Investigators Update Flu, RSV Mortality Rates

Respiratory syncytial virus (RSV) poses a greater threat to mortality of infants than influenza, according to new research.

A team, led by Chelsea L. Hansen, MPH, Division of International Epidemiology and Population Studies, Fogarty International Center, National Institutes of Health, evaluated excess mortality from RSV and influence in the US between 1999-2018.

Death Certificates

In the cross-sectional study, the investigators examined data from 50.3 million US death certificates between 1999-2018, accounting for 1043 weeks of data.

Doing this, they were able to create age-specific linear regression models to help assess weekly mortality fluctuations above a seasonal baseline associated with the 2 viruses. The mean age of the deaths was 72.7 years.

The investigators sought main outcomes of excess mortality associated with RSV and influenza estimated from the difference between observed and expected underlying respiratory mortality each season.

In addition, 1% of the deaths were individuals younger than 1 year, while 73.4% of the deaths were individuals aged 65 years or older. There was a mean of 6549 (95% CI, 6140-6958) underlying respiratory deaths associates with RSV annually. This included 96 (95% CI, 92-99) deaths among pediatric patients younger than 1 years.

In the influenza analysis, the investigators identified 10,171 (95% CI, 9652-10,691) underlying respiratory deaths annually, with 23 deaths (95% CI, 19-27) occurring among children younger than 1 year.

Mortality Rates

The highest mean mortality rate per 100,000 people for both viruses among adults aged 65 years or older was 14.7 (95% CI, 13.8-15.5) for RSV and 20.5 (95% CI, 19.4-21.5) for influenza.

There was also a lower proportion of influenza deaths in individuals 65 years or older compared to earlier estimates (75.1%; 95% CI, 67.4-82.8%), but influenza mortality was highest among those aged 65 years or older in seasons when A/H3N2 predominated (n = 18, 739 deaths in 2017-2018; 95% CI, 16,616-21 336). This was also true for those between 5-49 years when A/H1N1pdm2009 predominated (n = 1683 deaths in 2013-2014; 95% CI, 1583-1787).

The results were also sensitive to the choice of mortality outcome and method, with the broadest outcome linked to annual means of 23,352 excess deaths (95% CI, 21,814-24,891) for RSV and 27,171 excess deaths (95% CI, 25,142-29,199) for influenza.

“This study suggests that RSV poses a greater risk than influenza to infants, while both are associated with substantial mortality among elderly individuals,” the authors wrote. “Influenza has large interannual variability, affecting different age groups depending on the circulating virus.”

The investigators said 1 piece of data worth monitoring is the shift in mortality for middle-aged adults during the emergence of influenzas A/H1N1pdm2009 pandemic virus.

One of the reasons for a study like this is that it has been a long time since mortality estimates have been updated for influenza.

“Respiratory syncytial virus mortality estimates have not been updated since 2009, and no study has assessed changes in influenza mortality after the 2009 pandemic,” the authors wrote. “Updated burden estimates are needed to characterize long-term changes in the epidemiology of these viruses.”

The study, “Mortality Associated With Influenza and Respiratory Syncytial Virus in the US, 1999-2018,” was published online in JAMA Network Open.