Dec 8, 2011 (CIDRAP News) – The interplay between influenza and pneumococcal disease has been difficult to sort out, but the 2009 H1N1 pandemic offered researchers a unique chance to explore the impact of flu on pneumonia hospitalizations, which they found was significant.
Experts know that pneumococcal disease, with its broad winter peak, spikes during flu season. However, the co-circulation of other respiratory viruses, environmental conditions, and increased personal contacts during the holiday season make it difficult to untangle the relationship between flu and pneumonia.
Researchers from two federal health agencies and George Washington University seized on flu circulation in non-winter months during the 2009 H1N1 pandemic to explore the impact of influenza on pneumonia hospitalizations.
They found a tripling of pneumococcal pneumonia in those 5 to 19 years old during the 2009 H1N1 peak, according to their report yesterday in an early online edition of the Journal of Infectious Diseases (JID).
Using weekly hospitalization data from the Agency for Healthcare Research and Quality, the authors established baseline periods for pneumococcal disease from 2003 through 2009. They also tracked pneumococcal septicemia, influenza, and, as a control outcome, Escherichia coli septicemia.
They recorded the dates of hospital admission and the age of the patients. Then they calculated the rates of excess pneumococcal pneumonia hospitalizations, comparing the pre-pandemic period with the second wave of the 2009 pandemic, which took place between the end of August and the middle of December. The comparisons allowed them to evaluate if increases in pneumococcal disease were linked to increased flu incidence.
The researchers also looked for patterns in weekly data for 30 states.
The group found a significant increase in pneumonia hospitalizations during the second wave of the pandemic that corresponded to peak flu activity. School children, those ages 5 to 19, had the largest increase in pneumonia hospitalizations, a threefold increase from baseline. This increase was unusual for a group that has relatively low baseline levels of pneumococcal disease, the report says.
The largest absolute increase was seen in adults aged 40 to 64. The authors estimated 923 excess US pneumonia cases in the fall of 2009 in this age-group when calculated from a regression model and 1,255 excess cases using a baseline method. For comparison, excess cases in the 5-to-19 age-group were about a third of that.
The team found similar increases in pneumococcal septicemia in those aged 40 to 64.
No increase in pneumonia cases was found in those 65 and older, a group thought to have some protection because of previous exposure to similar flu strains.
The short spikes the researchers saw in E coli septicemia throughout the year did not become more numerous during the pandemic period, though there were sustained increases in pneumococcal septicemia during those months. Those findings suggest that changes in testing were not responsible for the increase in pneumococcal disease during the pandemic months, they noted.
When they analyzed patterns by state, they found that later increases in pneumococcal pneumonia hospitalizations occurred in states that had later influenza peaks, which they said further supports a link between the two diseases.
For school children, the magnitude of the flu-related pneumonia increase was two to three times higher than in any of the six earlier flu seasons. They attributed the increase to the age-group's high pandemic flu attack rate.
For young and middle-aged adults, the increases in 2009 were similar to those in the 2007-08 flu season and the severe H3N2 flu season of 2003-04.
The researchers concluded that although the number of pneumococcal infections they found didn't greatly exceed what would be expected during a typical winter season, a more severe pandemic with a rapid rise in bacterial infections could have major public health consequences, if the nation lacked adequate antibiotic stockpiles.
They added that the new formulation of the pneumococcal conjugate vaccine could tamp down the disease and transmission of invasive serotypes, which could reduce the impact of future pandemics.
In an editorial commentary in the same issue of JID, two researchers from Vanderbilt University School of Medicine's Department of Preventive Medicine, Dr Carlos Grijalva and Dr Marie Griffin, called the study an "elegant assessment" that helps clarify the contribution of flu to pneumonia hospitalizations during the 2009 H1N1 pandemic.
They acknowledged the challenge of sorting out the contributions of winter factors and scientific interactions that mediate the relationship between the two diseases. "Disentangling the direct contribution of influenza is complicated," they wrote, adding that looking at the relationship during the 2009 H1N1 pandemic setting helped minimize some of the factors.
"Overall, these data strongly support the previously suspected synergism between influenza virus and the pneumococcus," Grijalva and Griffin wrote.
The synergism the researchers found suggests other questions, such as whether or not the relationship holds for all flu virus types and pneumococcal serotypes, and Grijalva and Griffin speculated that, given the 2000 launch of the seven-valent pneumococcal vaccine, the increase observed during the pandemic might have involved serotypes not covered by the vaccine.
Weinberger DM, Simonsen L, Jordan R, et al. Impact of the 2009 influenza pandemic on pneumococcal pneumonia hospitalizations in the United States. J Infect Dis 2011 Dec 7 [Abstract]
Grijalva CG, Griffin MR. Unveiling the burden of influenza-associated pneumococcal pneumonia. J Infect Dis 2011 Dec 7 (Editorial) [Extract]
