Infection by the novel SARS-CoV-2 virus (also known as COVID-19) has tremendous social impact. Most of Western societies are at major or part lock-down whatever brings unpredictable financial and societal consequences. The urgent need for the reversal of this situation can only be met through the generation of an immune defence shield to protect the society from COVID-19. Many efforts for the development of a vaccine are under way without any specific outcome so for.

The stimulation of trained immune responses seems the only alternative to bridge the gap from the turn-on of the society until the entrance of a specific vaccine in the market. Trained immunity stands for the non-specific raise of defense shield for severe infections coming once tissue macrophages recognize a universal pathogen. The concept was successfully tested in healthy volunteers that were vaccinated with placebo or BCG (Bacillus Calmette-Guérin) vaccine. These volunteers were injected 14 days latter a tri-valent influenza A vaccine. Volunteers previously vaccinated by BCG developed significantly greater titers against hemagglutinin A of the influenza A virus whereas their circulating monocytes were more potent for the production of interferon-gamma.

It is proposed that this BCG vaccination triggering trained immune responses may play a role of protection against the COVID-19 pandemic. A solid background on this rationale came recently from the interim analysis of the ACTIVATE trial. ACTIVATE (A randomized Clinical trial for enhanced Trained Immune responses through Bacillus Calmette-Guérin VAccination to prevenT infections of the Elderly) was a prospective randomized open-label controlled trial conducted among patients hospitalized at the 4th Department of Internal Medicine of ATTIKON University General Hospital in Greece. The protocol was approved by the National Ethics Committee of Greece and the National Organization for Medicine of Greece (EudraCT number, 2017-000596-87; ClinicalTrials.gov NCT03296423). The trial is conducted and funded by the Hellenic Institute for the Study of Sepsis. In this trial hospitalized elderly patients were vaccinated on the day of hospital discharge with single doses of placebo or BCG. Every patient is under follow-up for 12 months. The last visit of the last patient is scheduled for August 2020. An interim analysis took place on April 29th 2020 by an independent committee of experts. The full interim analysis focused on the study primary endpoint that was the comparative time to a new infection between the two groups of treatment. Infections counting against this primary endpoint were respiratory or viral infections necessitating medical treatment, community-acquired pneumonias, hospital-acquired pneumonias, intraabdominal infections, urinary tract infections, soft tissue infections and bloodstream infections. Analysis revealed 53% decrease of the incidence of new infections in the BCG group compared to the placebo group. This decrease reached 80% for all respiratory tract infections. Multivariate analysis showed that most of benefit was for patients with coronary heart disease (CHD) and chronic obstructive pulmonary disease (COPD). This interim analysis clearly enhances the concept that BCG can be protective against COVID-19.