human infections. In some places, markets have been shut down. With H5N1, most infections have been in people with direct contact with live poultry, either in marketplaces or on poultry farms. These outbreaks are devastating for Asian poultry producers, who house approximately 4 billion chickens and are already feeling the sting of the H7N9 outbreak.
One reason to worry about both H5N1 and H7N9 is that the data we have on infections and deaths are available largely because they have made people very ill — ill enough to seek treatment at a hospital. As you might imagine, calculating infection and death rates based only on hospital data — where you'll find the sickest of the sick — is kind of like asking if someone in the vicinity of Yankee Stadium on game day is a baseball fan. You've artificially enriched your population with baseball lovers, so you may find that 95 percent of the population adores the sport — numbers that wouldn't hold up if you'd sampled more randomly throughout New York City. So, in reality, H5N1 (and possibly H7N9) likely don't have death rates of 60 percent and 30 percent, respectively. But we're probably missing more human cases — baseball fans who didn't make it to the game — which means that there are more spillover events from birds to humans than medical authorities are picking up. With H5N1, when researchers have gone to look for these undocumented infections, results have been mixed. Some groups didn't find mild or asymptomatic cases, while others found people who did have evidence of prior infection with H5N1 (they carried antibodies which suggested their bodies had seen the virus sometime in the past) but no evidence of clinical illness with symptoms befitting H5N1. If and when larger-scale studies of H7N9 are conducted, we'll probably see the same thing: It's likely that more people are being infected than are becoming seriously ill, but the mild or remote cases just aren't showing up on the radar of those who are doing surveillance right now.