ECDC: Planning Assumptions for the First Wave of Pandemic A(H1N1) 2009 in Europe

30 07 2009

The ECDC have just released a “Public Health Development” entitled “Planning Assumptions for the First Wave of Pandemic A(H1N1) 2009 in Europe”. Read it here (with full references). This paper seems to have been prompted by Norway and the UK re-assessing the assumptions they are using in their pandemic planning. The paper starts this way:

As it is summer in Europe the 2009 pandemic has yet to really accelerate in EU countries but the experience in temperate Southern Hemisphere countries suggests it is inevitable that Europe will be affected by a major first A(H1N1) 2009 pandemic wave in the autumn and winter.  The 2009 pandemic is less severe than might have been expected and ECDC has been made aware by two European Union countries (Norway and the UK) of the updating they have made of their planning assumptions specifically for a first wave of an A(H1N1) 2009 pandemic.

The paper is short, but makes interesting reading and starts to answer a number of questions.

What do we think the “clinical attack rate” might be?

The “clinical attack rate” is the proportion of the population that will catch Swine Flu at some point in the pandemic’s “wave” and display symptoms. The paper says it is assuming a clinical attack rate of 30% (and that another 30% of the population will catch Swine Flu but display no symptoms). By the way, I’m guessing that “displaying symptoms” means getting sick enough to take at least half a day off work – my understanding is that that’s the requirement to be “symptomatic”. Here’s what the paper says about the clinical attack rate it’s assuming:

This is 30 % (The UK clinical attack rate is based on an assumption that half of the infected become symptomatic so this would imply a total infection attack rate of about 60 %). WHO assumptions are that two thirds become symptomatic. Whether the UK or WHO is correct will be determined later when the results from serology become available.   The UK assumptions imply a basic reproductive number Ro in the interval 1.4 – 1.5 which seems to be the case at present in the UK. A Ro of value 1.4 implies a total infection attack rate of about 50 % (which would imply a clinical attack rate of 25 % in the UK planning assumptions). A higher value of Ro of 2.0 implies a total infection attack rate of about 80 % (hence a clinical attack rate of 40% in the UK planning assumptions).

What do we think the “case fatality rate” might be?

This is the proportion of people who catch the virus (and are symptomatic?) who then go on to die. The paper states:

This is one of the most eagerly sought parameters but it is also amongst the hardest to determine with any accuracy. The earliest studies of this pandemic gave a high CFR of about 0.4 % compared to lower rates for the 1957 and 1968 pandemics but higher rates for 1918. The UK estimates are of a CFR of 0.1-0.2 though values of up 0.35% cannot be ruled out as impossible.

Later, the ECDC comments:

Case Fatality Rates (CFR) will also change as more data become available and more stable estimates will take some time to emerge. As some cases will be very mild and not reported the reported figures from official tables of cases and deaths will most often be an over-estimate of the true CFR. Equally though many deaths which result from influenza (seasonal or pandemic) are not attributed to the infection in official causes of deaths and so officially reported influenza deaths are always an underestimate, sometimes grossly so. In previous pandemics it has only been computed with any accuracy once the pandemics were over. It is also important to appreciate that CFR is especially subject to social effects. In poor social settings such as Africa even seasonal influenza can result in CFR’s that are higher than seen in pandemics.

What level of absenteeism might we see?

In the comments section the ECDC discusses the UK’s assumption of a peak in absenteeism at 12%:

The UK predicted peak absenteeism rate of 12 % of the workforce is interesting and fits with the mild illness seen for most people. It suggests that the social disruption effects of the pandemic will be less than feared for other pandemics and that severe social interventions will not be necessary given good business continuity planning.

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2 responses

31 07 2009
Snake Oil Baron

I hope that they calculate CFR based on symptomatic cases using whatever criteria they use to calculate clinical attack rate. That way they don’t have to worry about missing large numbers of asymtomatic infections. It might make the pandemic seem a bit worse than it would be if CFR was calculated from every person who contracts the virus but it would probably make for a more accurate comparison with 1918 flu since they didn’t have the ability to detect antibodies in healthy people back then.

The fatality rate does seem to be much higher in certain places which suggests to me that the state of the health care system at the time you measaure CFR has a significant effect on the CFR value. When there is a significant number of cases where the patient survives because of a respirator, the CFR will change greatly depending on the availability of respirators. The same goes for ICU staff and supplies.

31 07 2009
aussieflublogger

It’s a good point – they probably do need to use symptomatic cases because of historical comparisons. I just need to find a bit of time to check this.

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