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Influenza is recognized both as an emerging and reemerging viral infection. It is described as an unvarying disease caused by a varying virus. The virus changes but its effects on health, lives, and manpower is consistently overwhelming.

There are three types of influenza virus. These are A, B and C. Influenza A and B are possible cause for epidemics. Types A and B circulate in human populations and changes constantly. This could result into a new strain and the need for a modified vaccine every year.

The Influenza A virus(H5N1) may undergo major genetic changes, resulting into new subtypes. This is called an antigenic shift. It occurs at irregular intervals. Some antigenic shifts result in local epidemics or global pandemics with high morbidity and mortality because no one has previously been exposed to the new sub-type.

Avian Influenza usally precedes an Influenza pandemic

Bird flu due to Influenza A/ H5N1 cause severe disease and mass deaths and destruction of poultry leading to serious economic consequences for the agricultural sector. This virus subtype has been shown to be transmitted from infected poultry to humans, causing serious illness and high mortality. The risks to human health remain so long as H5N1 continues to circulate in domestic poultry.

Widespread epidemics in birds increase opportunities for human exposure. In a simultaneous infection, the avian and human influenza viruses could interact and exchange genes to give rise to a totally new influenza virus. If that virus proves capable of spreading easily and sustainably from person to person against which most of the human population do not have natural immunity or cannot be protected by existing vaccines, then the conditions for the start of an influenza pandemic will have been met.

Since 1580, there were at least 31 documented pandemics affecting different segments of the population and with varying levels of impact. In the 20th century, the largest pandemic was in 1918, due to Influenza A/H1N1 which caused at least 20 million deaths worldwide. Adults (20-50) were extremely affected.

Other pandemics were in: 1950 due to A/H 1 N 1, a mild influenza pandemic, in 1957 caused byA/H2N2 which affected mainly infants and children and in 1968, due to recombination of avian and human influenza virus (A/ H3N2), which had mortality highest among the > 65 year old population and groups younger than 65 years old but with underlying medical conditions.

Stages for Preparedness to Avian and Pandemic Influenza

The government's Preparedness and Response Plan for Avian and Pandemic Influenza consists of four stages:

  • Stage 1 - Avian influenza-free Philippines
  • Stage 2 - Avian influenza in domestic fowl in the Philippines
  • Stage 3 - Confirmation of avian influenza transmission from poultry to humans in the Philippines
  • Stage 4 - Avian Influenza with human-to-human transmission, causing outbreaks in the country.

Possible scenarios based on the progression of stages are as follows:

  • Stage 1 is maintained and Philippines remains to have no case of Avian Influenza in poultry and humans
  • Stage 2 - Presence of Al in domestic fowls but is controlled and no transmission to humans
  • Stage 2 progresses to Stage 3 (Al outbreaks in fowl and cases of Al in humans exposed to sick fowl)
  • Stage 3 without progression to Stage 4
  • Stage 3 to Stage 4 – human outbreaks due to virus with person-to-person-transmission or pandemic influenza virus with or without outbreaks in poultry
  • Stage 1 directly to Stage 4 – from Al-free Philippines directly to outbreaks in humans because of a pandemic influenza virus


  • Bird Flu: Guidelines on Infection Control and Management in the Hospital Setting. Department of Health of the Philippines. (Accessed May 18, 2010).