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Public health folks are doing a terrific job staying on top and ahead of the H1N1 (swine flu) epidemic, which is both local and world-wide. As is often the case, public health officials have to walk a fine line between informing and inflaming the public. Right now there's reason for "cautious optimism" (which I share)

but the CDC says its own count is outdated almost as soon as it's announced. More cases are being confirmed daily. About one-third so far are people who had been to Mexico and probably picked up the infection there. Many newly infected people are getting the illness in the U.S., and the CDC says it probably still is spreading.

In this case, there are a lot of things that really need explaining:

  • what a pandemic is (based on spread, not severity)
  • why there are school closures with only a handful of cases
  • why this appears to be no worse than seasonal flu – yet isn't seasonal flu and has to be treated differently
  • why testing isn't instantly available (needs sophisticated PH lab testing)
  • why not everyone needs tamiflu even though tamiflu "works" (mild flu does not need treatment, regardless of seasonal or swine designation)

So let's tackle some of those difficult points, starting with school closure.

The rationale for school closure was developed in concert with the idea that a pandemic means a widespread geographic range — for the WHO, it has got to be on two continents:

Phase 5 is 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.

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.

For a given region, even in a pandemic, spread may have just started to get underway. See CDC graph for the initiation, acceleration and peak:

We are not at the peak now (but we might be at the acceleration interval.) It's early and worse things can come. So wouldn't that mean we are at phase 6? Well, the thought all along was that it all would start in Southeast Asia and then at Phase 6, come to San Diego. Acceleration matches "spread throughout United States" on the top of the CDC graph and sits within phase 6, and with 21 states involved, we properly might now describe things as exactly that. That's why WHO recently revised the phases. In the new method, phase 5 simply means widespread in one region. In fact, it's in San Diego, and it will be Phase 6 when it moves to Hong Kong or New Zealand or the UK. Therefore, moving to phase 6 doesn't affect what we already have in the US. Declaring a pandemic is for the rest of the world. We got what we got right here, right now, whatever it's called.


The peak means sick people, and hospitalizations, and some death. it would be helpful to shrink it, delay it, and mitigate it.

The three major goals of mitigating a community-wide epidemic through NPIs [non-pharmaceutical interventions] are 1) delay the exponential increase in incident cases and shift the epidemic curve to the right in order to "buy time" for production and distribution of a well-matched pandemic strain vaccine, 2) decrease the epidemic peak, and 3) reduce the total number of incident cases and, thus, reduce morbidity and mortality in the community (Figure 1).  These three major goals of epidemic mitigation may all be accomplished by focusing on the single goal of saving lives by reducing transmission.  NPIs may help reduce influenza transmission by reducing contact between sick persons and uninfected persons, thereby reducing the number of infected persons.  Reducing the number of persons infected will also lessen the need for healthcare services and minimize the impact of a pandemic on the economy and society.  The surge of need for medical care associated with a poorly mitigated severe pandemic can be only partially addressed by increasing capacity within hospitals and other care settings.  Thus, reshaping the demand for healthcare services by using NPIs is an important component of the overall strategy for mitigating a severe pandemic.

The less severe the pandemic, the less mitigation is needed. But as far as that peak goes, here's the goal:

Cool! But how do you do it?

You can wash you hands, wear a mask, telecommute... use social distancing (stay away from each other), and the key to that is kids (the humans who don't wash their hands.) Closing schools early before disease spreads is a community mitigation technique that has to be initiated before there is already spread underway. Typically, schools close between 7-30% ill. It is extremely atypical to ask them to close at anywhere near 1% ill, but that's what's needed to reduce spread. Wait any longer and it's too late to make a difference.

Here's a study showing that it might work (scenario 1 is doing nothing and scenarios 2 and 3 are intervening at 1% illness and 0.1% illness):

The attack rate drops the sooner you intervene with NPI's. Adding treatment and targeted anti-viral prophylaxis (TAP) adds further benefit. However, there are practical aspects of intervening too early, so there are always balances and trade-offs:

The timing of initiation of various NPIs will influence their effectiveness.  Implementing these measures prior to the pandemic may result in economic and social hardship without public health benefit and may result in compliance fatigue.  Conversely, implementing these interventions after extensive spread of a pandemic influenza strain may limit the public health benefits of an early, targeted, and layered mitigation strategy.  Identifying the optimal time for initiation of these interventions will be challenging, as implementation likely needs to be early enough to preclude the initial steep upslope in case numbers and long enough to cover the peak of the anticipated epidemic curve while avoiding intervention fatigue.

You can see from that (and the detailed description) that this is not an exact science. And let me repeat this: implementation likely needs to be early enough to preclude the initial steep upslope in case numbers. That's the answer to "but there's only one case! Why are we doing this?" If you wait for "the steep upslope", it's too late to matter.

But is it all theoretical? Not hardly. In 1918, during the terrible pandemic of that year, Philadelphia refused significantly delayed NPI (closing schools, canceling public gatherings), and look what happened compared to St. Louis, which did.

Isn't that a startling mirror of the "goals of community mitigation" curve? It's meant to be. Everything that we are describing is an attempt to be St. Louis and not Philadelphia, even though this year's version is so much milder.

Now, remember, the milder the virus, the less intervention and community mitigation is needed, and the less the plan calls for:

Note that in the above table, it's likely that even a mild pandemic will kill more people than an average seasonal flu. That's what makes it different than seasonal flu even when it's described as "mild". In this context, "mild" means only 3-10x the amount of deaths as usual. And it could turn out to be more.

But when you don't know, in the beginning, how things are sorting themselves out then you err on the side of caution. You can always pull back. And the degree of disease and death will dictate how long the schools close for. The more severe the pandemic, the longer the schools will stay closed.

Now, there are current and active interim recommendations from CDC as to when to close schools:

CDC recommends that affected communities with laboratory-confirmed cases of influenza A (H1N1) virus infection consider activating school dismissal and childcare closure interventions according to the guidelines below.  "Affected communities" may include a U.S. State or proximate epidemiological region (e.g., a metropolitan area that spans more than one State’s boundary).  These guidelines address a flexible and scalable approach that States and local jurisdictions can use based on the situation in their communities (e.g. number of cases, severity of illness, affected groups)...

Dismissal of students in a school and closure of childcare facilities should be considered in schools with one or more laboratory-confirmed or non-subtypable influenza A case among students, faculty or staff in order to decrease the spread of illness in the community.

Beyond that, the detailed guidance suggests close collaboration between school officials and public health officials.

State-wide closure would only happen in a Category 2-3 (where it can be "considered") or greater (it's "recommended for category 4-5"). and that can't happen until we determine that A) we have a pandemic and B) we know the severity and therefore assign a category. But as we discussed above, in the US we already have virus spread (if/when it spreads elsewhere, WHO will declare a pandemic), so health officials are acting now based on what we are seeing today. We don't have a declared pandemic and we don't know what category we are in (maybe a 2, maybe a good deal less). Until we know, the precautionary principle applies and the interim CDC guidelines will be followed.

There are no predictions that can be made about the fall. Whether this virus disappears, smolders, or returns, will have to be carefully monitored.

So, back to the job of public health officials. If they do their job, there'll sometimes be the appearance of over-reacting. If they don't there'll be the appearance of being asleep at the switch. Richard Besser (Acting Director, CDC) in particular, and the entire team is doing very well, but there's a lot more to explain, including that the virus could be back in the fall, that vaccine will take six months to produce (and even then, will need to be prioritized to essential workers first), and that CDC doesn't decide if your local school closes... and when it re-opens. Behind the scenes, communication has also been excellent. In a way, this is a country-wide tabletop for a worse event that still may come (maybe not for years, but eventually it will.) But don't judge things just yet. This story is still in the early chapters.

In the meantime, there's more to discuss, especially about surveillance, vaccines, health reform, and public health infrastructure. But for today, pay attention to school closings and understand why they are happening.

Originally posted to Daily Kos on Sun May 03, 2009 at 08:02 AM PDT.

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