Pre-Existing Immunity to COVID-19: Overview and Implications – Part 2

This review article comes in three parts: Part 1 introduced the subject and investigated how other coronaviruses provided pre-existing immunity to SARS-CoV-2. It considers pre-existing community from other microorganisms. Part 2 examined the evidence existing immunity from non-COVID vaccinations and will conclude the article as a whole.

Studies of exposure to some of the more common viruses found that they are generally associated with worsening of COVID-19, while malaria exposure correlated with lower disease incidence and severity. The effect of the host microbiome on COVID-19 seems particularly intriguing; multiple studies note microbiome diversity, lower levels of beneficial bacteria, especially short-chain fatty acid (SCFA)-producing bacteria, and increased inflammation were all associated with COVID-19 severity, although supplementation with beneficial oral bacteria appeared protective. Nevertheless, some studies showed specific strains of beneficial and pathogenic bacteria in the microbiome may correlate with less severe disease. These results tend to correlate with why other viruses are associated with more severe COVID-19, while a parasitic (malaria) and certain traditionally pathogenic bacteria in the microbiome are associated with protection. More research is needed to confirm findings and to determine the mechanism(s) producing these effects, but the studies suggest that, ideally, the influence of the human host’s other viral exposures and microbiome composition on a particular pathogen must be considered, just as we would consider other disease risk factors to assess vulnerability.

Keywords: COVID-19, SARS-CoV-2, pre-existing immunity, viruses, commensal bacteria, microbiome