ILI Forecasts for 2017-2018-Epidemic Week 48

This is week six of the CDC Flu Challenge.  During the previous epidemic week (EW 47) there was a significant slowdown in the increase of %ILI in Regions 1, 2, 7, and 8.  The %ILI stayed at about the same level in Regions 3 and 5 and it decreased in Regions 4, 9 and 10.  The only Region to exhibit a large increase in %ILI during EW 47 was Region 6. Consequently, the national %ILI profile (red line in top left panel below) showed only a minor increase during the past week.  This slowing/leveling/decrease in %ILI is often observed a week or two after the Thanksgiving holiday and is likely a result of the large increase in %ILI during the holiday week itself when people tend to go to the doctor only if they are truly sick.

Currently, four Regions (1, 4, 6 and 7) and the National have been at, or above their respective onset values for the past 1-4 weeks.  Our forecast for the national onset week is therefore peaked at EW 48 (second panel from left in top row below).  The Thanksgiving holiday ‘slowdown’ is often followed by a large increase in the number of ILI cases in subsequent weeks and this has guided our selection for the national (top left panel and bottom row panels) and Regional 4-week forward forecast. Given that the %ILI profiles for most Regions and the National are still significantly above their historic values for this time of year, our prediction for the national peak timing (second panel from right in top row) is about evenly split between an early (EW52) and late (EWs 6-7) times.

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As noted above, Region 6 was the only region to have a large increase in the previous week and it has crossed its %ILI onset value (dashed grey horizontal line in top left panel below).  Our forecast for onset (second panel from left in top row) is therefore centered at the current epidemic week, with a minimal inclusion of the historic NULL model which allows for the possibility of ‘downward’ data update (and consequently a later onset week).   Given the elevated  %ILI values for this region we predict (second panel from right in top row)  that this region is more likely to peak early in the season (EW 51-52  ~50%) than later (EW 6-7 15%). Our forecast for the next four weeks (top left panel and bottom row) is based on a school vacation and specific humidity time-dependent force of infection, and a data augmentation procedure which uses %ILI data from previous seasons to inform the prior distribution used in the MCMC procedure.

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ILI forecasts for 2017-2018-Epidemic Week 47

This is the fifth week of the CDC Flu Challenge.  Region 4 remained above its onset value (for three consecutive weeks) and Region 1 and the national have crossed their onset values in the past week. The %ILI values for Region 6 were adjusted downward, to just below the onset value of the region.  Currently, six regions (1, 2, 4, 5, 6 and 7) and the national are (mostly well) above their historic average %ILI values, and four regions (3, 8, 9 and 10) are tracking their average %ILI values closely.  As in the previous week our selected forecast for the national peak week (second from right panel in top row) comes from coupled spatial model with a force of infection that depends on the school vacation schedule and consequently shows a probability for either an early (EW 52) or later (EW 6-7) season peak. Our forecast for onset (second from left panel in top row) is based mostly on the observed data (EW 47) but includes also a historic NULL model to account for the possibility of a data update that will reduce the national %ILI to below its onset value of 2.2%. Our national forecast for the next four weeks is shown in the top left panel with the corresponding probabilities in the four panels of the bottom row. This forecast is based on a model of with a time-dependent (school vacation and specific humidity) force of infection.Picture1

Region 7 had a large increase in the previous week and has crossed its %ILI onset value (dashed grey horizontal line in top left panel below).  Our prediction for onset (second panel from left in top row) is therefore centered at the current epidemic week, but as in the national case we do allow for the possibility of ‘downward’ data update (and consequently a later onset week) by including the historic NULL model in our submitted forecast.   Similarly to the national case,  our prediction for this region season peak week is split between an early (EW 52) and a late (EWs 5-8) time.  Our forecast for the next four weeks (top left panel and bottom row) is based on school vacation and specific humidity time-dependent force of infection and a data augmentation procedure which uses %ILI data from previous seasons to inform the prior distribution used in the MCMC procedure.

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