Pre-Existing Immunity to COVID-19: Overview and Implications — Part 3

This review article is comprised of three parts; Part 1 introduced the subject and investigated how other coronaviruses provided pre-existing immunity to SARS-CoV-2, Part 2 looked at pre-existing immunity from other micro-organisms and this part (Part 3) investigates pre-existing immunity from non-COVID vaccinations and discusses the article as a whole. Observational studies found that the prior administration of live attenuated vaccines (BCG, influenza, oral polio, herpes zoster, MMR and DTP) were associated with lower incidence and/or severity of COVID-19. Clinical trials of vaccination during the COVID-19 pandemic, however, showed mixed results, with BCG trials proving to be indeterminate although a trial of MMR vaccination showed it was associated with reduced risk of symptomatic infection. More research is required to explain anomalous results and to determine the precise mechanism(s) of pre-existing immunity. Overall, the presence of pre-existing immunity from coronaviruses, malaria, commensal bacteria and vaccines has implications for the development of vaccines against COVID-19; if a significant proportion of the population already has some immunity, the need for those vaccines is reduced. Furthermore, it is unknown whether pre-existing immunity will enhance the anti-COVID-19 vaccines or will work against them to create adverse reactions.

Keywords: COVID-19, SARS-CoV-2, pre-existing immunity, cross-reactivity, vaccines