In both models, we estimated the protection from vaccine-acquired and infection-acquired
immunity as being markedly reduced against Omicron compared with Delta. Estimates of vaccine
effectiveness in recent recipients (at 14 days post-dose) were negative for full primary courses of
ChAdOx1 against Omicron and only 16% against Delta. For two doses of mRNA vaccines, vaccine
effectiveness was significantly lower for Omicron versus Delta; BNT162b2 (6.84% versus 56.53%) and
mRNA-1273 (8.83% versus 60.07%) (Fig.4C). These responses increased significantly following a third
booster dose of BNT162b2 or mRNA-1273 to 91.87% and 89.28% against Delta and 67.57% and
71.15% against Omicron. These estimates are similar to those reported against symptomatic
infection recently in England where vaccine effectiveness was estimated as 71.4% and 75.5% for
ChAdOx1 and BNT162b2 primary course recipients boosted with BNT162b2, respectively 18 .