Past pandemics
Explosive and unusually deadly outbreaks of influenza have occurred throughout recorded history, probably originating in the earliest cities where humans lived crowded together in close proximity to domestic animals. Pandemics behave as unpredictably as the viruses that cause them. True pandemics, characterized by sharp increases in morbidity and mortality and rapid spread throughout the world, have been reliably documented since the 16th century. Since then, each century has seen an average of three pandemics occurring at intervals ranging from 10 to 50 years.
1918–1919
Of all pandemics, the one that began in 1918 – in a world wearied by war – is generally regarded as the most deadly disease event in human history. Not only did it kill upwards of 40 million people, but it did so in less than a year. For comparison, total military deaths on all fronts during the First World War have been estimated at 8.3 million over four years.
The beginnings were inauspicious. The first simultaneous outbreaks were detected in March 1918 in Europe and in different states within the USA. The infection then travelled back and forth between Europe and the USA via ships carrying troops and then, by land and sea, to Asia and Africa. That first wave, which took place in the spring and summer, was highly contagious but not especially deadly; its significance as a warning signal was missed.
When the second wave began near the end of August, no country was prepared. The experience was unprecedented. That second wave, which began almost simultaneously in France, Sierra Leone and the USA, saw explosive outbreaks characterized by a 10-fold increase in the death rate.
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1957–1958
The pandemic that began in 1957 was caused by a milder virus than the one responsible for the 1918 pandemic. In addition, the world was much better prepared to cope. The speed of international spread was characteristically swift. Less than six months after the disease reached Hong Kong SAR, every part of the world had experienced cases. During the first wave, cases of illness were concentrated in school-aged children; this was attributed to their close contact in crowded settings, and not to a particular age-related vulnerability. In general, close contact and crowding of persons together, as also seen in military barracks, favoured the spread of infection.
In most countries, a second wave followed disappearance of the first from one to three months later, causing very high rates of illness and increased fatalities. The second wave was concentrated in the elderly, which helps to explain the increased mortality. Total excess mortality globally has been estimated at more than 2 million deaths.
1968–1969
Initial international spread did resemble that seen during 1957, but there the resemblance ended. Nearly everywhere, clinical symptoms were mild and mortality low. In most countries, the disease spread slowly rather than in the highly visible pattern of explosive outbreaks seen in previous pandemics. Although good mortality estimates are not available, global excess mortality was probably around 1 million.
Lessons learnt from past pandemics
One consistent feature important for preparedness planning is the rapid surge in the number of cases and their exponential increase over a very brief time, often measured in weeks. The severity of illness caused by the virus, which cannot be known in advance, will influence the capacity of health services, including hospitals, to cope, but a sudden sharp increase in the need for medical care will always occur.
Apart from the inherent lethality of the virus, its capacity to cause severe disease in non-traditional age groups, namely young adults, is a major determinant of a pandemic’s overall impact.
Milder pandemics are characterized by severe disease and excess deaths at the extremes of the lifespan (the very young and the elderly).
The epidemiological potential of a virus tends to unfold in waves. Age groups and geographical areas not affected initially are likely to prove vulnerable during the second wave. Subsequent waves have tended to be more severe, but for different reasons.
Virological surveillance, as conducted by the WHO laboratory network, has performed a vital function in rapidly confirming the onset of pandemics, alerting health services, isolating and characterizing the virus, and making it available to vaccine manufacturers.
Over the centuries, most pandemics have originated in parts of Asia where dense populations of humans live in close proximity to ducks and pigs. In this part of the world, surveillance for both animal influenza and clusters of unusual respiratory disease in humans performs an important early warning function.
Quarantine and travel restrictions have shown little effect. As spread within countries has been associated with close contact and crowding, the temporary banning of public gatherings and closure of schools are potentially effective measures. The speed with which pandemic influenza peaks and then disappears means that such measures would probably not need to be imposed for long.
The interval between successive waves may, however, be as short as a month.
In the best-case scenario, a pandemic will cause excess mortality at the extremes of the lifespan and in persons with underlying chronic disease. As these risk groups are the same as during seasonal epidemics, countries with good programs for yearly vaccination will have experience in the logistics of vaccine administration to at least some groups requiring priority protection during a pandemic.
The H5N1 outbreaks in 2004: a pandemic in waiting?
Historically, human infections with avian influenza viruses have been extremely rare. At some unknown time prior to 1997, the H5N1 strain of avian influenza virus began circulating in the poultry populations of parts of Asia. Like other avian viruses of the H5 and H7 subtypes, H5N1 initially caused only mild disease with symptoms, such as ruffled feathers and reduced egg production that escaped detection. After months of circulation in chickens, the virus mutated to a highly pathogenic form that could kill chickens within 48 hours, with a mortality approaching 100%. The virus first erupted in Hong Kong in its highly pathogenic form in 1997 and caused disease in 18 human of whom six died, but it did not appear again.
Then, towards the end of 2003, H5N1 suddenly became highly and widely visible. Near the end of January 2004, the situation in poultry exploded. Outbreaks in the Republic of Korea, Viet Nam, Japan, and Thailand were followed by reports of the same disease in Cambodia, Lao People’s Democratic Republic, Indonesia, and China. Most of these countries had never before experienced outbreaks of highly pathogenic avian influenza caused by any strain. In the Asian outbreaks, more than 120 million birds died or were destroyed within three months. That figure is higher than the combined total from all previous large outbreaks of avian influenza recorded throughout the world over four decades.
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Never before had highly pathogenic avian influenza caused outbreaks in so many countries at once. Never before had the disease spread so widely and rapidly to affect such huge geographical areas. Never before had it caused such enormous consequences for agriculture – from large commercial farms to the roots of rural subsistence agriculture.
High alert
In January 2004, WHO officials were understandably on high alert for any signs that H5N1 might again cross the species barrier to cause disease in humans. On 11 January 2004 a WHO reference laboratory announced detection of H5N1 in specimens from 2 of the fatal cases in Hanoi.
The confirmation of human cases gave the outbreaks in poultry a new dimension. They were now a health threat to populations in affected countries and, possibly, throughout the world. All prerequisites for the start of a pandemic had been met save one, namely the onset of efficient human-to-human transmission. Should the virus improve its transmissibility, everyone in the world would be vulnerable to infection by a pathogen – passed along by a cough or a sneeze – entirely foreign to the human immune system. Each human case that occurs in Asia is potentially a global threat.
Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO
17 June 2005
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Viet Nam
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Thailand
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Cambodia
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Total
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cases
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deaths
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cases
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deaths
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cases
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deaths
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cases
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deaths
|
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86
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38
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17
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12
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4
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4
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107
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54
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Notes Total number of cases includes number of deaths, WHO reports only laboratory-confirmed cases. Each human case that occurs in Asia is potentially a global threat.
Can the spread of a pandemic be delayed?
For the first time in history, the H5N1 situation in Asia has given the world a warning that a pandemic may be imminent. This warning has inevitably sparked questions about whether the right actions, taken at the right time, might do something to alter the historical pattern of rapid international spread.
Such an approach, which aims to forestall international spread and thus gain time to augment vaccine supplies, is linked to assumptions that the first chains of human-to-human transmission might not reach the efficiency needed to initiate and sustain pandemic spread. Should this happen, early detection of tell-tale clusters of cases, followed by aggressive containment measures, including the prophylactic use of antiviral drugs might hold the disease at bay, thus gaining time to increase preparedness. Should early containment fail, once a certain level of efficient transmission is reached, no interventions are expected to halt international spread, and priorities will need to shift to the reduction of morbidity and mortality.
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