POW: Increased Risk of Influenza-Related Respiratory Mortality with Obesity

A recently published study has found that the risk of respiratory mortality is up to 35% higher in elderly Chinese adults who are overweight or obese during seasonal influenza outbreaks, compared with adults with a normal BMI.

In 1998, a cohort of 66,820 adults aged 65 years and older was identified and assessed in Hong Kong. By 2012, 29% of these adults had died, and 6% of the group had died of respiratory causes.

Researchers Zhou and colleagues(external link) have recently analysed the data from this cohort to determine whether overweight and obesity affected the risk of death during times of seasonal influenza activity between 1998 and 2012. Influenza seasons were determined from sentinel clinics and hospitals in Hong Kong, defined as periods of at least two consecutive weeks with more than the established threshold of positive laboratory or clinical reports. For almost half the years examined, there were two influenza seasons, and one season in the remaining years.

At the start of the study, 3% of men and 7% of women were obese, with a body mass index (BMI) of 30 kg/m2 or more, and a further 34% were overweight, with a BMI of 25 to 29.9 kg/m2.

Overall, respiratory mortality followed the peaks and troughs of seasonal influenza activity, with a 13% increase in deaths seen after a lag of 4 weeks from peak influenza activity. There was also a statistically significant 4% increase in all-cause mortality, and a 13% increase in deaths specifically from pneumonia and influenza. There was no association between deaths from external causes and seasonal influenza activity, suggesting that the link with respiratory mortality may be real and not coincidental.

The association between respiratory deaths and influenza activity was 11% higher over the study period for adults who were overweight or obese compared with those of normal weight, with a time lag of 3 weeks. However, there was no statistically significant association between all-cause mortality and BMI. The hazard ratios for respiratory deaths by BMI group are shown in the chart below, with the red horizontal line representing the hazard ratio of 1.0 for the normal BMI group, who were used as the reference category. The hazard ratios for the obese group with 1 to 3 weeks’ lag were statistically significantly higher than the reference group, but none of the other values reached this level of significance.

Hazard ratio for respiratory mortality with 0 to 6 weeks lag after seasonal influenza activity for different BMI groups

When only those adults with better baseline health were assessed, the increased risk of respiratory mortality with influenza activity was 35% higher in the overweight and obese group than in those with normal body weight, with the greatest impact after a 5-week lag. In contrast, in adults with worst baseline health, those who were obese or overweight showed a 21% increase in risk of respiratory mortality, with just a 1-week lag.

A similar effect was also seen with age. In the younger cohort members, who were aged 65 to 69 years at baseline, those who were obese had a 2.5-fold increase in respiratory mortality risk associated with influenza activity, compared with those of normal BMI.

The association between obesity and likelihood of dying of respiratory disease during an outbreak of seasonal influenza was independent of other factors such as comorbidities, weather and other respiratory infections, and supported previous research that had identified a link between obesity and mortality during the H1N1 pandemic of 2009. The authors suggest that older adults who are overweight or obese should receive influenza vaccinations each year to reduce their risk. Looking forward, we can expect that, as our ageing populations show an increasing prevalence of obesity, the excess winter mortality related to seasonal influenza may also increase.

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