Effect of booster vaccination against Delta and Omicron variants in Iceland

By the end of July 2021, the majority of the Icelandic population had received vaccination against COVID-19. In mid-July a wave of SARS-CoV-2 infections, dominated by the delta variant, spread through the population, followed by an omicron wave in December. A booster vaccination campaign was initiated to curb the spread of the virus. We estimated the risk of infection for different vaccine combinations using vaccination data from 276,028 persons and 963,557 qPCR tests for 277,687 persons. We measured anti-Spike-RBD antibody levels and ACE2-Spike binding inhibitory activity in 371 persons who received one of four recommended vaccination schedules with or without an mRNA vaccine booster. Overall, we found different antibody levels and inhibitory activity between recommended vaccination schedules, which was reflected in the observed risk of SARS-CoV-2 infections. We observed an increased protection following mRNA boosters, against both omicron and delta variant infections, although BNT162b2 boosters provided greater protection against omicron than mRNA-1273 boosters.