Can a Patient with Recent Influenza A Contract Influenza B?
Yes, a patient who recently had influenza A can absolutely contract influenza B, as antibody against one influenza virus type confers limited or no protection against another type. 1, 2, 3
Immunologic Basis for Susceptibility
The fundamental reason patients remain vulnerable to both types lies in the strain-specific nature of influenza immunity:
Antibody responses to influenza A are primarily strain-specific and provide limited or no cross-protection against influenza B. 1, 2 The surface antigens (hemagglutinin and neuraminidase) that trigger protective antibody responses differ substantially between influenza A and B viruses.
Influenza A and B are distinct virus types that are not categorized into the same subtypes. 1 Since 1977, influenza A (H1N1), influenza A (H3N2), and influenza B viruses have all circulated globally, meaning patients can be exposed to multiple types within a single season. 1
Even T-cell responses, which target more conserved viral proteins and can provide some heterosubtypic immunity across different influenza A subtypes, do not provide meaningful cross-protection between influenza A and B. 2
Clinical Implications
Testing and treatment protocols explicitly recognize this lack of cross-protection:
The CDC recommends testing for both influenza A and B when influenza is suspected, precisely because infection with one type does not protect against the other. 3
Neuraminidase inhibitors (oseltamivir, zanamivir) are effective against both influenza A and B viruses, and treatment decisions should be made regardless of which type the patient previously had. 3
Important Clinical Considerations
Do not assume prior influenza A infection provides any meaningful protection against influenza B during the same season or subsequent seasons. 1, 2 Patients who have recovered from confirmed influenza A remain fully susceptible to influenza B infection and should:
- Continue appropriate infection control measures when exposed to influenza B. 1
- Receive influenza vaccination (which contains both A and B strains) regardless of recent influenza A infection, as the vaccine provides protection against types not yet encountered. 1, 2
- Be considered for antiviral prophylaxis if exposed to influenza B in high-risk settings, particularly if they are in high-risk groups. 1
The clinical severity of influenza A versus B infections is similar, with some studies showing influenza B may actually have higher mortality rates in hospitalized children. 4, 5 Therefore, having survived influenza A does not mean a subsequent influenza B infection will be milder.