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Pandemic | Informational Series | E3 | Transmission



Source: Metro

The World Health Organization (WHO) previously applied a six-stage classification to describe the process by which a novel influenza virus moves from the first few infections in humans through to a pandemic.

It starts when mostly animals are infected with a virus and a few cases where animals infect people, then moves to the stage where the virus begins to be transmitted directly between people and ends with the stage when infections in humans from the virus have spread worldwide.


WHO Phases of Transmission
Source: The Epoch Times


Phase 1 is actually a natural state. The viruses which cause influenza, circulate continuously among animals, especially birds. In this phase,  no viruses circulating among animals have been reported to cause infections in humans. These viruses might theoretically develop into pandemic in later phases.
Due to this presence, a number of scientists, continuously check for possible transmission into next phase. So that it could be prevented, or the health agencies are well prepared to deal with it. 



When a virus circulating among domesticated or wild animals causes an infection in humans, it goes into Phase 2. It is now considered a potential pandemic threat.



When an animal or human-animal influenza reassortant virus has caused sporadic cases or small clusters of disease in people, it is said to have moved into phase 3. But still the virus has not resulted in human-to-human transmission sufficient to sustain community-level outbreaks. Small scale human-to-human transmission may occur under special circumstances, for example, when there is proximate contact between an infected person and an unprotected caregiver. However it does not indicate that the virus has gained the level of transmissibility among humans which is necessary to cause a pandemic.


A verified human-to-human transmission of an animal or human-animal reassortant virus is able to cause
“community-level outbreaks”.
This ability to cause sustained disease outbreaks in a community marks a significant upwards shift in the probability of a pandemic occurrence. According to WHO guidelines,if a country suspects or has verified such an event has to urgently consult with WHO. This is done so that the situation can be jointly assessed and a decision made by the affected country if implementation of a rapid pandemic containment operation is required. This is what the recent controversy is all about China not providing correct information on time. This phase indicates a huge increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion.


Phase 5 is characterized by human-to-human spread of the virus into at least two countries in one WHO region. While many countries are not affected at this stage, the declaration of a virus achieving this stage is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short.



Phase 6, the pandemic phase, is characterized by community level outbreaks in at least one other country in a different WHO region in addition to the criteria defined in Phase 5. Designation of this phase will indicate that a global pandemic is under way.



 During the post-peak period, pandemic disease levels in most countries with adequate surveillance will have dropped below peak observed levels. The post-peak period signifies that pandemic activity appears to be decreasing; however, it is uncertain if additional waves will occur and countries will need to be prepared for a second wave.

 Previous pandemics have been characterized by waves of activity spread over months. Once the level of disease activity drops, a critical communications task will be to balance this information with the possibility of another wave. Pandemic waves can be separated by months and an immediate “at-ease” signal may be premature.



In the post-pandemic period,  disease activity will have returned to normal levels of seasonal influenza. The pandemic virus will be expected to behave as a seasonal influenza A virus. Now, it is important to maintain surveillance and update pandemic preparedness plans accordingly.

This phased approach is intended to help countries and other stakeholders to anticipate when certain situations will require decisions and decide at which point main actions should be implemented (see Table 3). As in the 2005 guidance, each of the phases applies worldwide once announced. However, individual countries will be affected at different times. In addition to the globally announced pandemic phase, countries may want to make further national distinctions based upon their specific situations. For example, countries may wish to consider whether the potential pandemic virus is causing disease within their own borders, in neighbouring countries, or countries in close proximity.

In the next Episode, we will take a look at what is exponential curve that we hear a lot about and look into world's worst pandemics in history!

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