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Table 1 Summary of WHO method and MEM concepts

From: Establishing seasonal and alert influenza thresholds in Morocco

Concepts

WHO method (5, 20)

MEM (20, 26)

Average epidemic curve

Find 3-week moving average of ILI%. Find median peak week for each season. Align the multiple seasons on median peak week. Calculate the average ILI% for each week. Indicates the usual level of influenza activity that occurs during a typical year.

MEM software produces an average curve, lower interval, and higher interval.

Alert threshold

Calculate the mean and standard deviation (SD) of the average epidemic curve. For each week, the alert threshold is 1.645 SD above the weekly ILI% mean. ILI% > 1.645 SD indicates high ILI activity or outbreaks and may be used to characterize a severe season.

 

Alert curve

A graph consisting of the alert thresholds for each epidemic week.

 

Seasonal threshold (WHO) or pre-epidemic threshold (MEM)

Median weekly ILI% over all weeks (i.e., the average epidemic curve is not used). Indicates the level of influenza activity that signals the start and end of the annual influenza season(s).

For prospective surveillance: upper limit of the 95% one-sided confidence interval of the arithmetic mean of the 30 highest pre-epidemic weekly ILI% values. Parameter value which marks the start of the epidemic period.

Post-epidemic threshold (MEM)

 

For prospective surveillance: upper limit of the 95% one-sided confidence interval of the arithmetic mean of the 30 highest post-epidemic weekly ILI% values.

Epidemic period start

The third of three consecutive weeks with ILI% above seasonal threshold. Indicates that influenza activity occurs consistently.

For retrospective analysis of individual season data: see “length of epidemic period”.

Epidemic period end

The third of three consecutive weeks with ILI% below seasonal threshold

For retrospective analysis of individual season data: see “length of epidemic period”.

Length of epidemic period

Weeks from epidemic start to end.

For retrospective analysis of individual season data: MEM software uses a “maximum accumulated proportions percentage (MAP)” algorithm to split the season into three periods: a pre-epidemic, an epidemic, and a post-epidemic period.

Epidemic percentage

Proportion of total cases that occurred during the epidemic period

Moderate (WHO) or medium (MEM) intensity

Upper 40% limit of 1-sided CI of mean of all peak values.

Upper 40% limit of the one-sided confidence interval of the geometric mean of the 30 highest epidemic weekly ILI% values.

High intensity

Upper 90% limit of 1-sided CI of mean of all peak values.

Upper 90% limit of the one-sided confidence interval of the geometric mean of the 30 highest epidemic weekly ILI% values.

Extraordinary (WHO) or very high (MEM) intensity

Upper 97.5% limit of 1-sided CI of mean of all peak values.

Upper 95% limit of the one-sided confidence interval of the geometric mean of the 30 highest epidemic weekly ILI% values.