The international scientific community and decision-makers on public health are debating how best to manage the anticipated vaccine shortage for the new pandemic strain of influenza A virus, recently emerged from the animal reservoir. Priority distribution of the first product batches must be to individuals at high risk and to crucial employees.

But decisions about priority distribution should also take into account the benefits to the international community of vaccination in developing countries. Viral 'shedding' (the expulsion of virus) is reduced in vaccinated individuals and therefore the risk of reassortment with animal viruses is decreased. This consideration is in line with the 'One Health' vision – a multidisciplinary initiative to improve the health of humans, animals and the environment that is endorsed by the United Nations Food and Agriculture Organization, the World Health Organization and the World Organisation for Animal Health (OIE).

There is a risk of generating novel influenza A viruses through reassortment of the eight genes that result in antigenic shift, which would give rise to strains to which the human population has no immunity. For example, reassortment occurred between avian and human influenza viruses to create the human pandemic viruses of 1957 and 1968 (K. Subbarao et al. in Influenza Virology Current Topics ed. Y. Kawaoka, 229–280, Caister Academic; 2006).

We are at present in a unique situation with the worldwide spread of the latest pandemic H1N1 virus, known as novel animal-origin H1N1 (nao-H1N1) virus. Concurrently, and possibly for the first time in history, several developing countries are experiencing widespread infections in poultry by avian influenza viruses of the H5N1 and H9N2 subtypes, which also infect humans. In addition, H5N1 viruses that are widespread in Africa, the Middle East and Asia contain genetic mutations that reflect increased virulence for humans (see, for example, E. De Wit and R. A. M. Fouchier J. Clin. Virol. 41, 1–6; 2008, and G. Cattoli et al. PLoS ONE 4, e4842; 2009).

In countries where animal husbandry practices fall short of accepted biosecurity standards and where immunologically naive animal caretakers infected with nao-H1N1 would shed large amounts of infectious virus, there is a significant risk of emergence of a reassortant virus. A reassortant virus containing a combination of genes, including a novel-human adapted influenza virus and H5N1 or H9N2, could result in chimeric viruses with unknown characteristics.

Fast-tracking vaccination of humans against pandemic influenza in developing countries where zoonotic flu in poultry is endemic would help prevent reassortment between naoH1N1 or other novel pandemic influenza strains and avian influenza viruses. That would deflect the unpredictable and serious consequences of viral reassortment to humankind worldwide.