A pandemic occurs when a new influenza virus spreads as easily among humans as the normal flu virus. Because the virus is new, humans will have no pre-existing immunity, and those who contract the virus will become more seriously ill than those with normal influenza.
The World Health Organization outlines six phases of pandemic alert. According to the WHO, “Phases 1–3 correlate with preparedness, including capacity development and response planning activities, while Phases 4–6 clearly signal the need for response and mitigation efforts.”
The current outbreak of H1N1 influenza is in Phase 5.
Phase 1: No human infections
No viruses circulating among animals are known to cause infections in humans. Although there may be viruses circulating among animals that have the potential to become pandemics, during this phase there is no animal-to-human transmission.
Phase 2: Potential pandemic threat
During this phase, an animal influenza virus has been known to spread to humans, making it a potential pandemic.
Phase 3: Small clusters of infection
Limited human-to-human transmission occurs, but only in clusters too small to sustain community-level outbreaks. The illness may spread between people in close contact, but it does not indicate a level of transmissibility high enough to cause a pandemic.
The majority of human infections in Phases 1-3 occur after contact with infected animals, or contact with someone who has had contact with infected animals.
Phase 4: Community-level outbreaks
In this phase, the virus is adapting to spread among humans, causing community-level outbreaks. The virus is becoming more widespread rather than dissipating, increasing the threat of a pandemic.
The H1N1 influenza virus was upgraded to Phase 4 on April 27. It was the first time the phase of pandemic has reached the fourth level since the system was adopted in 2005.
A WHO statement released Monday said, “The change to a higher phase of pandemic alert indicates that the likelihood of a pandemic has increased, but not that a pandemic is inevitable.”
It also said that containing the virus was not feasible, and the focus should be on mitigation measures.
Phase 5: Virus spreads to other regions
The illness enters the fifth phase when the virus has spread among humans in two countries within one WHO-designated region. The WHO designates six regions: Africa, the Americas, South-East Asia, Europe, The Eastern Mediterranean and the Western Pacific.
At this point, the virus is becoming increasingly adapted to humans, and a pandemic is likely imminent.
The H1N1 influenza virus was upgraded to Phase 5 on April 29.
Phase 6: Pandemic
A pandemic is underway when the community-level outbreaks spread to another WHO region, in addition to those from Phase 5.
The flu was upgraded to Phase 6 on June 11.
Post-peak period
During this phase, infection levels in most countries drop below their peak and the spread of the virus decreases. There is always the possibility of another wave of infections, which can occur months later. Pandemics have historically occurred in two or even three waves over a number of months.
Post-pandemic period
Influenza infection has dropped to regular seasonal levels.
Comments
Want to discuss? Please read our Commenting Policy first.