What does the WHO's pandemic scale mean? And why is anyone worried about this?

I've been seeing a lot of comments mocking the current outbreak of H1N1, and a lot of people (and journalists) who don't understand what "big deal" is about the "snoutbreak" of swine influenza, or don't get what the raising of the World Health Organization's pandemic alert phase up to 5 means. I noted here what the alert level meant, but wanted to discuss it a bit more in a full post; after the jump.

So, some comments I've seen thrown about:

This is all just media hype! Lies about the Mexican cases! The WHO just revised their numbers down to 26 cases and 7 deaths! Why should I believe the crap about this aporkalypse?

First, indeed, the Mexican cases stand at 26 cases and 7 deaths--*laboratory confirmed* cases. Even for regular influenza, the number of cases that are confirmed--much less to the level that these are, by the CDC or other high-level labs--is extremely small. We always expect many fewer confirmed cases than reported cases, because for a number of reasons, some people who display all the symptoms of a disease will be difficult to confirm. Maybe they were unable to get to a hospital in time for a sample to be taken; maybe they live in a remote area where it's difficult to ship samples from; maybe they came in too late after their disease had already cleared, and no live virus was present in their system anymore; and on and on. Most of the Mexican cases (the thousands reported) have been suggested to be H1N1 based on symptoms and timing. The current WHO report isn't saying that these 26 cases are the ONLY H1N1 cases in Mexico--these are just the ones they're absolutely, positively, 100% sure are swine flu.

Ordinary flu kills 36,000 Americans a year. Why should I care about this "swine flu," when only a hundred or so cases have been reported in the US?

Yes, ordinary flu is a bitch, and most people don't worry about it one iota. (Mike laments this situation). However, ordinary flu kills these 36,000 people *despite* many of us having some pre-existing immunity to the circulating strains, and a percentage of the high-risk groups receiving vaccination. With these new strains, it's unlikely that the population as a whole will have much cross-protective immunity, meaning higher numbers of people will get sick and die than a "regular" flu year, and all the impact to society and the economy that goes along with an ill population. As rhozack notes:

I live in a town with 65,000 people and somewhere around 300 hospital beds. If 20,000 people have the flu at the same time its like not having hospitals at all. Plus all the services those 20,000 people provide will be stopped at the same time and because of the nature of the thing, a large portion of your health care workers will be in that 20,000.

Now of course, we have no idea currently what kind of levels of illness or hospitalization H1N1 would cause if it becomes established in the population, but this is a situation where there's a small chance of a very bad situation, and so we need to prepare for it as best we can.

Does 7 deaths REALLY constitute a pandemic? Are we that wussy and panic-prone as a society that the WHO freaks out over a couple deaths from the freakin' flu?

And this is what I was getting at with my comment on pandemic phases. The number of deaths don't matter--it's the transmission in the human population that's key. Let's look at their scale:

Phase 1-3 go from no animal viruses reported causing infections to humans up to the presence of a new animal or human-animal reassortant that's caused sporadic cases or small clusters of disease in people, but no sustained human-to-human transmission or community-level outbreaks.

Phase 4 signifies a new virus capable of causing community-level outbreaks. "Phase 4 indicates a significant increase in risk of a pandemic but does not necessarily mean that a pandemic is a forgone conclusion." This is where we were with the beginning of the outbreak in Mexico.

Phase 5 (and I'll quote directly from WHO here): characterized by human-to-human spread of the virus into at least two countries in one WHO region. While most countries will not be affected at this stage, the declaration of Phase 5 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. This is the key--Phase 5 is a signal to governments to get their act together, because the shit is about to hit the fan.

Phase 6, then, is the full-blown pandemic phase. This has not been declared yet. Maybe we won't get to this, the individual outbreaks will burn themselves out, and we'll head back down to a 4 or even 3. But for now, we're still seeing cases increase, and the prudent thing to do is be careful and prepared. Weren't any of these nay-sayers ever Boy Scouts?


More like this

What is your take on this article http://www.latimes.com/features/health/la-sci-swine-reality30-2009apr30… that states this strain is pretty mild

"There are certain characteristics, molecular signatures, which this virus lacks," said Peter Palese, a microbiologist and influenza expert at Mt. Sinai Medical Center in New York. In particular, the swine flu lacks an amino acid that appears to increase the number of virus particles in the lungs and make the disease more deadly.

"Sequences of viral RNAs from 20 swine flu isolates have now been posted on the NCBI website. Included are isolates from California, Texas, New York, Ohio, Kansas, and Germany (taken from a tourist who returned from Mexico). It is difficult to understand why RNA sequences of none of the Mexican isolates have been posted, which would enable us to determine if the viruses in that country are different from the others...."

Hunh? Is this still true??

Link: http://www.virology.ws/2009/04/30/swine-influenza-amexico2009-h1n1-upda…

To further add a little neighing to the saying,
Dr. Olsen is quoted later in the article as noting that . . . . "Just because the virus is being identified in a growing number of places -- including Austria, Canada, Germany, Israel, New Zealand, Spain and Britain -- doesn't mean it's spreading particularly quickly, Olsen said."

I think when much more analysis has been done, this H1N1 strain won't be found to be unusually infectious or transmissable either.

Professor, my posts haven't been intended to "mock" but to maintain a sense of proportion over this episode, if for no other reason than the health experts running the system need to maintain credibility else they won't be listened to when needed - and unfortunately, that's the path they're going down so far in my opinion.

Remember how awful the experience was in 1976 when reactions to the swine flu vaccine created a mini-epidemic of Guillane-Barre syndrome? There may be another horrendously awful flu virus coming in the distant or not-so-distant future that has the killing potential of the 1918 bug, and we'll need all the health system guru's to be on top of their game and with maximum credibility to deal with that prospective crisis -- but getting too worked up over a novel new flu virus with run-of-the-mill symptoms is not going to help.

Just my two cents' worth.

Thank you for your blog. It helps make things clear.

MarketBlogic, what you're missing is that this is a novel virus, meaning that nobody has much or any immunity to it. This means it might spread quite easily through populations even if trasmissibility is comparable to ordinary flu. And this kind of spread will create ample opportunity for it to evolve into a more virulent strain, and to acquire Tamiflu resistance.

Even a novel virus with low virulence could cause such surges in illness to overwhelm hospitals and disrupt their normal functions. This even happens to an extent with ordinary seasonal flu.

This is also a big deal because even if this one turns out to never become anything more at all, it is extremely illustrative of the ways in which we are unprepared for a nasty pandemic, and how one can emerge right underneath our noses with our current surveillance strategies.

Exclusive Interview: CDC Head Virus Sleuth


Q: How does it tie to the current outbreak?

R.D.: Where does all this talk about avian and human genes come from? I was describing a fully swine virus. For [the] last 10 years, this has been a fully swine virus. Can you tell I have an accent? Iâm a U.S. citizen but I have the roots in Argentina. Itâs like me. Iâve been in the U.S. since 1980. Iâm a U.S. citizen but I have an accent.

By Vulariter (not verified) on 30 Apr 2009 #permalink

Thanks for posting this. I've been telling people that while there's no need to panic, there's certainly cause for concern with this new virus, and I've been having a hard time making that case. The argument that regular flu kills X many people per year hasn't worked for me, but I haven't been able to articulate why. Now I can.

Of the 159 or so ?suspected influenza deaths in Mexico (what is the time frame here) - only 8 are confirmed to be 'swine flu'. So what are the other 151 deaths caused by - ordinary, this year's nasty old flu - what is the hold up? And really what is the real denominator. The CDC and the WHO as well as Mexican public health officials appear to be everywhere doing everything (at least on the news programs). We read that there are thousands of Mexicans sickened with influenza - which is only 'suspected' to be the 'swine flu' strain. How many are confirmed and what are the demographics. Among the hundreds confirmed in the US - there has been only one confirmed death - a Mexican toddler who was described as having underlying health problems. Until we know real numbers - we can't say much about the virulence of this influenza variant.
I have been looking without much luck for government guidelines on procedures for post-mortem examination of suspected influenza cases - such as what kind of sample to collect (upper respiratory tract versus lung) and any report on the post-mortem findings on the 8 + 1 'confirmed swine flu fatalities'. Believe me - guidelines on what to collect in SARS and autopsy findings were available very quickly. Where are the 'swine flu' guidelines? I have moved from deeply concerned to more skeptical.
We need real numbers to justify the enormous impact this is having on Mexico and its really battered economy. Try to imagine Americans accepting the measures imposed on the residents of Mexico City.

By robin abaya (not verified) on 30 Apr 2009 #permalink

Tara, I haven't been mocking the flu but I have been criticizing the press for not understanding what the various bureaucratic steps are in dealing with things. However, that's what concerns me about the WHO's going to Phase 4, then 5 with this particular organism.

Does the current situation really satisfy their own requirements? In particular, is there sustained human-to-human transmission? As I understand things without sustained human-to-human transmission they shouldn't have declared Phase 4 let alone Phase 5.

If the WHO declared Phase 5 because they weren't satisfied with how some governments were reacting, isn't that beyond their competence?

Dave, as I read the news WHO has a rather loose definition of "sustained transmission."

One instance of influenza A(H1N1) in an individual who has not been to Mexico or been in contact with pigs seems to be sufficient. I would think that someone else would have to develop the condition from contact with that first non-Mexico, non-pig case in order to term transmission "sustained," but that does not appear to be what is happening.

By that loose usage of sustained, we have sustained transmission in New York. And I've seen it suggested we have it in California and Spain as well. (In the California case the argument was that since we did not know where the condition came from it must be human to human transmission. Pretty fuzzy thinking in my book.)

Perhaps someone more knowledgeable than I will reply.

By Paul Heikkila (not verified) on 01 May 2009 #permalink

You should be worried about the swine flu because it's highly contagious, and everybody reacts to the virus differently.

What's counterintuitive with this particular virus, it's in the people who have robust immune systems. As their body starts to respond, to try and fight off that virus, they produce tons of inflammatory cells. Those inflammatory cells can sort of flood the lungs. So, in essence, it's not the virus itself that's so problematic, but the body's reaction to it.

By Still Fly (not verified) on 01 May 2009 #permalink

Learning about the WHO pandemic scales and other measuring tools used by the science industry for contagion hazard levels is important for students working on medical or biological science fair projects. Thank you for talking about these things in your blog.

The origin, and properties of this virus, seem to be questionable at least. At the onset, It was reported if one listened carefully that this virus was a combination of the northamerican swine flu and a european form of swine flu, never before seen here. There is also a property to this virus which makes it penetrate the human tissues in a way never before seen in any virus.Since this, info was quickly replaced by the bird/ pig/man concept. Only now are some very pronound scientists confirming what was originaly reported. why then are we still being told that there is abird flu combo. And why are they not able to test the pigs on the american operated pig farms in veracruz, where this obviosly started.

By therimmers@shaw.ca (not verified) on 01 May 2009 #permalink

I think your post here is an excellent and balanced summary of the current status (and I do mean current as a very relative term - the interpretation may very well likely be different tomorrow - and that is reality).

I teach a zoonotic disease elective at the OSU vet school, and unfortunately for teaching, this outbreak began after I could teach on it. My thoughts - FWIW - have been somewhat circumspect - I understand the realities and potentialities, but also perceive a bit of an over-reach, and I'm not sure what engenders it. Is it a naive public? a government trying to develop a near zero risk society? A little of both? For example, the entire metro school district in Fort Worth, TX, is shut down (a week or so?) due to one reported case. Why the whole school district? I could understand the single school, but an entire district?

By the way, two thoughts I'd appreciate a specific comment upon, if time allows:
1. I understand the WHO Pandemic phase definitions, but are there epidemiologic computer models in use by international authorities that are refining their predictions and recommendations?
2. I've discussed this next point with other vet colleagues. It seems that with the definitions of the WHO Pandemic phases, that the general public is going to perceive them in a very linear fashion (phase 5 of 6) when the events defining the phases are fairly non-linear. After all, who amongst us has ever read a report from WHO (if such reports even exist) advising us about a new virus reassortant at Pandemic phase 1,2,or 3? In reality, phase 4-6 are pretty important and actionable. If we were told we were at level 2 out of 3 instead of 5 out of 6, I'm just guessing that the psychological ramifications would be different.

Enjoy your blog!
Tim Snider

By Tim Snider (not verified) on 01 May 2009 #permalink

The NY Times this morning indicates less than half of the suspect cases in Mexico have turned out to be confirmed cases. The confusion in the press and the popular and political imagination over numbers brings to mind a Times article 10 (or maybe even 20 years ago) on newspapers' reporting of science news. My recollection (foggy) is that 9 of the year's top 10 science stories were reported with considerable inaccuracy!

I digress. From today's Times:

Outbreak in Mexico May Be Smaller Than Feared
The swine flu outbreak in Mexico may be considerably smaller than originally feared, test results released there on Friday indicate.
Of 908 suspected cases that were tested, only 397 people turned out to have the virus, officially known as influenza A(H1N1), Mexican health officials reported at a news conference. Of those, 16 people have died.

Mexico had reported about 2,500 suspected cases as of Friday, but the number of real cases could turn out to be less than half the suspected number if further testing follows the same pattern as the original round. Officials said that the tests were being done quickly, and that 500 more would be completed Friday. ...

By Paul Heikkila (not verified) on 02 May 2009 #permalink

Again, not surprising that fewer are confirmed than suspected, but at the same time, keep in mind that the confirmed cases are only the tip of the iceberg as far as the total cases. When was the last time you went to the hospital for the flu?


This indeed is bad news. If 16 people have died from 397 cases, we have a 5% mortality rate, a very high number! But, of course, the real number of mild cases is higher so that the mortality rate is lower.

Tara, it seems that most of people does not understand what is an exponential grow. The use of expressions like "small number at this point of time" etc reflects this.

I have fitted the WHO numbers with a nice exponential with a daily grow factor of 1.51. This means that we will see 250.000 by two weeks from now and 80 million cases at the end of May (but of course, there is not sufficient time or kits to diagnose them all). See here:


Of course, this is a conservative number based only on oficial data. And signs of sigmoidal saturation will appear in June.