Via The Lancet Infectious Diseases, an editorial: Pandemic potential of emerging influenza. Excerpt:
Throughout the 20th century, four pandemics primed our current approaches to influenza surveillance. The 1918—20 pandemic was by far the most devastating on a global scale. At the end of World War 1, with many parts of the globe reeling from enormous loss of life, this particularly severe illness was caused by influenza A H1N1, with high rates of haemorrhage adding to the pneumonia death toll—the disease is estimated to have killed upwards of 50 million people with a particularly high death rate among young adults.
The next four pandemics—H2N2 in 1957—58, H3N2 in 1968—69, and a much milder H1N1 in 1977—78—were less severe than the 1918—20 event, no doubt mitigated in part by improvements in health care and the development of vaccines, but the threat of a global pandemic lingers.
The 2009 pandemic was a truly modern affair: the case count was delivered to the world's media in real-time from the first seven cases identified in the southwest USA, the spread of the disease, facilitated by the ease of global travel, was monitored with near daily updates of confirmed cases.
The 2009 influenza A H1N1, like the 1918—20 virus, disproportionately affected young adults, but was less severe, causing an estimated 300 000 deaths in the first year. The 2009 pandemic aside, the influenza strains that have dominated research and the news in recent years behave very differently to the pandemic viruses of the 20th century. They are the particularly severe avian viruses, which thankfully rarely infect human beings, but when they do, they have exceptionally high case fatality.
Highly pathogenic avian influenza H5N1, first identified in Hong Kong in 1997 and circulating more widely since 2003, has been recorded in 649 patients in 16 countries, almost 60% of whom have died. The predictions for a pandemic caused by this virus were terrifying, but despite research showing that the virus can become transmissible in mammals with just a few mutations, it remains an avian virus that only very rarely affects people who have had close contact with infected poultry. The pathogenic effect of this virus on poultry also enables early warning.
By contrast, H7N9, identified in March 2013, does not cause illness in infected poultry, making it hard to detect before people become sick. Despite the recent surge in the number of cases, the disease remains confined to China (including Hong Kong and Taiwan), and closure of so-called wet markets seems to be fairly effective in halting its spread.
Influenza viruses are some of the most dynamic pathogens, and their changeable nature makes them fascinating subjects for research. A key question for people investigating and tracking the emergence and spread of new viruses will be whether we can predict pathogenicity and pandemic potential—and importantly, which novel strains combine both.
Modern surveillance, typing, and sequencing technologies provide unprecedented opportunities to identify new strains among the first few human cases. We must be careful not to overstate the pandemic potential of novel strains but we should also embrace insights into pandemic potential that they afford.