Avian Flu Diary: WHO H7N9 Study: Preliminary Age & Sex Distribution

# 7173

From the Western Pacific Surveillance and Response Journal (WPSAR) we’ve a preliminary review of the age and gender demographics of H7N9 infection among China’s first 63 cases reported through April 16th.

You’ll recall we touched on a similar analysis by CIDRAP of some of these same issues on Friday in H7N9: The Riddle Of The Ages.

Unlike the previous infection patterns we’ve seen with the H5N1 avian flu in China – which has had its biggest impact in those 15-39 years of age – the H7N9 patients have been conspicuously older.

Figure 1. Age group and sex distribution of reported human infections with avian influenza A(H7N9) and A(H5N1) viruses, China, as of 16 April 2013

Click to download Figure 1.
jpg, 394kb

Another notable trend : H7N9 patients have been predominantly male, by a ratio of 2 to 1.

This skewing towards elderly males runs counter to China’s population demographics, which is heavily weighted towards young and middle-aged adults and where there are more women than men among the elderly.

Today’s study doesn’t provide us with any answers for why these trends are appearing, but it does provide us with three possible avenues of investigation:

(1) differential exposure between males and females due to gender-associated practices and norms;

(2) biological differences between males and females in the clinical course post exposure/infection; and

(3) differential healthcare-seeking/access behaviour between male and females, leading to surveillance/detection bias.

The study, which was conducted by scientists working with the World Health Organization is open access. Below you’ll find some excerpts (slightly reparagraphed for readability) - follow the link to read it in its entirety.

Human infections with avian influenza A(H7N9) virus in China: preliminary assessments of the age and sex distribution

Perspective

Yuzo Arima,^a Rongqiang Zu,^a Manoj Murhekar,^a Sirenda Vong,^b Tomoe Shimada^a and the World Health Organization Regional Office for the Western Pacific Event Management Team^*

(EXCERPTS)

Since 31 March 2013, the government of China has been notifying the World Health Organization (WHO) of human infections with the avian influenza A(H7N9) virus, as mandated by the International Health Regulations (2005).

While human infections with other subgroups of H7 influenza viruses (e.g. H7N2, H7N3, and H7N7) have previously been reported, the current event in China is of historical significance as it is the first time that A(H7N9) viruses have been detected among humans and the first time that a low pathogenic avian influenza virus is being associated with human fatalities.

In this rapidly evolving situation, detailed epidemiologic and clinical data from reported cases are limited—making assessments challenging—however, some key questions have arisen from the available data. Age and sex data, as one of the first and most readily available data, may be an important proxy for gender-specific behaviours/conditions and an entry point for response.

Here, we describe the age and sex distribution of the human cases of avian influenza A(H7N9) to better inform risk assessments and potential next steps.

Between 31 March and 16 April 2013, there were 63 reported cases of avian influenza A(H7N9). The median age was 64 years (range 4–87), and 45 cases (71%) were male. Notably, 39 of the 63 cases (62%) were ≥ 60 years of age.

When stratified by age and sex, elderly men were the most affected demographic group (Figure 1).

< SNIP lengthy discussion of possible factors that might influence the age/gender demographics of H7N9 infection>

At this time, it is clear that there are more questions than answers. Based on the basic age and sex distribution, we identify several critical questions and options to guide the ongoing investigation:

What are the societal norms and common social practices among elderly men in the affected provinces? Qualitative approaches and involvement of anthropologists/ sociologists specializing in the sociology of health of the Chinese population may be beneficial.

What is the age and sex distribution of severe acute respiratory illnesses and key risk factors for respiratory illness (e.g. smoking) in the underlying population in the affected provinces? While detailed case-based clinical information is pending, data from the general population may be helpful for initial assessments.

What is the age and sex distribution of healthcare utilization in the Chinese population in the affected provinces? Ruling out any possible selection bias will be an important initial step in understanding both the clinical and epidemiologic spectrum of infection.

In these situations, it is easy to dismiss preliminary epidemiologic assessments as being too low in numbers or with too few variables of interest.

There is a need for further case-based information, such as zoonotic exposures and underlying medical conditions. However, for public health workers engaged in rapid response to acute events, it is essential to operate as observational scientists and assess available information to help formulate the next steps.

Following age and sex distributions closely over time may detect important changes in the epidemiology of this virus and with better understanding, high-risk populations, targeted interventions (e.g. gender-specific risk communication messages), prevention and control measures (e.g. vaccination) and treatment options (e.g. antivirals) may be identified.

While this brief and rapid communication cannot offer answers, we hope that public health practitioners involved in similar responses – at various capacities around the world – may consider these key concerns and questions to help them respond to not only the current virus but also other emerging infectious threats.

Over the past 15 years the study of the H5N1 avian flu virus has produced its share of unusual and unexpected findings (see Study: What Makes Avian Flu So Deadly. Similarly, the 2009 H1N1 pandemic virus behaved in ways that were not anticipated (see There’s No Flu Like A New Flu).