Influenza Pandemic Preparation and Response: A Citizen's Guide
by Dr. David L. Heymann World Health Organization, Executive Director, Communicable Diseases
Most public health specialists from around the world believe that there will be another human influenza pandemic, a pandemic caused by an avian influenza virus that can cause human illness and has mutated to a form that spreads from person to person.
Such a random event has occurred three times during the past century, causing three different influenza pandemics.
Since 2003 three major classes of avian influenza virus - H5, H7 and H9 - have caused sporadic human infections, and because of the instability of the influenza virus, any one of these viruses is thought to be capable of mutating in such as way as to cause a human pandemic.
Presently the most widespread of these viruses is the H5N1 avian influenza virus, or simply H5N1. Since an H5N1 outbreak in chickens in Hong Kong was first reported to infect humans in 1997, the H5N1 virus has spread in poultry populations throughout Asia, the Middle East and some parts of Africa and Europe causing a pandemic of influenza in chickens; and occasional human infections in persons who have come into contact with infected chickens.
Since 12003 there have been just over 300 reported human infections with H5N1, all having caused severe illness, with an overall death rate of 61%.
Two of the three influenza pandemics of the past century - one that occurred from 1957-1958, and one in 1968-1969 - are still in the memory of many persons living today.
These pandemics spread rapidly throughout the world, causing severe illness in persons of all ages, massive absenteeism from school and the workplace, and an estimated 2.5 million deaths, mostly in persons over the age of 60 years.
The third pandemic - that of 1918-1919 - caused an estimated 40 million deaths in persons of all ages. Articles published in scientific and medical journals of the time speak of severe illness and death, with a breakdown of routine health and mortuary services in almost all major cities, closure of public gathering places, and quarantine and isolation of those infected or exposed to infected persons in an attempt to stop the spread of infection.
Recent examination of records from the years 1918-1919 in many cities across the United States has shown that communities that put into practice social distancing measures such as closure of schools and public gathering places before infections were first detected, were able to maintain lower levels of infection than others.
Those persons at greatest risk were those who lived closely together in confined spaces, such as men serving in the military.
During the inter-pandemic period since 1968, six levels of alert for pandemic influenza have been defined by the World Health Organization in order to best describe the pandemic risk: phase 1 and 2 during which no new influenza virus is infecting humans: and phases 3 to 6 when there is human infection with a new influenza virus - phase 3 when there is no human to human transmission to phase 6 when there is increased and sustained transmission of the new influenza virus in human populations.
The world is currently at phase 3 - a new (avian) influenza virus, H5N1, that occasionally infects humans and causes severe illness, but that is not capable of sustained human to human transmission.
Should the H5N1 virus mutate in such a way that it can readily transmit from human to human in a limited geographic area, a collective international response would be made in an attempt to contain the outbreak by stopping human to human transmission.
The objective of such a containment activity would be to circle the focus of human infection by using an antiviral medicine, and/or a vaccine should one be available, in all persons with the potential of exposure to the H5N1 virus.
Such a containment activity would be conducted under the International Health Regulations (2005), an international law that requires countries to work together collectively in assessing and responding to any public health emergency of international concern, such as the current threat of an H5N1 pandemic.
The International Health Regulations (2005) came into effect on 15 June 2007, four years after the outbreak of Severe Acute Respiratory Syndrome (SARS). The worldwide response to SARS, that was led by the World Health Organization where I work as head of the communicable disease programmes, permitted development of control strategies using information collected in real time by epidemiologists working in all affected countries.
Within five months the SARS outbreak was fully contained using these strategies, and the virus disappeared from human populations. Though a pandemic of influenza could not be contained using the same strategies, an attempt at early containment would require early detection of a focus of human to human transmission, and effective use of antiviral drugs and/or a vaccine to "ring fence" the outbreak and prevent further spread.
Because containment has never before been tried as a measure to prevent or slow the spread of an influenza pandemic, the success of this strategy cannot be predicted.
If containment activities did not cover an area wide enough to stop transmission, it would be only a matter of weeks or months until the virus had spread throughout the world.
We are all vulnerable to the risk of pandemic influenza no matter where live, work or go to school.
The most important public health measure at present, however, has nothing to do with human infections.
That measure is to prevent a pandemic by eliminating the H5N1 virus from chicken populations either by culling of infected flocks, or by preventing infection in flocks through various measures that include vaccination of chicks and limiting exposure of chickens to possible sources of infection.
As long as H5N1 continues to circulate anywhere in animals, there is a potential for the virus to mutate in such as way that it could cause a human pandemic.