A pandemic is an outbreak of global proportions. It happens when a novel virus emerges among humans - it causes serious illness and is easily human transmissible (spreads easily from person-to-person).
This article discusses the difference between epidemics and pandemics, how pandemics start, the way they are described and future concerns.
Here are some key points about pandemics. More detail and supporting information is in the main article.
- Pandemics are generally caused by a novel infectious agent
- The death tolls of pandemics are generally much higher than epidemics
- Influenza pandemics can be caused by an "antigenic shift"
- The Black Death was the worst pandemic in history, killing 75 million people
- The World Health Organization has a six stage pandemic plan
- Because people are now more mobile, epidemics are more likely to spread
- Antibiotic resistance poses the threat of future pandemics
- If a pandemic were to strike today, vaccines, antiviral agents, and antibiotics would rapidly dwindle.
Pandemic or epidemic?
The death toll of a pandemic is usually much greater than that of an epidemic.
Put simply, a pandemic covers a much wider geographical area, often worldwide.
A pandemic also infects many more people than an epidemic. An epidemic is specific to one city, region or country, while a pandemic goes much further than national borders.
An epidemic is when the number of people who become infected rises well beyond what is expected within a country or a part of a country. When the infection takes place in several countries at the same time it then starts turning into a pandemic.
A pandemic is usually caused by a new virus strain or subtype - a virus humans either have no immunity against, or very little immunity. If immunity is low or non-existent the virus is much more likely to spread around the world if it becomes easily human transmissible.
In the case of influenza, seasonal outbreaks (epidemics) are generally caused by subtypes of a virus that is already circulating among people. Pandemics, on the other hand, are generally caused by novel subtypes - these subtypes have not circulated among people before.
Pandemics can also be caused by viruses, in the case of influenza, that perhaps have not circulated among people for a very long time.
Pandemics generally cause much higher numbers of deaths than epidemics. The social disruption, economic loss, and general hardship caused by a pandemic are much higher than what an epidemic can cause.
A pandemic can emerge when the influenza A virus changes suddenly - what experts call an antigenic shift. The HA and/or NA proteins, which are on the surface of the virus, have new combinations; resulting in a new influenza A virus subtype.
This new influenza subtype needs one characteristic to cause a pandemic - it must be easily human-transmissible (it can easily spread from one person to another).
After the pandemic has emerged and spread, the virus subtype circulates among humans for several years, causing occasional flu epidemics. These will not usually become more than epidemics because humans have developed some immunity over time. Various bodies around the world, such as the Health Protection Agency (UK), the World Health Organization (WHO), and the Centers for Disease Control and Prevention (CDC) monitor the behavior and movements of the virus.
Below you can see the dates of the most famous pandemics throughout history.
The Black Death pandemic is considered to have been the worst in history, claiming the lives of more than 75 million people.
Some of the following may have been epidemics which crossed borders, but did not go right around the world. In earlier times, travel was much slower and less frequent which naturally halted the spread of infections:
The Black Death killed 30-60% of Europe's total population.
- Antonine Plague (possibly smallpox) 165-180 A.D.
- Plague of Cyprian 251-266 A.D.
- Plague of Justinian 541 A.D.
- Black Death 1300s A.D.
- Typhus 1501-1587 A.D.
- Influenza 1732-1733 A.D.
- Influenza 1775-1776 A.D.
- Cholera 1816-1826 A.D.
- Cholera 1829-1851 A.D.
- Influenza 1847-1848 A.D.
- Cholera 1852-1860 A.D.
- Bubonic Plague 1855 A.D.
- Influenza 1857-1859 A.D.
- Cholera 1863-1875 A.D.
- Influenza 1889-1892 A.D.
- Cholera 1899-1923 A.D.
- Spanish Flu (avian flu) 1918-1920 A.D.
- El Tor (Vibrio cholerae - cholera) 1960s A.D.
- HIV/AIDS 1980s - now
- Swine Influenza - 2009.
The etiology of pandemics can be split into six stages.
The World Health Organization has a six-stage influenza program:
No animal influenza virus circulating among animals have been reported to cause infection in humans.
An animal influenza virus circulating in domesticated or wild animals is known to have caused infection in humans and is therefore considered a specific potential pandemic threat.
An animal or human-animal influenza reassortant virus has caused sporadic cases or small clusters of disease in people, but has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks.
Human-to-human transmission of an animal or human-animal influenza reassortant virus able to sustain community-level outbreaks has been verified.
The same identified virus has caused sustained community level outbreaks in two or more countries in one WHO region.
In addition to the criteria defined in Phase 5, the same virus has caused sustained community level outbreaks in at least one other country in another WHO region.
Lost peak period
Levels of pandemic influenza in most countries with adequate surveillance have dropped below peak levels.
Post pandemic period
Levels of influenza activity have returned to the levels seen for seasonal influenza in most countries with adequate surveillance.
According to the WHO, if an influenza pandemic were to emerge today, we could expect the following to occur:
- As people today are highly internationally mobile, the pandemic virus would spread rapidly around the world
- Vaccines, antiviral agents, and antibiotics to treat secondary infections would rapidly be in short supply
- Several months would be needed before any vaccine became available; this is because pandemic viruses are novel agents
- Medical facilities would be overwhelmed
- There would be sudden and potentially considerable shortages of personnel to provide vital community services as the illness became widespread.
Pandemics involve novel agents; because of this, medical science must keep on its toes.
Although medical science is constantly making leaps and bounds in our defense against disease, because of the novel nature of pandemics, we will never be fully protected.
The categories below all hold a certain level of concern for experts:
Viral hemorrhagic fevers
These diseases require being close to an infected person, and, as the infected individual tends not to live particularly long after infection, their spread is naturally curbed.
Additionally, the short time interval between infection and symptoms allows health care professionals to quarantine new cases swiftly.
Antibiotic resistance is never far from the front page of medical news; it is a genuine, major concern. Resistant strains of tuberculosis are among the most worrying.
Each year, almost half a million new cases of multidrug-resistant tuberculosis (MDR-TB) are estimated to occur globally.
SARS, caused by the coronavirus, recently came close to generating a pandemic. Health agencies and government bodies prevented the disease from becoming more than localized epidemics. SARS has not been eradicated, however, and may return.
Wild aquatic birds are a natural host for a variety of influenza strains. In rare cases these influenza species can pass from bird to human, sparking epidemics with the potential to turn into pandemics, if left unchecked.
The recent discovery of the H5N1 (Avian Flu) is an example of this. The strain was spotted in Vietnam in 2004 and, although it never reached even epidemic levels, the ability of the virus to potentially combine with human flu viruses is a concern to scientists.
The largest Ebola epidemic the world has ever seen is still ongoing. Huge efforts to contain the problem have prevented it from turning into a pandemic.
Doctors at the Royal Free Hospital in London, UK, have discharged Pauline Cafferkey from their care - for the second time - and say she has made a full recovery from Ebola.
A new study suggests a class of drugs already being tested in clinical trials for the treatment of cancer may also hold the key to complete eradication of HIV.
Written by Christian Nordqvist