“Objective: To estimate the effectiveness of mRNA vaccines against moderate and severe covid-19 in adults by time since second, third, or fourth doses, and by age and immunocompromised status…
Results: 45,903 people admitted to hospital with covid-19 (cases) were compared with 213,103 people with covid-like illness who tested negative for SARS-CoV-2 (controls), and 103,287 people admitted to emergency department or urgent care with covid-19 (cases) were compared with 531,168 people with covid-like illness who tested negative for SARS-CoV-2. In the omicron period, vaccine effectiveness against covid-19 requiring admission to hospital was 89% (95% confidence interval 88% to 90%) within two months after dose 3 but waned to 66% (63% to 68%) by four to five months. Vaccine effectiveness of three doses against emergency department or urgent care visits was 83% (82% to 84%) initially but waned to 46% (44% to 49%) by four to five months. Waning was evident in all subgroups, including young adults and individuals who were not immunocompromised; although waning was more in people who were immunocompromised. Vaccine effectiveness increased among most groups after a fourth dose in whom this booster was recommended.
Conclusions: Effectiveness of mRNA vaccines against moderate and severe covid-19 waned with time after vaccination.”