Swine Flu: Bringing the Pandemic Out of the Box

I didn’t want to jump on the swine flu bandwagon as this current global pandemic is getting so much coverage at the moment. However, I feel obliged to comment on the H1N1 influenza type A virus as production and testing rushes novel swine flu vaccines into readiness. Human trials of these vaccines are currently underway in Australia and USA; European vaccines will bypass large scale human trials. However, all of these novel vaccines should be set for injection before the winter flu season. Some investigators argue that trials won’t be rigorous enough to fully inform vaccine implementation, others that with or without human trials these vaccines will be safe and essential to saving countless lives  But how much of a menace is H1N1? There appears to be not so much cause for concern at the moment, but a lot of money and influence behind these novel interventions. So what’s all the fuss?

Influenza types A and B are the typical flu viruses which infect humans, resulting in 2 annual seasonal flu transmission periods, three to five million cases of severe illness and 250,000 to 500,000 deaths. The majority of seasonal flu deaths occur among adults over 65 years.¹ Currently an estimated 134,503 individuals are infected with swine flu, 816 deaths have resulted.² Those at risk of death and severe disease outcome have been younger compared to seasonal influenza viruses, and include children, pregnant women and those with underlying medical conditions. The additional burden of H1N1 for the forthcoming flu season is difficult to predict.

Reassortment Events: 2009 Influenza A (H1N1) Virus

Although much of the genetic sequence of swine flu virus originates from a viral strain which infects pigs, to date this particular H1N1 influenza A strain has not been identified in swine populations. The swine flu H1N1 pandemic virus is a combination of four influenza virus sequences from North American swine and avian influenzas, human influenza, and Eurasian swine influenza.³ This novel virus illustrates how cross species transmission of a virus can occur effectively as a result of viral recombination and re-assortment. Many organisms which cause some of today’s global pandemics such as malaria and HIV/AIDS are also known to have had their genetic origin as infectious organisms in other mammals. The current swine flu virus is a new H1N1 virus which differs markedly from the virus which caused Spanish flu. However, the 1918 -1919 Spanish flu pandemic, which resulted in up to 40 million global deaths, was also caused by an H1N1 influenza A virus.⁴

Another diverse influenza A virus, H5N1 (bird flu) caused global alarm in recent years. Like swine flu, bird flu can infect humans. However, there are dramatic differences between disease outcomes in humans for these flu viruses. The elevated H5N1 mortality rates are much more alarming compared to those with H1N1. There have been 436 reported cases of H5N1 infections in humans; amongst these 262 deaths have occurred ⁵. Fortunately the H5N1 bird flu virus strain has proven to be not easily transmitted between humans.

There are limitations to swine flu mortality rate estimates as H1N1 appears readily transmissible between humans, so the number of infected individuals remains highly speculative. The basic reproductive rate (R₀) describes ‘the number of individuals infected by a single infected individual during his or her entire infectious period, in a population which is entirely susceptible.’⁶ Basic reproductive rates above 1 suggest that infection can spread within a population; large R₀ values indicate an epidemic. Recent epidemiological estimates suggest a reproductive rate between 1.4 and 1.6 for H1N1, which is higher than the expected rate for seasonal flu,^7,8 but lower compared with rates (2 to 4) for the 1918-1919 Spanish flu pandemic.⁴ However, that the time and original human host of the current swine flu H1N1 virus can be precisely defined is both impressive and important in determining the genetic and epidemic history of this virus. Both genetic and epidemiological factors will help to limit the future impact of the swine flu pandemic.

So, I get it. In a nutshell, the reasons for the H1N1 fuss as I see it are: the novelty of the virus, the apparent ease with which the virus is spread, the expanded high risk groups for severe flu and mortality, the risk of viral mutation and recombination to a more virulent H1N1 virus for the winter flu season.

A lot of people are very excited about H1N1; it’s a hot topic! But really, although the swine flu mortality cases are devastating for families and communities, the data so far are telling us not to worry too much about this particular virus.

‘Catastrophic disruption of societal function, as anticipated in some planning scenarios for a severe pandemic, is unlikely.’⁹

Swine flu is not likely to become a severe pandemic, but it is important to be cautious. The readiness of vaccine supplies globally and potential adverse effects of interventions must not be overlooked in the rush for vaccine sales.¹⁰ It’s also important to be mindful that infectious diseases do not respect the sovereignty of individual nations; neglecting this fact may spell a public health disaster for all.

I suggest that while the money and political will exist, we should get on the bus and ride with the H1N1 global pandemic preparedness efforts – just in case! Whatever happens with the swine flu pandemic, let’s be sure to become enlightened and perhaps also pick up some other compelling, public health passengers along the way.

1. WHO. Influenza (Seasonal). 2009;

http://www.who.int/mediacentre/factsheets/fs211/en/

2. WHO. Pandemic (H1N1). 2009; Update 59   http://www.who.int/csr/don/2009_07_27/en/index.html

3. Reuters. Special Focus; Influenza A (H1N1). Thompson Reuters 2009;    http://science.thomsonreuters.com/pharma/h1n1/

4. Mills CE, Robins JM, Lipsitch M. Transmissibility of 1918 pandemic influenza. Nature 2004; 432 (7019) 904-6

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=15602562

5. WHO. Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) 2009; http://www.who.int/csr/disease/avian_influenza/country/ cases_

table_2009_07_01/en/index.html

6. Heffernan JM, Smith RJ, Wahl LM. Perspectives on the basic reproductive ratio. J R Soc Interface 2005; 2 (4) 281-93 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=16849186

7. Coburn BJ, Wagner BG, Blower S. Modeling influenza epidemics and pandemics: insights into the future of swine flu (H1N1). BMC Med 2009; 7  30 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=19545404

8. Fraser C, Donnelly CA, Cauchemez S, Hanage WP, Van Kerkhove MD, Hollingsworth TD, Griffin J, Baggaley RF, Jenkins HE, Lyons EJ, Jombart T, Hinsley WR, Grassly NC, Balloux F, Ghani AC, Ferguson NM, Rambaut A, Pybus OG, Lopez-Gatell H, Alpuche-Aranda CM, Chapela IB, Zavala EP, Guevara DM, Checchi F, Garcia E, Hugonnet S, Roth C. Pandemic potential of a strain of influenza A (H1N1): early findings. Science 2009; 324 (5934) 1557-61 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&dopt=Citation&list_uids=19433588

9. NBDB. H1N1 Countermeasures Strategy & Decisionmaking: A Report Prepared By The Pandemic Influenza Working Group (PIWG) National Bioscience Defense Board 2009; http://www.hhs.gov/aspr/conferences/nbsb/panflu-wg-execrpt-090717.pdf

10. Couzin-Frankel J. Retrospective: What Happened With Swine Flu in 1976. Science Insider 2009; http://blogs.sciencemag.org/scienceinsider/2009/04/retrospective-w.html

Photo: Pandemic preparedness (WHO)