Pneumococcus infection boosts risk of severe illness in H1N1 patients

Dec 31, 2009 (CIDRAP News) – H1N1 flu patients who were also infected with a common cause of pneumonia were 125 times more likely to develop life-threatening disease, a team of American and Argentinean researchers reported Thursday.

The association between severe disease and coinfection with Streptococcus pneumoniae was so strong, especially among those not in high-risk groups, that the researchers do rapid strep tests on flu patients to identify the bacteria's presence in time to ramp up their care.

The possibility that bacterial coinfections can increase flu's morbidity and mortality has been a concern since the beginning of the H1N1 pandemic, driven by findings from earlier pandemics of bacterial pneumonias among the dead. The Centers for Disease Control and Prevention (CDC) announced in September and also in November that bacterial infections were playing a role in serious and fatal flu cases, particularly among young adults, and urged people to consider pneumococcal vaccination.

Today's analysis by researchers from the Mailman School of Health at Columbia University in New York City and the Instituto Nacional de Enfermedades Infecciosas in Buenos Aires draws on nasopharyngeal swabs and patient records from 199 H1N1 patients who fell ill in late June and early July, when the pandemic's first wave reached Argentina. The study was prompted by recognition that Argentina's mortality rate at the time was 4.5%, anomalously high compared with a concurrent World Health Organization estimate of 0.6%.

After testing for underlying risk factors in the patients and virulent mutations in the H1N1 virus, researchers at the Argentinean national agency turned to the possibility of bacterial or viral coinfection as a possible explanation for the unexpected severe illnesses.

Writing in the online journal Public Library of Science ONE, the researchers say that at least one additional bacterium or virus was found in 76% of the patients, with Haemophilus influenzae; S pneumoniae, or pneumococcus; and Staphylococcus aureus the most common (also found: respiratory syncytial virus, human rhinovirus, human coronaviruses, Klebsiella pneumoniae, Acinetobacter baumanii, Serratia marcescens, and methicillin-resistant S aureus, or MRSA.)

But of all the organisms, pneumococcus had by far the strongest association with severe disease and death. It was found in 56% of the 20 patients who died and 19 who were hospitalized, and also in 25% of 160 milder cases, a higher correlation than existed for any of the other coinfecting pathogens.

In a particularly interesting signal, pneumococcus was found in approximately half of those who fell ill and were less than 6 years old or over 55, groups that the Argentinean government considered high risk. But it was present in two thirds of those in the low-risk age range (6 to 55 years) who fell severely ill, and in one fourth of that age range who were mildly ill. In the low-risk age-group, the odds ratio for severe illness in the presence of S pneumoniae coinfection was 125.5.

Because of that correlation, diagnosis of bacterial coinfection can serve as a signal of flu patients' increased risk of serious illness, and rapid bacterial detection methods such as PCR should be considered, the authors said.

Palacios G, Hornig M, Cisterna D, et al. 2009 Streptococcus pneumoniae coinfection is correlated with the severity of H1N1 pandemic influenza. PLoS ONE 2009 Dec 31;4(12):e8540

See also:

Sep 29 MMWR article "Bacterial coinfections in lung tissue specimens from fatal cases of 2009 pandemic influenza A (H1N1)—United States, May-August 2009"

Nov 25 CIDRAP News article "CDC warns about rise in serious pneumococcal disease"

Newsletter Sign-up

Get CIDRAP news and other free newsletters.

Sign up now»


Unrestricted financial support provided by

Bentson Foundation Gilead 
Grant support for ASP provided by


  Become an underwriter»