The pandemic potential of avian influenza A(H7N9) virus: a review

Abstract

In March 2013 the first cases of human avian influenza A(H7N9) were reported to the World Health Organization. Since that time, over 650 cases have been reported. Infections are associated with considerable morbidity and mortality, particularly within certain demographic groups. This rapid increase in cases over a brief time period is alarming and has raised concerns about the pandemic potential of the H7N9 virus. Three major factors influence the pandemic potential of an influenza virus: (1) its ability to cause human disease, (2) the immunity of the population to the virus, and (3) the transmission potential of the virus. This paper reviews what is currently known about each of these factors with respect to avian influenza A(H7N9). Currently, sustained human-to-human transmission of H7N9 has not been reported; however, population immunity to the virus is considered very low, and the virus has significant ability to cause human disease. Several statistical and geographical modelling studies have estimated and predicted the spread of the H7N9 virus in humans and avian species, and some have identified potential risk factors associated with disease transmission. Additionally, assessment tools have been developed to evaluate the pandemic potential of H7N9 and other influenza viruses. These tools could also hypothetically be used to monitor changes in the pandemic potential of a particular virus over time.

Keywords: Avian flu; influenza; influenza A; pandemic; risk assessment.

Fig. 1.

Number of H7N9 cases reported weekly since April 2013; as of May 2015. Number of cases each week are based on dates of case reports from the Hong Kong Centre for Health Protection. Data source: Hong Kong Centre for Health Protection weekly Avian Influenza Report [32]. * Nineteen cases were reported on 10 March 2015, with onset dates in the 5 weeks preceding 25 February. These cases were added to others reported 10 March 2015, thus the distribution of H7N9 cases over this time period is slightly distorted.

Fig. 2.

Relative weights of each IRAT element as they relate to pandemic influenza emergence (sustained human-to-human transmission) and the potential impact on public health, in the case of an outbreak. Based on IRAT weights in Cox et al. [111]. Arrow thickness is proportional to the contributive weight of the IRAT element to pandemic influenza virus emergence or public health impact.