Flu & Pneumonia: Understanding the Connection & Risks

The order in which a person contracts influenza A and the bacterium Streptococcus pneumoniae significantly alters the body’s immune response, according to research published today by the University CEU San Pablo in collaboration with the Instituto de Salud Carlos III (ISCIII).

The study, detailed in the journal Frontiers in Immunology, reveals that the immune system reacts differently to simultaneous infection – known as coinfection – versus sequential infection, or superinfection, where influenza precedes a Streptococcus pneumoniae infection. Researchers analyzed the behavior of macrophages, critical cells within the immune system, when exposed to both pathogens.

When a coinfection occurs, macrophages activate an inflammatory program closely resembling the response triggered by Streptococcus pneumoniae alone. In this scenario, the bacterial signal dominates the immune response, leading to a strong activation of inflammatory pathways dependent on NF-κB. This suggests the body prioritizes fighting the bacterial threat when both are present at the same time.

However, the research demonstrates a stark contrast when infection occurs sequentially. If the influenza virus infects first, macrophages are “programmed” by the virus, fundamentally changing their reaction to a subsequent bacterial infection. In this superinfection model, the immune response is dominated by the virus, influencing how the body reacts to Streptococcus pneumoniae.

Javier Sanz Herrero, Estanislao Nistal Villán, and Jordi Cano Ochando led the research team. Their operate focused on understanding how macrophages respond to influenza A and Streptococcus pneumoniae, a leading cause of bacterial pneumonia following influenza infection. The findings indicate that the order of pathogen infection determines which microorganism ‘dominates’ the immune response.

This “viral primacy” alters macrophage behavior and can amplify inflammatory responses associated with lung damage and respiratory complications, according to the study. The research highlights the complex interplay between viral and bacterial infections and the importance of understanding these interactions to improve treatment strategies.

The ISCIII confirmed the research findings, noting the implications for understanding and potentially mitigating the severity of respiratory illnesses caused by these common co-occurring pathogens.