Objective To determine whether the novel avian influenza H7N9 virus can transmit from person to person and its efficiency.
Design Epidemiological investigations conducted after a family cluster of two patients with avian H7N9 in March 2013.
Setting Wuxi, Eastern China.
Participants Two patients, their close contacts, and relevant environments. Samples from the patients and environments were collected and tested by real time reverse transcriptase-polymerase chain reaction (rRT-PCR), viral culture, and haemagglutination inhibition assay. Any contacts who became ill had samples tested for avian H7N9 by rRT-PCR. Paired serum samples were obtained from contacts for serological testing by haemagglutination inhibition assays.
Main outcomes measures Clinical data, history of exposure before the onset of illnesses, and results of laboratory testing of pathogens and further analysis of sequences and phylogenetic tree to isolated strains.
Results The index patient became ill five to six days after his last exposure to poultry. The second patient, his daughter aged 32, who provided unprotected bedside care in the hospital, had no known exposure to poultry. She developed symptoms six days after her last contact with her father. Two strains were isolated successfully from the two patients. Genome sequence and analyses of phylogenetic trees showed that both viruses were almost genetically identical. Forty three close contacts of both patients were identified. One had mild illness but had negative results for avian H7N9 by rRT-PCR. All 43 close contacts tested negative for haemagglutination inhibition antibodies specific for avian H7N9.
Conclusions The infection of the daughter probably resulted from contact with her father (the index patient) during unprotected exposure, suggesting that in this cluster the virus was able to transmit from person to person. The transmissibility was limited and non-sustainable.
We believe that the most likely explanation for this family cluster of the two patients with novel avian influenza H7N9 virus infection is that the virus transmitted directly from the index patient to his daughter. Firstly, the diagnosis of the two patients was confirmed virologically, and the clinical features―fever, pneumonia with lymphopenia, and rapid progression to acute respiratory distress syndrome—all correspond to the cardinal features of reported cases in humans in China.17 18 Secondly, it was fortuitous for the investigation that the daughter did not visit the markets to buy foodstuffs and cook for the family and had no exposure to animals or history of visiting live poultry markets. She did, however, have prolonged, direct, and unprotected exposure to her father. Thirdly, two strains were isolated successfully from the two patients. Further sequence analysis showed that both possessed high degrees of similarity between nucleotide (99.6%-99.9%) and amino acid (99.0%-100%) sequences. Finally, before this study, A (H7N7) among H7 subtype influenza viruses and H5N1 virus was known to have the ability to transmit from person to person.19 20 21 Animal (ferrets and pigs) experiments indicated that the H7N9 virus possessed the capability to bind to both avian and human receptor and to transmit itself by droplet under certain conditions.22 23
Our findings, however, indicated that the virus has not gained the ability for efficient sustained transmission from person to person. From 14 March, the daughter came into direct contact with the oral secretions of the index patient without any personal protective equipment, contacting the patient at a much higher rate than other individuals in contact with the patient. Similar to other available human strains, the characteristics of the two strains showed no adaptation change in the receptor binding site from the avian 2,3-linked pattern toward the human 2,6-linked pattern of sialic acid receptor.1 7 Phylogenic tree analysis of all eight genomic segments indicated that the two isolates were of avian origin and that there was no reassortment with human or swine influenza viruses.24 2526 Furthermore, no asymptomatic or subclinical infections were identified among 43 close contacts by haemagglutination inhibition testing. Recent studies also indicated that avian H7N9 tends to bind lower pulmonary epithelial cells rather than those of the upper respiratory tract, which makes it difficult to transmit between humans.7
The two patients were blood related. The index patient’s son in law also provided bedside care for the index patient in the mornings and nights between 11 and 13 March without any personal protective equipment. Both biological and serological evidence showed that he was not infected with the H7N9 virus. These findings suggest that potential genetic susceptibility might be one of determinants and that avian influenza viruses, like H5N1, are more easily transmitted between individuals with genetic connection.27 28
The possible source of infection for the index case was likely to be from the live poultry market that the patient used to visit or the six quails that he bought but were slaughtered by the seller in the market one week before illness onset. One strain of avian H7N9 was isolated from environmental samples from the live poultry markets. Visiting a wet poultry market has been identified as a risk factor for human infections.8 9
Important strengths and differences in relation to other studies
With respect to the possible infection period in the second patient, we cannot ascertain when person to person transmission occurred. The most likely period was from 11 to 15 March during the index patient’s admission to hospital before transfer to intensive care, especially the periods between the afternoon of 14 March and the morning of 15 March, when the index patient began to expectorate abundantly. The daughter had direct and close contact with the index case without any protection during this period. During 8-10 March, the daughter provided only conventional care for the index patient, such as taking his temperature. Transmission is unlikely to have occurred during this period as the patient had not yet to start expectoration. Thus, the most likely incubation period was six to seven days (range 6-13 days) based on the daughter’s unprotected exposure to her father. The putative incubation period was a relatively longer than that reported by Cowling and colleagues, who estimated the average incubation period to be around three days based on Weibull model as well as live poultry to human transmission.29 We estimated the incubation period based on person to person transmission and one case in cluster.
Implications of the study
Possible transmission routes include contact while cleaning up infected oral secretions and subsequent inoculation of mucous membranes or the respiratory tract. Some researchers suggested that the daughter might have acquired her infection during the process of washing her father’s diarrhoea-soiled underwear.11 We thought this to be unlikely. Firstly, clinical data showed that the index patient did not develop diarrhoea during course of disease. Secondly, the daughter wore gloves while washing. Thirdly, no nucleic acids specific for avian H7N9 were detected from the faecal samples from the father.
We noted that 39 healthcare workers were identified as close contacts in this cluster, which was a little unusual. Both the two hospitals where the two patients were admitted were general hospitals rather than hospitals specialising in infectious disease. The awareness of personal protection of the healthcare workers was relatively weak. They used common surgical masks instead of N95 masks while providing medical services for the two patients. Another important factor was the relatively long time between the patients’ admission to a clear diagnosis with H7N9 virus infection. After the cluster was identified, infection control measures were initiated to prevent potential nosocomial transmission. Patients were isolated. Healthcare workers were required take standard respiratory and contact protection.
Weaknesses of the study
There are several limitations to our study. Firstly, we could not interview with the two patients as they were both critically ill. Therefore, the possibility that the daughter acquired her infection from the environment or other sources could not be completely ruled out, although we believe that it is less likely. Environmental investigation discovered that in addition to two swans raised by an employee of the property management, there were no other birds or poultry in her living environment. No positive results were detected from the cloacal swab and faecal samples from the two swans. Secondly, the sensitivity of haemagglutination inhibition assay was not satisfied. Thus, subclinical or asymptomatic infections could not be excluded among close contacts. A more sensitive method such as micro-neutralisation assay should be considered in the future. Finally, whole sequence alignment showed that the two isolates from patients were not identical. It is reasonable to expect slight differences between the two strains. Variations of the viral genomes occur constantly because of the high error rate of the viral polymerase. We sequenced viral genomes from the strain from the father that underwent three successive passages using MDCK cell lines. Evolution of the strain might have occurred in vivo because of the long time interval as well as the use of antiviral drugs. A previous study showed that influenza viruses often differ from those present in clinical specimens after isolation in MDCK cell culture as adaptive changes occur during virus transmission from the human host to cells of heterologous origin.30
Unanswered questions and future research
To the best of our knowledge, this is the first report of probable transmissibility of this novel virus from person to person with detailed epidemiological, clinical, and virological data. The importance of an isolated case of such transmission means there is potential for greater human to human transmission. Thus, timely detection as well as rapid investigation and risk assessments of clusters is critically important as the increase in clusters might indicate potential transmissibility of a novel virus.
What is already known on this topic
- Most cases of novel avian H7N9 occur sporadically
- Animal experiments indicated that the H7N9 virus can transmit itself by droplet under certain conditions
- No definite evidence indicates that the novel virus can transmit itself from person to person
- To our best knowledge, this is the first report of probable transmissibility of the H7N9 virus from person to person with detailed epidemiological, clinical, and virological data
- Our findings reinforced that the novel virus possesses the potential for pandemic spread
What this study adds