Study finds urban centers can serve as flu virus sources

Nov 15, 2011 (CIDRAP News) – New seasonal flu viruses can launch from one of many world urban centers, not just from tropical areas in Southeast and East Asia, as previously thought, a research group reported yesterday.

The international team of scientists also found that any of the urban centers can be the source of flu viruses for any other area. Their findings were published yesterday in an early online edition of Proceedings of the National Academy of Sciences (PNAS).

To gauge seasonal dynamics and migration patterns from 2003 to 2006, they analyzed RNA sequences of influenza A (H3N2) samples from Australia, Europe, Japan, New York State, New Zealand, and Southeast Asia. They also included 105 newly sequenced viruses from Hong Kong.

Phylogeographic analysis showed that flu virus populations in tropical Southeast Asia and Hong Kong had relatively low levels of genetic diversity and no seasonal fluctuations when compared with viruses responsible for outbreaks in more temperate regions.

They found high rates of viral migration between different urban centers, the group reported.

Gavin Smith, PhD, the study's senior author and associate professor of the Duke University–National University of Singapore (NUS) Program in Emerging Infectious Diseases, said in a press release that urban centers are a network connected by air travel, which can contribute to a series of flu epidemics that overlap in time.

He noted that other examples of global virus movement facilitated by air travel include the SARS epidemic and the 2009 H1N1 influenza pandemic.

Viral population persisted across time in Southeast Asia and Hong Kong, and infection patterns were influenced by viruses from temperate regions that have distinct flu seasons. "Larger regions with greater connectedness may potentially contribute more to the global diversity of influenza viruses circulating," he said.

According to the analysis, multiple lineages of a virus could seed yearly flu epidemics, with the ability of each region to function as a possible source.

Justin Bahl, PhD, lead author of the study and assistant professor in the Duke-NUS Program in Emerging Diseases, said in a press release that current flu strain selection strategies are generally effective, but that the group's findings might improve the process by shedding new light on virus migration and connections between regions.

The researchers say they plan to expand on their study by including new data from areas that currently have little or no genetic information available. Bahl said this work is part of a larger effort to better understand the patterns and mechanisms of respiratory virus transmission in humans.

Other studies over the years have proposed different theories regarding the spread of flu, including flu virus migration between the Northern and Southern hemispheres, from tropical areas, or from China. In 2008 a group from Cambridge University analyzed 13,000 H3N2 samples from six continents in 2002 to 2007 and suggested that new variants appear in Asian countries first.

The World Health Organization's (WHO's) flu vaccine selection advisers meet twice a year to recommend the three seasonal flu strains to include in upcoming seasonal flu vaccines. They meet in September to make the recommendation for the Southern Hemisphere and in February to issue the same type of guidance for the Northern Hemisphere. They base their decision on year-round analysis of virologic and epidemiologic data and surveillance forecasts.

In June 2010 the WHO held an information consultation to improve the influenza vaccine virus process. It will hold a follow-up meeting on the topic Dec 7 to Dec 9.

According to background materials on the meeting, experts are slated to discuss ways to improve surveillance data, new assays and modeling approaches, the relationships between virus characteristics and vaccine efficacy, and regulatory considerations. The agenda also notes that the group will discuss new bioinformatics tools for genetic analysis and vaccine virus selection.

Bahl J, Nelson MI, Chan KH, et al. Temporally structured metapopulation dynamics and persistence of influenza A H3N2 virus in humans. Proc Natl Acad Sci 2011 Nov 14 [Full text]

See also:

Nov 14 EurekAlert press release

Apr 16, 2008, CIDRAP News story "Study: new seasonal flu strains launch from Asia"

WHO background information

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