U.S. Flu Forecasts --- 2009 Week 41 (Produced 16 October)
Regional Influenza Activity for week 41 (ending 17 October)
2009 Week 41 (nowcast)
Color coding is based on region-specific baselines. Green and yellow:
below baseline; orange: between baseline and 1.5 times baseline; red:
above 1.5 times baseline; dark red: above 3 times baseline; very dark red:
above 6 times baseline.
1 Summary for Week 41 (Produced 16 October)
The CDC reports in week 39 that the %weighted ILI
(the percentage of outpatient visits for influenza-like illness) stands at 5.1.
Google Flu Trends estimates ILI% for week 38 at 4.9.
The influenza activity in the US has increased substantially
in week 39, following the decrease in week 38.
Based on the resumption of growth in ILI%, the M2 week 40 nowcast is 5.4%,
for week 41 it is 5.6%.
ILI% in the US is well above the national baseline of 2.4%, and
higher than the peak of last season.
Flu activity in 8 regions is above their region-specific baseline, based on week 40 and
Regions 1 and 2 are below its baseline. Regions 7 to 10 are above
three times baseline.
2 Current Influenza Activity
Robustified Google Flu Trends (RGFT) shows a further increase of ILI%
in weeks 40 and 41: 5.6% and 6.0% respectively. This uses the revised
GFT data which show 5.4 and 5.7% respectively.
RGFT is based on the changes in the logit of Google Flu Trends (GFT),
applied to the ILI% level reported in the CDC influenza report for week 39.
The rapid growth of weeks 33-35 is very well described by the
addition of a local linear trend to model M2: the logit increased by
0.2, 0.4, 0.6 respectively.
This corresponds approximately with increases of 20%, 40% and 60% in
flu activity (ILI%). Week 36 shows a slower increase at 0.3 and week 37 at 0.1.
Week 38 had a sudden decline which matched the model, corresponding
to a zero in the trend. However, week 39 is back to a 20% increase
over the model.
In comparison, during the winter peak in a normal flu season (weeks 50-51 and 3-6),
there is an average weekly increase of about 20%.
This trend could not be clearly seen two week ago, but is now estimated
with high significance in the model M2.
The nowcasts make the assumption that the trend follows a 1,2,3,1.5,0.5,0,1,1
pattern (multiplied by the estimated coefficient of 0.2) for
Model M2 nowcasts, based on this trend, report a resumption of growth in
5.4% and 5.6% for weeks 40 and 41 respectively.
The average nowcasts for week 40 and 41, based on pooling a dynamic model with
calendar effects (M2) and RGFT, show an increase in ILI%:
5.5% and 5.8% respectively.
3 Expected Influenza Activity
Historically, there tends to be a fairly slow increase in ILI% until
The Thanksgiving holidays are associated with an additional increase of 15%.
It is clear that this season is already developing in a very different way.
4 Flu Season Forecasts of Influenza Activity
The scenarios have not been updated, and are as in the week 38 report.
The dynamic model M2 describes normal flu seasons quite well, but is
inappropriate during pandemic flu. For that reason, scenarios are reported.
The one-year ahead forecasts use
the average nowcasts for week 37 and 38 as the starting point for forecasting.
Then three assumptions are used to capture pandemic flu activity:
Medium 20%: the period of elevated activity lasts until Thanksgiving Day.
This is captured by a trend effect estimated over weeks 33 to 37
which is increasing until week 35 then decreasing. From week 38
until Thanksgiving Day 0.2 is added to the intercept (a 20% increase in ILI).
This is the dotted blue line.
Note that Thanksgiving Day is usually associated with an increase in flu activity.
Long 20%: the period of elevated activity is extended until the week
before Christmas, the solid blu line.
Long 25%: the coefficient is increased from 0.2 to 0.25, and runs
until the week before Christmas (the dotted purple line).
Note that the dynamics in the model effectuates a moderating effect
after a longer period of very rapid increase.
If these assumptions hold, very high levels of ILI% should be expected.