BCG vaccination scar reactivation presents as redness, swelling, or ulceration at the BCG injection site months to years after inoculation. Although erythema and induration of the BCG scar are not included in the diagnostic criteria of Kawasaki disease, likely due to variable vaccine requirements in different countries, these findings are largely recognized as specific for Kawasaki disease and present in approximately half of affected patients who received the BCG vaccine.2
Heat Shock Proteins—Heat shock proteins (HSPs) are produced by cells in response to stressors. The proposed mechanism of BCG vaccination scar reactivation is a cross-reaction between human homologue HSP 63 and Mycobacterium HSP 65, leading to hyperactivity of the immune system against BCG.7 There also are reports of reactivation of a BCG vaccination scar from measles infection and influenza vaccination.2,8,9 Most prior reports of BCG vaccination scar reactivation have been in pediatric patients; our patient is an adult who received the BCG vaccine more than 40 years ago.
Mechanism of Reactivation—The mechanism of BCG vaccination scar reactivation in our patient, who received the Moderna COVID-19 Vaccine, is unclear. Possible mechanisms include (1) release of HSP mediated by the COVID-19 vaccine, leading to an immune response at the BCG vaccine scar, or (2) another immune-mediated cross-reaction between BCG and the Moderna COVID-19 Vaccine mRNA nanoparticle or encoded spike protein antigen. It has been hypothesized that the BCG vaccine might offer some protection against COVID-19; this remains uncertain and is under further investigation.10 A recent retrospective cohort study showed that a BCG vaccination booster may decrease COVID-19 infection rates in higher-risk populations.11
We present a case of BCG vaccine scar reactivation occurring after a dose of the Moderna COVID-19 Vaccine, a likely underreported, self-limiting, cutaneous adverse effect of this mRNA vaccine.