Results
Between
November 27, 2021, and January 12, 2022, a total of 886,774 eligible
persons infected with the omicron variant, 204,154 eligible persons
infected with the delta variant, and 1,572,621 eligible test-negative
controls were identified. At all time points investigated and for all
combinations of primary course and booster vaccines, vaccine
effectiveness against symptomatic disease was higher for the delta
variant than for the omicron variant. No effect against the omicron
variant was noted from 20 weeks after two ChAdOx1 nCoV-19 doses, whereas
vaccine effectiveness after two BNT162b2 doses was 65.5% (95%
confidence interval [CI], 63.9 to 67.0) at 2 to 4 weeks, dropping to
8.8% (95% CI, 7.0 to 10.5) at 25 or more weeks. Among ChAdOx1 nCoV-19
primary course recipients, vaccine effectiveness increased to 62.4% (95%
CI, 61.8 to 63.0) at 2 to 4 weeks after a BNT162b2 booster before
decreasing to 39.6% (95% CI, 38.0 to 41.1) at 10 or more weeks. Among
BNT162b2 primary course recipients, vaccine effectiveness increased to
67.2% (95% CI, 66.5 to 67.8) at 2 to 4 weeks after a BNT162b2 booster
before declining to 45.7% (95% CI, 44.7 to 46.7) at 10 or more weeks.
Vaccine effectiveness after a ChAdOx1 nCoV-19 primary course increased
to 70.1% (95% CI, 69.5 to 70.7) at 2 to 4 weeks after an mRNA-1273
booster and decreased to 60.9% (95% CI, 59.7 to 62.1) at 5 to 9 weeks.
After a BNT162b2 primary course, the mRNA-1273 booster increased vaccine
effectiveness to 73.9% (95% CI, 73.1 to 74.6) at 2 to 4 weeks; vaccine
effectiveness fell to 64.4% (95% CI, 62.6 to 66.1) at 5 to 9 weeks.
Conclusions
Primary
immunization with two doses of ChAdOx1 nCoV-19 or BNT162b2 vaccine
provided limited protection against symptomatic disease caused by the
omicron variant. A BNT162b2 or mRNA-1273 booster after either the
ChAdOx1 nCoV-19 or BNT162b2 primary course substantially increased
protection, but that protection waned over time. (Funded by the U.K.
Health Security Agency.)
