U.S. Flu Forecasts --- 2009 Week 42 (Produced 22 October)
Regional Influenza Activity for week 42 (ending 24 October)
2009 Week 42 (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 42 (Produced 22 October)
The CDC reports in week 40 that the %weighted ILI
(the percentage of outpatient visits for influenza-like illness) stands at 6.1.
Week 40 is the start of the 2009-2010 flu season.
Unfortunately, the numerical data on the CDC web site does not
correspond with the charts, and seems all wrong.
The week 35-39 data points in the regional charts are very different from last week,
and could be wrong too.
The regional data have been read of the graph, which is not very accurate.
The influenza activity in the US has increased substantially
in week 40, following a similar increase in week 39.
Based on the continuation of growth in ILI%, the M2 week 41 nowcast is 6.2%,
for week 42 it is 6.3%.
ILI% in the US is well above the national baseline of 2.4%, and
higher than the peak of last season.
Flu activity in all regions is above their region-specific baseline, based on week
42 nowcasts.
Regions 5,7,8,10 are above three times baseline.
2 Current Influenza Activity
Robustified Google Flu Trends (RGFT) shows a further increase of ILI%
in weeks 41 and 42: 6.7 and 7.3% respectively. This uses the revised
GFT data which show 6.0 and 6.5% 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 40.
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, and week 40 shows 30%.
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%.
The nowcasts make the assumption that the trend follows a 1,2,3,1.5,0.5,0,1,1,1
pattern (multiplied by the estimated coefficient of 0.2) for
weeks 33-42.
Model M2 nowcasts, based on this trend, report a resumption of growth in
ILI%:
6.2% and 6.3% for weeks 41 and 42 respectively.
The average nowcasts for week 41 and 42, based on pooling a dynamic model with
calendar effects (M2) and RGFT, show an increase in ILI%:
6.5% and 6.8% respectively.
3 Expected Influenza Activity
Historically, there tends to be a fairly slow increase in ILI% until
Thanksgiving Day.
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.
The results reported here are based on forecast, and
are therefore uncertain. These results are my personal opinion, based on extensive
modeling, and not endorsed by either the CDC or Google.
