Real-world data on heterologous boosting with messenger RNA (mRNA)-1273 (Moderna) after inactivated COVID-19 vaccination are limited. We report mRNA-1273 boosting in heavily SARS-CoV-2-exposed Indonesian healthcare workers who received a two-dose CoronaVac 6 months prior. Between August and November 2021, we measured SARS-CoV-2 spike-specific IgG binding antibody (Bab) titers in all 304 participants, and neutralizing antibody titers in a random subset of 71 participants, on stored paired serum samples taken before and 28 days after a full-dose (100-mg) mRNA-1273 booster. At the time of the mRNA-1273 boost, 107 participants (35.2%) were not previously infected (naive vaccinated), 42 (13.8%) were infected before CoronaVac (infected vaccinated), and 155 (51.0%) were infected after CoronaVac (mostly during the Delta wave; vaccinated infected). At time of the mRNA-1273 boost, neutralizing antibodies could still be detected in 83% of participants (59 of 71) overall, 60% of naive-vaccinated participants (15 of 25), 95.7% of vaccinated-infected participants (22 of 23), and 95.7% of infected vaccinated participants (22 of 23). After the mRNA-1273 boost, 100% of participants (71 of 71) had neutralizing antibody activity, with increases in median Bab and neutralizing antibody serum titers of 9.3- and 27.0-fold overall, 89.1- and 2,803.4-fold in naive-vaccinated participants, 15.9- and 19.9-fold in infected-vaccinated participants, and 2.2- and 18.4-fold in vaccinated-infected participants. In the multivariable analysis, Bab titers after the mRNA-1273 boost were greatest in individuals who had a previous virus breakthrough post-CoronaVac, and when a longer time period (. 4 months) had elapsed since the most recent prior “spike antigen exposure” (either second CoronaVac or virus breakthrough). Overall, adverse reactions were mild and short-lived. In conclusion, a full-dose mRNA-1273 booster after CoronaVac was well tolerated and immunogenic after 28 days, including in those with very low antibody levels.