London School of Hygiene & Tropical Medicine

Writing in an editorial in the BMJ, Dr James Rudge and Professor Richard Coker say that although the study, led by the Jiangsu Province Center for Disease Control and Prevention, provides the strongest evidence yet of H7N9 transmission between humans, it probably does not mean that the virus has come one step closer towards adapting fully to humans.

The case in question involves a 60 year old man who is thought to have been infected at a live poultry market, and subsequently developed a severe and ultimately fatal respiratory illness. His 32 year old daughter, who provided prolonged bedside care for her father before his admission to intensive care, later also became fatally infected. As there was no indication that the daughter was exposed to live poultry within the days before becoming ill, and because there was almost 100% genetic similarity between the viruses isolated from each patient, the evidence points to transmission from father to daughter.

“Since the new avian influenza virus, H7N9, first emerged in China, a primary concern has been whether it might spread between humans,” write Dr Rudge and Professor Coker. “There are some limitations to the study but, on balance, human to human transmission looks probable.

“Crucially, there is still no evidence of sustained transmission among humans - all 43 close contacts of these two patients, including a son in law who also helped care for the father, tested negative for infection. In addition, the receptor binding sites of the viruses from the two patients are no more adapted towards humans than those of other available H7N9 isolates. In many ways, the evidence corroborates, rather than challenges, previous assertions that the transmissibility of H7N9 between humans is currently low.”

The researchers say that the occasional case of human to human transmission “appears to be the norm rather than the exception for influenza viruses that sporadically cross the species barrier to humans.” They cite examples of other such reported cases with the highly pathogenic avian influenza H5N1, and another bird flu sub-type, H7N7.

“To observe some transmission of H7N9 from human to human is therefore not surprising, and does not necessarily indicate that the virus is on course to develop sustained transmission among humans” they write.

However, Dr Rudge and Prof Coker do highlight several traits of H7N9 that are of particular concern. Previous studies have shown airborne transmissibility of H7N9 between ferrets in the lab (a mammalian model), and it’s also acknowledged that the virus can spread undetected through avian populations as it is not lethal to birds.

In addition to this, Chinese surveillance data suggests that the number of human cases may be much greater than currently reported, as many mild cases are likely to have passed undetected. This does mean that the fatality rate from the virus is likely to be substantially lower than in confirmed cases.  However, the incidence of human infections also gives H7N9 much greater opportunity than other bird flu viruses to adapt to humans or to re-assort through mixed influenza infections.

The researchers conclude that while the study “might not suggest that H7N9 is any closer to delivering the next pandemic, it does provide a timely reminder of the need to remain extremely vigilant: the threat posed by H7N9 has by no means passed.”

Speaking to BBC News, Dr Rudge added: “It would be a worry if we start to see longer chains of transmission between people, when one person infects someone else, who in turn infects more people, and so on. And particularly if each infected case goes on to infect, on average, more than one other person, this would be a strong warning sign that we might be in the early stages of an epidemic.”

Publication

• James Rudge, Richard Coker, Human to human transmission of H7N9, BMJ. Doi: 10.1136/bmj.f4730