What's a Phase 6 pandemic?

WITHOUT A VACCINE, CDC ESTIMATES GRIM FOR SWINE FLU Denver Post, July 25, 2009

Grim?!? Whoa, that's a pretty strong word. In this weekly flu update, we'll check out the ease with which the H1N1 (formerly known as swine) flu spreads from person to person, and why the CDC is hoping for a timely vaccine against the virus.

What's a Phase 6 pandemic?

Sounds serious, and indeed it is insofar as it speaks of the ease with which this little hummer spreads from one human to the next.

The World Health Organization (WHO) has developed a 6-tiered approach that classifies the worldwide threat from strains of animal influenza newly arrived in human airways. Each higher level indicates an increased penetrance of the virus into human populations. By Phase 6--the current global level of the H1N1 flu--the virus has shifted into a lean, mean, human-to-human infecting machine in at least two countries of one WHO region and with spread detected in at least one country of a different region. This particular influenza virus, therefore, reached Phase 5 once it spread from person to person in Mexico and the United States; its subsequent spring '09 appearance in the Southern hemisphere then raised the threat to level 6.

Experts agree so far that this variety of influenza has a 'substantially higher' rate of transmissibility than the usual seasonal flu although this in itself does not mean that it's more lethal.

So what's transmissibility and what's the H1N1 score?

Influenza is quite contagious as it thrives in infected respiratory secretions and effectively spreads those viral laden droplets into the environment via juicy sneezes and coughs. The smaller the droplet, the longer it floats through the air and the further it penetrates into the airway of the unlucky recipient who inhales it. If the virus is novel--meaning that large portions of the population have never encountered this particular bug before--the risk of contagion is even higher.

Epidemiologists express transmissibility as reproductive number (R0 or R-zero) which is the number of cases caused by one infected person. If the R0 is less than one, the disease fades away, whereas rising R0 numbers mean that the spread of the disease is increasingly harder to contain. Experts think that an R0 more than 2 renders such measures as closing schools and screening visitors from other countries for signs of the disease as virtually useless.

So what's the R0 of H1N1? Compared to the seasonal flu with an average R0 of 1.3, estimates for H1N1 vary between 1.3 to more than 2. Calculating an accurate R0 for novel H1N1 flu is very complicated considering the many unknowns about this pandemic such as the incubation time and the percentage of cases mild enough to escape official notice. But we must assume that H1N1 is sporting a fairly robust R0 considering its current worldwide status, and, for that reason, we desperately need a vaccine to slow the spread of infection.

Upcoming info

The important thing in containing this pandemic, therefore, is the development and widespread use of an effective vaccine. More on that subject next week.