There are two main influenza Type A viruses that cause annual seasonal illness. These, and other strains, can sometimes cause pandemics. Type B viruses also circulate each year, causing milder respiratory illness than Type A.
The natural host for influenza Type A viruses is wild waterfowl, not human beings, though the viruses also infect people, pigs and other mammals. Influenza viruses routinely mutate; Type A human influenza viruses commonly derive from animal viruses.
Type A virus subtypes are known by their "H" and "N" numbers. The letters refer to two different proteins on the virus surface. The numbers refer to 16 "H" and 9 "N" varieties. Of these, only H1, H2, H3, and N1 and N2 have circulated in people. In recent years, seasonal disease has been caused by influenza A H1N1 and H3N2 subtypes, and influenza B.
Incidence of disease
Influenza viruses spread by coughing, sneezing or touching contaminated surfaces, followed by touching one's eyes, nose or mouth. According to the World Health Organization (WHO), influenza infects about 5-15% of the world's population and causes up to 500,000 deaths each year. However, most people recover from seasonal flu within a week.
Why influenza viruses cause pandemics
Flu viruses, especially Type A, mutate rapidly; this change in their genetic makeup can be either big or small. Mutation produces new virus strains within an A subtype or it produces a new influenza B strain. When a human or animal influenza virus mutates in such a way that the new strain is completely unfamiliar to human immune systems and capable of spreading readily, it may trigger a pandemic—a worldwide spread of the disease.
Small mutations are most common. Immunity from prior infection or vaccination partially protects people from new flu strains; however, their ability to rapidly mutate necessitates equally rapid action to develop a fully protective flu vaccine each year. Such quick-change medical artistry is not necessary for any other human vaccine.
Governments need to be alert to the emergence of new influenza viruses that could trigger a pandemic and leave wide swaths of illness and death. Influenza pandemics have occurred periodically over the centuries, with three pandemics of different severity occurring during the 1900s. The 1918-1919 pandemic, caused by a particularly deadly H1N1 strain, resulted in an estimated 50 million deaths worldwide. The 1957 H2N2 pandemic virus was less deadly, killing about two million people; the 1968 H3N2 pandemic killed one million.
The first pandemic scare of this century emerged in late 2003 but did not trigger a human pandemic. A new avian influenza—subtype H5N1—spread among birds and infected some people. In the last seven years, this virus has claimed millions of birds in countries across the Eastern Hemisphere. In humans, the virus caused about 500 confirmed cases of illness and 300 deaths. To date, the H5N1 virus has not mutated into a form that spreads easily from person to person.
Last year an entirely new Type A flu strain of swine origin emerged in people in North America. Because this 2009-H1N1 virus was totally new and readily transmissible, it quickly reached pandemic proportions. Two waves of disease—one in spring and one in autumn—ran their natural courses. By 30th December, the WHO had received reports of at least 12,220 deaths worldwide. In August 2010, the WHO announced the transition from pandemic to post-pandemic period. The 2009-H1N1 virus "has largely run its course" as a pandemic, but will likely circulate during influenza seasons for years to come. Seasonal flu vaccines now include protection against this virus.
Prevention and mitigation
Defence against pandemics begins with the same behaviours we use to control seasonal flu. Individuals need to practice good personal hygiene—regular hand washing and respiratory etiquette. That means sneezing or coughing into a tissue and staying away from others when ill. Governments can promote these behaviours through annual education campaigns.
Vaccine provides another key defence. Experts convene annually at the WHO to examine circulating flu strains and to decide which should be included in the next season's vaccine. Each year's vaccine includes a selected strain each of H1N1, H3N2 and influenza B. Manufacture of vaccine takes about six months.
Antiviral drugs have been developed to treat flu. However, influenza viruses, like many other diseasecausing organisms, can become resistant to treatments. Constant monitoring of the virus's sensitivity to antiviral drugs is critical.
The 2009-H1N1 pandemic spread quickly throughout the world but caused milder illness and fewer deaths than the 1918 H1N1 pandemic. That was due to differences in the two pandemic H1N1 strains and to the availability of drugs for treating the secondary pneumonia that often infects flu patients. Last year a vaccine was produced fast but not fast enough. It was not available until after the first two waves of disease had hit Europe and the Americas.
Pandemic preparedness planning triggered by the H5N1 avian flu virus also helped countries quickly communicate and respond. Government public health agencies tracked cases and risks worldwide and educated their citizens to take proper precautions to prevent disease. Governments also mobilised their medical and public health systems. All this was facilitated by the WHO and the US Centers for Disease Control and Prevention in collaboration with other public health agencies.
In a world where viruses travel as fast as jets, it becomes important for governments to share timely information and accelerate the production and delivery of vaccines.
Melinda Moore, Physician and Senior Researcher at the RAND Corporation, charts the occurrence and severity of the influenza virus, and what can be done to prevent its spread.
This commentary originally appeared in Public Service Review: European Union on September 22, 2010.