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ImportancePeople over 60 developed less protection after two doses of inactivated COVID-19 vaccine than younger people. Heterologous vaccination might provide greater immunity and protection against variants of concern. ObjectiveTo assess the safety and immunogenicity of a heterologous immunization with an adenovirus type 5-vectored vaccine (Convidecia) among elderly who were primed with an inactivated vaccine (CoronaVac) previously. DesignAn observer-blind, randomized (11) trial, conducted from August 26 to November 13, 2021. SettingA single center in Jiangsu Province, China. Participants299 participants aged 60 years and older, of them 199 primed with two doses of CoronaVac in the past 3-6 months and 100 primed with one dose of CoronaVac in the past 1-2 months. InterventionConvidecia or CoronaVac as boosting dose Main Outcomes and MeasuresGeometric mean titers (GMTs) of neutralizing antibodies against wild-type SARS-CoV-2, and Delta and Omicron variants 14 days post boosting, and adverse reactions within 28 days. ResultsIn the three-dose regimen cohort (n=199; mean (SD) age, 66.7 (4.2) years; 74 (37.2%) female), 99 and 100 received a third dose of Convidecia (group A) and CoronaVac (group B), respectively. In the two-dose regimen cohort (n=100; mean (SD) age, 70.5 (6.0) years; 49 (49%) female), 50 and 50 received a second dose of Convidecia (group C) and CoronaVac (group D), respectively. GMTs of neutralizing antibodies against wild-type SARS-CoV-2 at day 14 were 286.4 (95% CI 244.6, 335.2) in group A and 48.2 (95% CI 39.5, 58.7) in group B, with GMT ratio of 6.2 (95% CI 4.7, 8.1), and 70.9 (95% CI 49.5, 101.7) in group C and 9.3 (95% CI 6.2, 13.9) in group D, with GMT ratio of 7.6 (95% CI 4.1, 14.1). There was a 6.3-fold (GMTs, 45.9 vs 7.3) and 7.5-fold (32.9 vs 4.4) increase in neutralizing antibodies against Delta and Omicron variants in group A, respectively, compared with group B. However, there was no significant difference between group C and group D. Both heterologous and homologous booster immunizations were safe and well tolerated. Conclusions and RelevanceHeterologous prime-boost regimens with CoronaVac and Convidecia induced strong neutralizing antibodies in elderly, which was superior to that induced by the homologous boost, without increasing safety concerns. Trial RegistrationClinicalTrials.gov NCT04952727 Key Points QuestionDoes a heterologous immunization with recombinant adenovirus type 5-vectored vaccine (Convidecia) produced a non-inferior or superior response of neutralizing antibodies among elderly primed with two doses of inactivated COVID-19 vaccine (CoronaVac), compared to the homologous boosting FindingsIn this randomized clinical trial, a heterologous third dose of Convidecia resulted in a 6.2-fold (geometric mean titers 286.4 vs 48.2), 6.3-fold (45.9 vs 7.3) and 7.5-fold (32.9 vs 4.4) increase in neutralizing antibodies against wild-type strain, Delta and Omicron variants 14 days post boosting, respectively, compared to the homologous boost with CoronaVac MeaningHeterologous prime-boost regimens with CoronaVac and Convidecia induced strong neutralizing antibodies in elderly, which was superior to that induced by the homologous boosting.