ECDC posts the UK planning assumptions for pandemic flu

The European Centre for Disease Prevention and Control posted the UK planning assumptions for pandemic waves in the fall and winter.

Clinical attack rate	30%
Peak clinical attack rate	6.5% (local planning assumptions 4.5% to 8%) per week
Complication rate	15% of clinical cases
Hospitalisation rate	2% of clinical cases
Case fatality rate	0.1% to 0.2% (cannot exclude up to 0.35%) of clinical cases
Peak Absence Rate	12% of workforce

"UK parameters and their broader applicability

The UK paper is based on a model using parameter estimates from the UK and abroad on the 2009 strain and fitted using real data on UK cases over the period when the majority of cases were confirmed and reported daily.

Clinical attack rate
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 [5]. 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).

Peak clinical attack rate
This can depend on a number of factors such as seasonality, immunity in the population and interventions that might prolong the epidemic but also reduce the peak attack rate [6]. A particularly important point to note is that local epidemics are often shorter and sharper in a pandemic than national rates and so there is a higher value for the peak clinical attack rates for local application [1,3].

Case fatality rate
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 % [7] compared to lower rates for the 1957 and 1968 pandemics but higher rates for 1918 [8]. The UK estimates are of a CFR of 0.1-0.2 though values of up 0.35% cannot be ruled out as impossible [3]. The CFR number reported in the UK are thus as stated the reasonable worst case scenario unless the virus changes its characteristics in terms of lethality while the Norwegian figure is more based on what has been directly observed, adjusted for assumed underreporting."

For the full discussion, see: http://www.ecdc.europa.eu/en/health_content/phdev/090729_ph.aspx