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H1N1 Dual Infections

By Amesh A. Adalja, MD, September 4, 2009

As the world is currently in the midst of a WHO stage VI pandemic with the 2009 H1N1 influenza virus, it remains a possibility that this virus could reassort with other flu strains—most notably, the WHO pandemic stage III H5N1 avian influenza virus.

Soon after the novel H1N1 virus began to circulate in areas in which human infections with H5N1 have occurred (Asia, Egypt, etc.), the prospect arose of a dual infection with the highly transmissible H1N1 strain and the highly lethal H5N1 strain. Given the genetic characteristics of influenza viruses, this event was feared because of a dual event’s capacity to spawn a reassorted virus with the pathogenicity of H5N1 and the transmissibility of H1N1. While reassortment with the H5N1 virus is the most feared event, reassortment with less severe seasonal strains could also produce a virus more adept at causing disease.

2009 H1N1 Reassortment with Seasonal Influenza Thought Unlikely

PLoS Currents recently published results of a study conducted by a University of Maryland research team that assessed the ability of the 2009 H1N1 influenza virus to reassort with seasonal H1N1 and H3N2 viruses. The researchers found no evidence of virus reassortment in ferrets that were infected with both the pandemic virus and one of the seasonal variants. Additionally, the team discovered that the novel influenza strain has growth advantages over ordinary seasonal strains (H1N1, H3N2), and reassortment is not favored in vivo. However, the researchers did not test the reassorting potential of H5N1 influenza viruses.1

H5N1 Can Reassort with Seasonal Influenza but Has Poor Transmissibility

Researchers have, in the past, constructed reassorted H5N1/seasonal influenza variants that were able to replicate efficiently, but were not highly transmissible.2,3

Will H5N1 and the 2009 H1N1 Reassort to Form a Highly Lethal and Contagious Influenza Strain?

Almost simultaneous with the publication of the study noted above, an erroneous report from Egypt describing a dual H5N1 and 2009 H1N1 infection appeared.4 Many may be relieved that the report from Egypt is false; however, the prospect of reassortment between the two viruses cannot be overlooked. While the 2009 H1N1 strain may not be able to recombine with seasonal variants, studies are needed to ascertain the ability of the virus to reassort with H5, H7, H9, and H10 avian influenza viruses. Finally, the reports from Egypt may have proved false, but they do underscore the need for rapid influenza diagnostics, including sequence analysis, to detect sentinel events that may forestall further spread of viruses with the potential to spark a lethal pandemic.


  1. Perez D, Sorrell E, Angel M, et al. Fitness of pandemic H1N1 and seasonal influenza A viruses during co-infection. PLoS Currents: Influenza 2009. Accessed September 2, 2009.

  2. Maines TR, Chen L, Matsuoka Y, et al. Lack of transmission of H5N1 avian–human reassortant influenza viruses in a ferret model. PNAS 2006;103:12121-12126.
    . Accessed September 2, 2009.

  3. Jackson S, Van Hoeven N, Chen L, et al. Reassortment between avian H5N1 and human H3N2 influenza viruses in ferrets: a public health risk assessment. J. Virol 2009; 83:8131-8140. Accessed September 2, 2009. 

  4. ProMed-mail. H5N1/H1N1 VIRUS CO-INFECTION - EGYPT (02): ex SAUDI ARABIA, NOT.
    . Accessed September 3, 2009.