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Summary: Background: After primary vaccination schemes with rAd26-rAd5 (Sputnik V), ChAdOx1 nCoV-19, BBIBP-CorV or heterologous combinations, the effectiveness of homologous or heterologous boosters (Sputnik V, ChAdOx, Pfizer-BioNTech, Moderna) against SARS-CoV-2 infections, hospitalisations and deaths has been scarcely studied. Methods: Test-negative, case–control study, conducted in Argentina during omicron BA.1 predominance, in adults ≥50 years old tested for SARS-CoV-2 who had received two or three doses of COVID-19 vaccines. Outcomes were COVID-associated infections, hospitalisations and deaths after administering mRNA and vectored boosters, < or ≥60 days from the last dose. Findings: Of 422,124 individuals tested for SARS-CoV-2, 221,993 (52.5%) tested positive; 190,884 (45.2%) and 231,260 (54.8%) had received 2-dose and 3-dose vaccination schemes, respectively. The 3-dose scheme reduced infections, hospitalisations and death (OR 0.81 [0.80–0.83]; 0.28 [0.25–0.32] and 0.25 [0.22–0.28] respectively), but protection dropped after 60 days to 1.04 [1.01–1.06]; 0.52 [0.44–0.61] and 0.38 [0.33–0.45]). Compared with 2-dose-schemes, homologous boosters after primary schemes with vectored-vaccines provided lower protection against infections < and ≥60 days (0.94 [0.92–0.97] and 1.05 [1.01–1.09], respectively) but protected against hospitalisations (0.30 [0.26–0.35]) and deaths (0.29 [0.25–0.33]), decreasing after 60 days (0.59 [0.47–0.74] and 0.51 [0.41–0.64], respectively). Heterologous boosters protected against infections (0.70 [0.68–0.71]) but decreased after 60 days (1.01 [0.98–1.04]) and against hospitalisations and deaths (0.26 [0.22–0.31] and 0.22 [0.18–0.25], respectively), which also decreased after 60 days (0.43 [0.35–0.53] and 0.33 [0.26–0.41], respectively). Heterologous boosters protected against infections when applied |
