Statistical estimates of absenteeism attributable to seasonal and pandemic influenza from the Canadian Labour Force Survey

Abstract

Background: As many respiratory viruses are responsible for influenza like symptoms, accurate measures of the disease burden are not available and estimates are generally based on statistical methods. The objective of this study was to estimate absenteeism rates and hours lost due to seasonal influenza and compare these estimates with estimates of absenteeism attributable to the two H1N1 pandemic waves that occurred in 2009.

Methods: Key absenteeism variables were extracted from Statistics Canada's monthly labour force survey (LFS). Absenteeism and the proportion of hours lost due to own illness or disability were modelled as a function of trend, seasonality and proxy variables for influenza activity from 1998 to 2009.

Results: Hours lost due to the H1N1/09 pandemic strain were elevated compared to seasonal influenza, accounting for a loss of 0.2% of potential hours worked annually. In comparison, an estimated 0.08% of hours worked annually were lost due to seasonal influenza illnesses. Absenteeism rates due to influenza were estimated at 12% per year for seasonal influenza over the 1997/98 to 2008/09 seasons, and 13% for the two H1N1/09 pandemic waves. Employees who took time off due to a seasonal influenza infection took an average of 14 hours off. For the pandemic strain, the average absence was 25 hours.

Conclusions: This study confirms that absenteeism due to seasonal influenza has typically ranged from 5% to 20%, with higher rates associated with multiple circulating strains. Absenteeism rates for the 2009 pandemic were similar to those occurring for seasonal influenza. Employees took more time off due to the pandemic strain than was typical for seasonal influenza.

Figure 1

Workplace absenteeism attributed to influenza: seasonal, 1997/98-2008/09, and pandemic 2009. Influenza seasons differed by predominant subtype (H1N1 vs H3N2), the co-circulation of influenza B strains and the number of antigenic strains. The pandemic waves were more remarkable for the number of hours lost than the number of employees taking time off work. Estimates of the seasonal absenteeism rate attributable to influenza and proportion of hours lost due to influenza infection were based on separate models, though a strong association between these estimates is evident.

Figure 2

Percent of potential hours worked annually that were lost due to influenza illness by age group. The dashed line shows the increase in the proportion of hours lost due to own illness or disability with increasing age. The estimated proportion of hours lost due to an infection with the pandemic strain was similar for all age groups.

Figure 3

Estimated percent of potential hours worked that were lost due to influenza per season. The proportion of potential hours worked annually that were lost due to influenza over the two pandemic waves was 0.19% (95% CI 0.15-0.23) compared to 0.08% (0.06-0.10) for seasonal influenza. Confidence intervals were estimated based on the coefficient of variation of the corresponding parameter for the proxy variable for influenza activity and includes excess variation estimated by the inclusion of a dispersion parameter. As a result, the CIs were quite broad and differences by employment characteristics were not statistically significant. The CIs for the Urban/Rural split were included to illustrate. While, the proportion of hours lost varied significantly with the specific employment characteristics, these employment characteristics had less impact on hours lost due to influenza.

Figure 4

Model fit showing the estimated baseline and attribution to influenza. The actual data, a) the proportion of hours lost, and b) absenteeism rates are plotted against the reference week along with the model predicted values, the model estimated baseline and the attribution to influenza. Influenza is responsible for much of the seasonal variation and contributes significantly to peak absenteeism rates.

Figure 5

Comparison of the attribution of hours lost to influenza: assessing model fit. The actual hours lost less baseline (excess) is compared with the model predicted hours lost less baseline. The difference between the two curves are known as model residuals (and equal to actual - baseline). Residuals represent the variation not explained by the model. The influenza-attributed curve is smoother as the residuals, or unexplained variation, are not included in this time series. The residuals will average out over a season. The model fit is reasonable, though the model seems to miss the occasional dip in hours lost over the summer period.

Figure 6

A Comparison of the seasonal baseline for absenteeism rates and percent of potential hours worked that were lost due to own illness or disability. The seasonal baselines in the absence of influenza activity for the two measures of time off work: absenteeism rate and hours lost due to own illness or disability, were estimated statistically. The baseline curves are distinct, with considerably more seasonal variation in the absenteeism rate than in the proportion of hours lost due to own illness or disability.