Effectiveness of ChAdOx1-S COVID-19 Booster Vaccination against the Omicron and Delta variants in England

BackgroundDespite the potential widespread global use of the ChAdOx1-S booster, to date there are no published data on the real-world effectiveness. VE studies have found one and two doses of the ChAdOx1-S vaccine to be highly effective, and clinical trial data have demonstrated enhanced immunity following a ChAdOx1-S booster. In England, some individuals received a ChAdOx1-S booster where vaccination with mRNA vaccines was clinically contraindicated. MethodsThe demographic characteristics of those who received a ChAdOx1-S booster were compared to those who received a BNT162b2 booster. A test-negative case control design was used to estimate vaccine effectiveness of the ChAdOx1-S booster against symptomatic disease and hospitalisation in England. FindingsThose who received a ChAdOx1-S booster were more likely to be female (adjusted odds ratio (OR) 1.67 (1.64-1.71)), in a clinical risk group (adjusted OR 1.58 (1.54-1.63)), in the CEV group (adjusted OR 1.84 (1.79-1.89)) or severely immunosuppressed (adjusted OR 2.05 (1.96-2.13)). Protection against symptomatic disease in those aged 65 years and older peaked at 66.1% (16.6 to 86.3%) and 68.5% (65.7 to 71.2%) amongst those who received the ChAdOx1-S and BNT162b2 booster vaccines, respectively. Protection waned to 44.5% (22.4 to 60.2%) and 54.1% (50.5 to 57.5%) after 5-9 weeks. Protection against hospitalisation following Omicron infection peaked at 82.3% (64.2 to 91.3%) after receiving a ChAdOx1-S booster, as compared to 90.9% (88.7 to 92.7%) for those who received a BNT162b2 booster. InterpretationDifferences in the population boosted with ChAdOx1-S in England renders direct comparison of vaccine effectiveness by manufacturer challenging. Nonetheless, this study supports the use of the ChAdOx1-S booster for protection against severe disease with COVID-19 in settings that have not yet offered booster doses and suggests that those who received ChAdOx1-S as a booster in England do not require re-vaccination ahead of others. FundingUKHSA

COVID-19 Vaccine Effectiveness against the Omicron BA.2 variant in England

The BA.1 sub-lineage of the Omicron (B.1.1.529) variant, first detected in the UK in mid-November 2021, rapidly became the dominant strain partly due to reduced vaccine effectiveness. An increase in a second Omicron sub-lineage BA.2 was observed in early January 2022. In this study we use a test-negative case control study design to estimate vaccine effectiveness against symptomatic disease with BA.1 and BA.2 after one or two doses of BNT162b2, ChAdOx1-S or mRNA-1273, and after booster doses of BNT162b2 or mRNA-1273 during a period of co-circulation. Overall, there was no evidence that vaccine effectiveness against symptomatic disease is reduced following infection with the BA.2 sub-lineage as compared to BA.1. Furthermore, similar rates of waning were observed after the second and booster dose for each sub-lineage. These data provide reassuring evidence of the effectiveness of the vaccines currently in use against symptomatic disease caused by BA.2.