Annals of Long Term Care

September 12-16, 2009
Vienna, Austria

Update on the H1N1 Influenza Epidemic

The European Respiratory Society held its 19th annual congress in Vienna, Austria, September 12-16, 2009. This meeting has become the most important annual gathering of clinicians and researchers in the respiratory field (over 18,000 in attendance this year). This year’s congress was met with great anticipation because of its unique timing with respect to developments in the H1N1 influenza epidemic. The highlight of the congress was an up-to-the-minute symposium on the H1N1 pandemic, including sessions on the history and epidemiology of the influenza virus, clinical experience with the current pandemic, strategies for prevention and containment, and recommendations for health system preparedness. Key points from the symposium are summarized below. For more extensive review of the H1N1 pandemic developments and updates on clinical recommendations, visit the Centers for Disease Control and Prevention (CDC) website at www.cdc.gov and the World Health Organization (WHO) website at www.who.int.

H1N1 Influenza: Panic or Pandemic

Background and Epidemiology

An epidemiologist from WHO presented an interesting historical review of influenza epidemics and the epidemiology of the current H1N1 pandemic. Virtually all influenza disease that affects humans originates in bird species. The influenza virus is well adapted to infecting and transmitting through avian hosts. The influenza virus can also infect mammalian species wherever birds and mammals co-exist, but the virus must then mutate to successfully transmit from mammal to mammal. When the virus has made that mutation and adapts to transmission in the mammalian host, that animal is then a reservoir of a new strain of influenza that is potentially more infective to other mammals.

The current H1N1 virus has made the successful mutation from bird to swine, and then to humans, and has proven exceptionally successful in human-to-human transmission. In contrast, the H5N1 influenza virus, “avian influenza,” which is very infectious among bird species and is transmitted by both domestic and migratory birds, has shown only limited infectivity and ability to spread from human to human. Human cases of H5N1 influenza have been reported in Asia, Africa, Europe, and the Pacific, and it is extremely pathogenic, 60% fatal. Fortunately, it has not yet made the mutation for successful mammalian infection and transmission.

As of September 2009, there have been approximately 300,000 documented cases of H1N1 influenza worldwide and over 3000 deaths. Although this appears to be a mortality rate of about 1%, the denominator of total cases is likely much higher than 300,000 since many countries only report severe cases. To put these mortality statistics in perspective, the “Spanish flu” of 1918 was estimated to have killed over 40 million persons, and the “Asian flu” of 1957 and the “Hong Kong flu” of 1968 each killed 1-4 million persons worldwide. The annual seasonal influenza typically kills 250,000-500,000 persons each year.

These mortality statistics might seem to bode well for this pandemic; however, the CDC is forecasting up to 2 billion cases of H1N1 influenza worldwide. Even if the mortality rate is less than 1%, the huge denominator of cases could result in a high number of fatalities. Epidemiologists also caution that the greatest numbers of fatalities with the 1918 pandemic, also an H1N1 virus, came later in the second and third waves of the outbreak. The biggest difference between this pandemic and that of 1918 is that we appear to have an effective vaccine that is soon going to be widely available, as well as antiviral drugs to treat infected persons.