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Leading Indicators and the Evaluation of the Performance of Alerts for Influenza Epidemics.

Schanzer DL, Saboui M, Lee L, Domingo FR, Mersereau T - PLoS ONE (2015)

Bottom Line: However, the difference in timing exceeded 1 week and was statistically significant at the significance level of 0.01 in 5 out of 28 regional seasons.After allowing for a reporting delay of 2 weeks, the alert period included 80% of all influenza-confirmed hospitalizations.Though differences in timing were statistically significant, neither time-series consistently led the other.

View Article: PubMed Central - PubMed

Affiliation: Centre for Communicable Diseases and Infection Control, Infectious Disease Prevention and Control Branch, Public Health Agency of Canada, Ottawa, Ontario, Canada.

ABSTRACT

Background: Most evaluations of epidemic thresholds for influenza have been limited to internal criteria of the indicator variable. We aimed to initiate discussion on appropriate methods for evaluation and the value of cross-validation in assessing the performance of a candidate indicator for influenza activity.

Methods: Hospital records of in-patients with a diagnosis of confirmed influenza were extracted from the Canadian Discharge Abstract Database from 2003 to 2011 and aggregated to weekly and regional levels, yielding 7 seasons and 4 regions for evaluation (excluding the 2009 pandemic period). An alert created from the weekly time-series of influenza positive laboratory tests (FluWatch, Public Health Agency of Canada) was evaluated against influenza-confirmed hospitalizations on 5 criteria: lead/lag timing; proportion of influenza hospitalizations covered by the alert period; average length of the influenza alert period; continuity of the alert period and length of the pre-peak alert period.

Results: Influenza hospitalizations led laboratory positive tests an average of only 1.6 (95% CI: -1.5, 4.7) days. However, the difference in timing exceeded 1 week and was statistically significant at the significance level of 0.01 in 5 out of 28 regional seasons. An alert based primarily on 5% positivity and 15 positive tests produced an average alert period of 16.6 weeks. After allowing for a reporting delay of 2 weeks, the alert period included 80% of all influenza-confirmed hospitalizations. For 20 out of the 28 (71%) seasons, the first alert would have been signalled at least 3 weeks (in real time) prior to the week with maximum number of influenza hospitalizations.

Conclusions: Virological data collected from laboratories was a good indicator of influenza activity with the resulting alert covering most influenza hospitalizations and providing a reasonable pre-peak warning at the regional level. Though differences in timing were statistically significant, neither time-series consistently led the other.

No MeSH data available.


Related in: MedlinePlus

a) Weekly number of influenza positive tests and influenza admissions to hospital for the Prairies region, 2003/04 season. b) Corresponding cumulative distribution functions (CDF).The two time series are in close agreement with a correlation coefficient (r) of 0.97 (95% CI: 0.94, 0.98). However, influenza hospital admissions continued for many months after the epidemic subsided in this region, and the impact of these later admissions is highlighted by the CDF comparison. As a result, the average date of hospital admissions lagged influenza positive tests by an average of 12 days. This season is of interest due to the early epidemic peak (week of Nov 9, 2003). The pre-peak alert period is 4 weeks (the first alert was set based on laboratory data for the week of Sept 28, with the alert period starting operationally in the week of Oct 12), well ahead of peak influenza activity for the region, thereby providing significant advanced warning at a key time.
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pone.0141776.g003: a) Weekly number of influenza positive tests and influenza admissions to hospital for the Prairies region, 2003/04 season. b) Corresponding cumulative distribution functions (CDF).The two time series are in close agreement with a correlation coefficient (r) of 0.97 (95% CI: 0.94, 0.98). However, influenza hospital admissions continued for many months after the epidemic subsided in this region, and the impact of these later admissions is highlighted by the CDF comparison. As a result, the average date of hospital admissions lagged influenza positive tests by an average of 12 days. This season is of interest due to the early epidemic peak (week of Nov 9, 2003). The pre-peak alert period is 4 weeks (the first alert was set based on laboratory data for the week of Sept 28, with the alert period starting operationally in the week of Oct 12), well ahead of peak influenza activity for the region, thereby providing significant advanced warning at a key time.

Mentions: In the 2003/04 season a novel strain (A⁄Fujian⁄411⁄02) emerged to dominate the season (Fig 1). Agreement was good between the two curves. The alert was first set based on virological data for the week of Nov 16, and available for planning 2 weeks later (week of Nov 30), or 3 weeks before the peak in influenza hospitalizations (week of Dec 21). In Fig 2, the correlation between curves was poorer (r = 0.55), though coverage was still good (77%). However, the alert status was on for only 2 weeks in real time before the peak in influenza hospitalizations. In the 2003/04 season, the A⁄Fujian⁄411⁄02 strain emerged very early in the season in the Prairies (Fig 3). An unusually long tail for hospitalizations resulted in an estimated average lag of 12 days (95% CI: 7.5, 17.3), though the epidemic midpoint (CDF = 50%) occurred in the week of Nov 9 for both time-series. A pre-peak alert period of 4 weeks (set based on laboratory data for the week of Sept 28, and reportable in the week of Oct 12), would have provided significant advanced warning. Fig 4 illustrates the variation corresponding to a mixed season (A/Fujian/411/2002(H3N2) and A/California/7/2004) with two separate peaks. The alert period was long (18 weeks), though the pre-peak warning was short (2 weeks). Fig 5 is an example of a season where two influenza A strains (A/Solomon Islands/03/2006(H1N1) followed by A/Brisbane/10/2007 (H3N2)) and a B strain (B/Florida/4/2006, B/Yamagata lineage) circulated. The epidemic curves are very similar (r = 0.95), however the pre-peak alert period was 15 weeks. Fig 6 is an example where peak influenza activity occurred over the last week of December and first week of January, a time when advanced warning would be helpful for resource planning in the hospital setting. In this example, the alert provided a 3 week pre-peak warning period.


Leading Indicators and the Evaluation of the Performance of Alerts for Influenza Epidemics.

Schanzer DL, Saboui M, Lee L, Domingo FR, Mersereau T - PLoS ONE (2015)

a) Weekly number of influenza positive tests and influenza admissions to hospital for the Prairies region, 2003/04 season. b) Corresponding cumulative distribution functions (CDF).The two time series are in close agreement with a correlation coefficient (r) of 0.97 (95% CI: 0.94, 0.98). However, influenza hospital admissions continued for many months after the epidemic subsided in this region, and the impact of these later admissions is highlighted by the CDF comparison. As a result, the average date of hospital admissions lagged influenza positive tests by an average of 12 days. This season is of interest due to the early epidemic peak (week of Nov 9, 2003). The pre-peak alert period is 4 weeks (the first alert was set based on laboratory data for the week of Sept 28, with the alert period starting operationally in the week of Oct 12), well ahead of peak influenza activity for the region, thereby providing significant advanced warning at a key time.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4626042&req=5

pone.0141776.g003: a) Weekly number of influenza positive tests and influenza admissions to hospital for the Prairies region, 2003/04 season. b) Corresponding cumulative distribution functions (CDF).The two time series are in close agreement with a correlation coefficient (r) of 0.97 (95% CI: 0.94, 0.98). However, influenza hospital admissions continued for many months after the epidemic subsided in this region, and the impact of these later admissions is highlighted by the CDF comparison. As a result, the average date of hospital admissions lagged influenza positive tests by an average of 12 days. This season is of interest due to the early epidemic peak (week of Nov 9, 2003). The pre-peak alert period is 4 weeks (the first alert was set based on laboratory data for the week of Sept 28, with the alert period starting operationally in the week of Oct 12), well ahead of peak influenza activity for the region, thereby providing significant advanced warning at a key time.
Mentions: In the 2003/04 season a novel strain (A⁄Fujian⁄411⁄02) emerged to dominate the season (Fig 1). Agreement was good between the two curves. The alert was first set based on virological data for the week of Nov 16, and available for planning 2 weeks later (week of Nov 30), or 3 weeks before the peak in influenza hospitalizations (week of Dec 21). In Fig 2, the correlation between curves was poorer (r = 0.55), though coverage was still good (77%). However, the alert status was on for only 2 weeks in real time before the peak in influenza hospitalizations. In the 2003/04 season, the A⁄Fujian⁄411⁄02 strain emerged very early in the season in the Prairies (Fig 3). An unusually long tail for hospitalizations resulted in an estimated average lag of 12 days (95% CI: 7.5, 17.3), though the epidemic midpoint (CDF = 50%) occurred in the week of Nov 9 for both time-series. A pre-peak alert period of 4 weeks (set based on laboratory data for the week of Sept 28, and reportable in the week of Oct 12), would have provided significant advanced warning. Fig 4 illustrates the variation corresponding to a mixed season (A/Fujian/411/2002(H3N2) and A/California/7/2004) with two separate peaks. The alert period was long (18 weeks), though the pre-peak warning was short (2 weeks). Fig 5 is an example of a season where two influenza A strains (A/Solomon Islands/03/2006(H1N1) followed by A/Brisbane/10/2007 (H3N2)) and a B strain (B/Florida/4/2006, B/Yamagata lineage) circulated. The epidemic curves are very similar (r = 0.95), however the pre-peak alert period was 15 weeks. Fig 6 is an example where peak influenza activity occurred over the last week of December and first week of January, a time when advanced warning would be helpful for resource planning in the hospital setting. In this example, the alert provided a 3 week pre-peak warning period.

Bottom Line: However, the difference in timing exceeded 1 week and was statistically significant at the significance level of 0.01 in 5 out of 28 regional seasons.After allowing for a reporting delay of 2 weeks, the alert period included 80% of all influenza-confirmed hospitalizations.Though differences in timing were statistically significant, neither time-series consistently led the other.

View Article: PubMed Central - PubMed

Affiliation: Centre for Communicable Diseases and Infection Control, Infectious Disease Prevention and Control Branch, Public Health Agency of Canada, Ottawa, Ontario, Canada.

ABSTRACT

Background: Most evaluations of epidemic thresholds for influenza have been limited to internal criteria of the indicator variable. We aimed to initiate discussion on appropriate methods for evaluation and the value of cross-validation in assessing the performance of a candidate indicator for influenza activity.

Methods: Hospital records of in-patients with a diagnosis of confirmed influenza were extracted from the Canadian Discharge Abstract Database from 2003 to 2011 and aggregated to weekly and regional levels, yielding 7 seasons and 4 regions for evaluation (excluding the 2009 pandemic period). An alert created from the weekly time-series of influenza positive laboratory tests (FluWatch, Public Health Agency of Canada) was evaluated against influenza-confirmed hospitalizations on 5 criteria: lead/lag timing; proportion of influenza hospitalizations covered by the alert period; average length of the influenza alert period; continuity of the alert period and length of the pre-peak alert period.

Results: Influenza hospitalizations led laboratory positive tests an average of only 1.6 (95% CI: -1.5, 4.7) days. However, the difference in timing exceeded 1 week and was statistically significant at the significance level of 0.01 in 5 out of 28 regional seasons. An alert based primarily on 5% positivity and 15 positive tests produced an average alert period of 16.6 weeks. After allowing for a reporting delay of 2 weeks, the alert period included 80% of all influenza-confirmed hospitalizations. For 20 out of the 28 (71%) seasons, the first alert would have been signalled at least 3 weeks (in real time) prior to the week with maximum number of influenza hospitalizations.

Conclusions: Virological data collected from laboratories was a good indicator of influenza activity with the resulting alert covering most influenza hospitalizations and providing a reasonable pre-peak warning at the regional level. Though differences in timing were statistically significant, neither time-series consistently led the other.

No MeSH data available.


Related in: MedlinePlus