Can a patient be co-infected with both influenza A and influenza B viruses?

Co-infection with Influenza A and B Viruses

Yes, a patient can be co-infected with both influenza A and influenza B viruses simultaneously, though this is a relatively rare occurrence that may be associated with more severe disease outcomes.

Evidence for Co-infection

Co-infection with both influenza A and B viruses has been documented in clinical practice and research studies, confirming this is a naturally occurring phenomenon 1, 2, 3.

• Documented cases include: dual infections with A(H1N1)pdm09 and influenza B, A(H3N2) and influenza B, and even triple infections involving A(H3N2), A(H1N1)pdm09, and influenza B 2.
• Frequency: Among 8,011 influenza-positive patients in a large Brazilian surveillance study (2009-2018), 18 cases of dual influenza virus infection were identified, representing approximately 0.2% of influenza cases 2.
• Confirmation methods: Co-infection can be confirmed through viral culture in MDCK cells, immunofluorescence assays, and real-time RT-PCR testing 1.

Clinical Significance and Risk Factors

Co-infection appears to be associated with worse clinical outcomes compared to monoinfection:

• Mortality risk: Dual influenza infection was significantly associated with cardiopathy and death when compared to monoinfected patients 2.
• High-risk populations: Documented cases include immunocompromised patients (such as transplant recipients) and children with severe respiratory illness 1, 3.
• Immunosuppressed patients: Multiple respiratory co-infections (including dual influenza A and B) have been reported in hospitalized immunosuppressed patients receiving immunosuppressive treatment 3.

Diagnostic Considerations

Clinicians should maintain awareness that co-infection is possible, particularly in severe cases:

• Testing approach: Multiplex PCR methods that detect multiple respiratory viruses simultaneously are essential for identifying true co-infection rates 3.
• Common pitfall: Many clinicians order diagnostics only for influenza A or specific subtypes, potentially missing co-infections with influenza B or other respiratory pathogens 3.
• Rapid screening: Both influenza A and B should be tested when influenza is suspected, as antibody against one type confers limited or no protection against the other type 4.

Treatment Implications

Neuraminidase inhibitors are effective against both influenza A and B viruses:

• Oseltamivir and zanamivir are active against both influenza A and B viruses and are FDA-approved for treatment within 48 hours of symptomatic onset 4.
• Treatment duration: Standard 5-day courses are indicated, though longer courses (10 days) and treatment until symptom resolution can be considered in immunocompromised patients 4.
• Alternative agents: Peramivir (intravenous) and baloxavir (polymerase inhibitor) also have activity against both influenza A and B viruses 4.

Key Clinical Points

• Co-infection does not appear to significantly alter clinical presentation, hospitalization rates, or ICU admission compared to monoinfection, but may increase mortality risk 5, 2.
• Both patients in documented co-infection cases fully recovered without significant differences from monoinfected patients when appropriately treated 1.
• The clinical severity of influenza A versus B monoinfections is similar in hospitalized children, with no significant differences in outcomes or management 5.