Bacterial Pneumonia-Complicating Influenza: Risks, Morbidity, and Mortality

An Introduction to Viral and Bacterial Pneumonia Coinfection

Both community- and hospital-acquired pneumonia (CAP, HAP, respectively) are responsible for many hospital admissions, morbidity, mortality, and a large outlay in medical resources.¹ Each year in the United States, 5% to 20% of the population will get influenza.² When CAP and HAP occur with influenza, the consequences of bacterial and viral coinfections can portend poor outcomes, longer hospitalizations, higher readmission rates, and greater mortality.¹ Viral infection can predispose patients to secondary bacterial infections and likewise, bacterial infections can induce viral shedding.¹

In patients with influenza, commonly occurring bacteria include Staphylococcus aureus, Streptococcus pneumoniae, Haemophilus influenzae, and Streptococcus pyogenes.¹ Up to 40% of pneumonia cases in hospitalized patients can be traced to viral infections.¹ In both children and adults with CAP, the most common viral causes were influenza, respiratory syncytial virus, human metapneumovirus, parainfluenza, and coronaviruses.¹ In children, the adenovirus was more prevalent vs rhinovirus, which was more prevalent in adults.¹

Bacterial pneumonia is a complication in approximately one third of patients with severe influenza, resulting in intensive care unit (ICU) admission.² Diagnosing coinfection can be challenging because of timing of the sample collection and false-negative results when the viruses replicate in the lower respiratory tract.² Containing the infections is also difficult because so little is understood about how S aureus and S pneumoniae affect viral transmission.²

Risk Factors for Coinfection

Because clinicians do not yet have a complete understanding of the pathogenesis of coinfection with bacterial and viral pathogens, Martin-Loeches and colleagues conducted a literature review to characterize patients who have the highest risk for coinfection³:

• Immunosuppression and aging are the leading factors for patient outcomes in influenza coinfection.
• The most frequently occurring bacteria in critically ill patients with influenza coinfection is S pneumoniae.
• Although P aeruginosa occurred in only 8.3% of patients with severe CAP in one of the studies, the pathogen had a case fatality rate of 100%.
• In the last few seasons, coinfection in patients in the ICU with influenza has emerged as an independent risk factor for death.

When Should Patients Not Receive Broad-Spectrum Antibiotics?

For people with suspected pneumonia seen in the emergency department, clinicians frequently prescribe anti-methicillin-resistant S aureus (MRSA) antibiotics.⁴ Self and colleagues, however, found that in a multicenter study of 2259 adult patients who were admitted to the hospital for CAP, only 1.6% were infected with S aureus, and of those patients, only 0.7% had MRSA.⁴ Yet one third of these patients admitted received anti-MRSA antibiotics because the symptoms of MRSA-related CAP overlapped with those of pneumococcal CAP.

Despite the similarity of clinical features in patients with both types of CAP — hemoptysis, multilobar infiltrates, influenza, and antibiotics prior to hospitalization — patients with MRSA CAP were more likely to be admitted to the ICU than were patients with pneumococcal CAP (86.7% vs 34.8%, respectively) and had higher hospital mortality rates (13.3% vs 4.4%, respectively).⁴