WHO: Developing a Pandemic Severity Scale to Use with Alert Phases

WHO is developing a revised pandemic severity scale to accompany the pandemic alert phases for the current H1N1 virus (which is not considered a pandemic yet). There are many different considerations for determining the severity of a pandemic. WHO's initial comments on these considerations are noted below:

A. Properties of Virus Spread and Severity:

• The virus is new or hasn't circulated among humans in recent memory - meaning the population has little or no immunity to the illness
• Contagiousness of the virus will determine how many fall ill and how quickly it spreads
• Large numbers of people will fall ill (see above), causing economic and social disruption even if the virus isn't severe
• The virulence of the virus will determine the severity of symptoms and the proportion of people who are hospitalized or die from the virus
• The population most at risk (very young and very old, similar to seasonal flu, or young, healthy adults) [it's concerning when young, healty adults become ill and not only the most vulnerable young and old]

All these properties combine in different ways to determine the ultimate severity of a pandemic caused by some new virus.

B. Pandemic "Waves"
Past pandemics have often started at the end of a seasonal flu season, taper off during the warm summer months, and re-emerge at the beginning of the next seasonal flu season, sweeping the globe in two or three waves. Wave severity differs, sometimes dramatically, because the influenza virus mutates and shifts between waves, meaning that the characteristics of the virus can also change. While one wave may be mild, subsequent waves may be much more virulent and less contagious (or any combination thereof).

C. Response Capacity
"The same virus that causes only mild symptoms in countries with strong health systems can be devastating in other countries where health systems are weak" (e.g. lack of medicine, human resources, equipment).

Current Situation Assessment
A. Characteristics of H1N1

• H1N1 is an entirely new virus
• The virus apperas to very contagious - at twice the attack rate of seasonal influenza, 15 - 33% of those people that come in contact with someone ill with H1N1 will contract the virus
• The virus appears to be very mild (similar to seasonal flu) EXCEPT in Mexico (the difference in virulence is not understood at this time)
• While the ill range in age from newborn to elderly, many have been young, healthy adults

B. Possibility of Mutations & Genetic Shifts:
"In these early days of the outbreaks, some scientists speculate that the full clinical spectrum of disease caused by H1N1 will not become apparent until the virus is more widespread ... Scientists are concerned about possible changes that could take place as the virus spreads to the southern hemisphere and encounters currently circulating human viruses as the normal influenza season in that hemisphere begins."

C. Response Capacity
The world is better prepared now than ever before to deal with a pandemic - see previous blog posts about WHO statement to this effect.

http://www.who.int/csr/disease/swineflu/assess/disease_swineflu_assess_20090511/en/index.html