Pandemic influenza

Influenza epidemics occur each winter, due to subtle mutations in the influenza virus as it evolves. Influenza arrives suddenly each year, spreads rapidly to peak within 2 to 3 weeks, and lasts 5 to 10 weeks. Community attack rates range from 10% to 20%, but can be more than 50% in closed settings such as long-term care facilities or schools. Peak hospitalization levels occur in the very young and very old, and average as much as 70 000 and 75 000 annually in Canada. Although 90% of deaths due to influenza occur among the elderly, individuals of all ages with underlying conditions are at increased risk.[1]

As threatening as influenza is on an annual basis, it has the potential to be much worse. Pandemics have been documented since the 16th century as unpredictable events that occur every 10 to 40 years.[1] Pandemic influenza subtypes typically emerge from the large reservoir of influenza A viruses that exist naturally in wild waterfowl.[2] Three pandemics occurred last century, the last in 1968. The first, and most severe, was the Spanish pandemic of 1918–1919, which caused 20 million to 40 million deaths in just a few months—more than in all four years of the first World War. Nearly half of the victims were young, healthy adults.[3]

Several influenza subtypes have recently emerged as potential pandemic candidates—most notably avian influenza H5N1. First arising in Hong Kong in 1997, this virus caused poultry outbreaks and 18 human infections, including six deaths.[4] Although rapid destruction of the entire Hong Kong poultry population reduced opportunities for further transmission to humans, the virus re-emerged in late 2003 and since then has caused widespread poultry outbreaks in Japan, South Korea, Cambodia, Vietnam, Thailand, China, Indonesia, and Laos. To date, efforts to halt these outbreaks have been unsuccessful, despite the death or culling of over 100 million birds. Of significant concern are the associated human infections. Between 28 January 2004 and 26 January 2005, 54 human infections were confirmed in Vietnam and Thailand, 41 of whom died. This virus is distinguished by its alarming virulence and predilection for young people—67% of the infections were in people 20 years of age or younger. Human infections have, so far, resulted primarily from direct contact with infected birds, and there is no evidence of efficient person-to-person spread. However, through mutation or genetic reassortment, the virus may acquire this ability and culminate in a pandemic.[1]

Preparation and prevention

The BC Centre for Disease Control has coordinated the development of the BC Pandemic Influenza Preparedness Plan, which provides information for pandemic planning. Clinicians are asked to be vigilant, use proper precautions, and elicit a travel history from those presenting with severe influenza-like illness. Public health should be notified of any concerns.

The cornerstone of influenza prevention is vaccination. Increased annual immunization will help build capacity in advance of the pandemic, when millions of people will have to be immunized in a very short time. In the early stages, vaccine is likely to be delayed or in short supply. Basic infection control precautions can help prevent spread, including proper disposal of used tissues, covering nose and mouth when coughing or sneezing, and frequent hand washing.[1] Antiviral drugs such as oseltamivir may also help to treat and/or prevent influenza infections. Antiviral drug supplies will also be very limited. Stockpile options and priority groups for its use should be determined well in advance.

The best way to prepare for the next pandemic is to optimize the use of prevention and control measures between pandemics. Since cumulative mortality due to influenza is still greater in the intervening years between pandemics, improved use of inter-pandemic control measures will yield multiple returns.[1]

—Aleina Tweed, MSc
—Danuta Skowronski, MD

References

1. Skowronski DM, Stiver HG, McGeer A. Preparing for the next influenza pandemic: What every clinician should know. Royal College Outlook 2004;1:5-8.
2. Trampuz A, Prabhu RM, Smith TF, et al. Avian influenza: A new pandemic threat? Mayo Clin Proc 2004;79:523-530. PubMed Abstract
3. Katz JM. Preparing for the next influenza pandemic. ASM News 2004;70:412-419.
4. Chan PK. Outbreak of avian influenza A(H5N1) virus infection in Hong Kong in 1997. Clin Infect Dis 2002;34(suppl 2):s58-s64. PubMed Abstract Full Text