BCG vaccine: Worrying proposal for COVID-19

Area of Science:

• Immunology
• Infectious Diseases
• Vaccinology

Background:

• The Bacille Calmette-Guérin (BCG) vaccine is primarily known for its efficacy against severe tuberculosis.
• Emerging evidence suggests BCG may confer non-specific protection against other infectious diseases through trained immunity.
• The duration and extent of this non-specific protection, particularly against novel pathogens like SARS-CoV-2, are not well-established.

Purpose of the Study:

• To critically evaluate the evidence for BCG's non-specific protective effects against infections beyond tuberculosis.
• To assess the potential role of BCG in mitigating COVID-19 severity or mortality.
• To caution against premature recommendations for BCG revaccination due to potential risks and confounding factors.

Main Methods:

• Review of experimental models, cohort studies, and epidemiological data on BCG's non-specific effects.
• Analysis of factors influencing tuberculosis and COVID-19 epidemiological data across different countries.
• Consideration of the limitations in current research, including uncontrolled variables and small study populations.

Main Results:

• The duration of BCG-induced protection against non-tuberculosis infections is uncertain, potentially ranging widely.
• Evidence supporting BCG's protective role against COVID-19 is inconclusive, with many confounding socio-economic and healthcare system factors.
• Revaccination with BCG carries risks, especially for individuals with immunodeficiency.

Conclusions:

• The current evidence for BCG's significant protective role against COVID-19 is insufficient.
• The hypothesis of BCG-induced trained immunity requires more robust, controlled studies before clinical recommendations can be made.
• Caution is advised regarding BCG revaccination due to potential risks and the complex interplay of factors influencing infectious disease outcomes.