Background
We aimed to systematically review the magnitude and duration of the protective effectiveness of prior infection (PE) and hybrid immunity (HE) against Omicron infection and severe disease.

Methods
We searched pre-print and peer-reviewed electronic databases for controlled studies from January 1, 2020, to June 1, 2022. Risk of bias (RoB) was assessed using the Risk of Bias In Non-Randomized Studies of Interventions (ROBINS-I)-Tool. We used random-effects meta-regression to estimate the magnitude of protection at 1-month intervals and the average change in protection since the last vaccine dose or infection from 3 months to 6 or 12 months. We compared our estimates of PE and HE to previously published estimates of the magnitude and durability of vaccine effectiveness (VE) against Omicron.

Findings
Eleven studies of prior infection and 15 studies of hybrid immunity were included. For prior infection, there were 97 estimates (27 at moderate RoB and 70 at serious RoB), with the longest follow up at 15 months. PE against hospitalization or severe disease was 82·5% [71·8-89·7%] at 3 months, and 74·6% [63·1-83·5%] at 12 months. PE against reinfection was 65·2% [52·9-75·9%] at 3 months, and 24·7% [16·4-35·5%] at 12 months. For HE, there were 153 estimates (78 at moderate RoB and 75 at serious RoB), with the longest follow up at 11 months for primary series vaccination and 4 months for first booster vaccination. Against hospitalization or severe disease, HE involving either primary series vaccination or first booster vaccination was consistently >95% for the available follow up. Against reinfection, HE involving primary series vaccination was 69·0% [58·9-77·5%] at 3 months after the most recent infection or vaccination, and 41·8% [31·5-52·8%] at 12 months, while HE involving first booster vaccination was 68·6% [58·8-76·9%] at 3 months, and 46·5% [36·0-57·3%] at 6 months. Against hospitalization or severe disease at 6 months, hybrid immunity with first booster vaccination (effectiveness 95·3% [81·9-98·9%]) or with primary series alone (96·5% [90·2-98·8%]) provided significantly greater protection than prior infection alone (80·1% [70·3-87·2%]), first booster vaccination alone (76·7% [72·5-80·4%]), or primary series alone (64·6% [54·5-73·6%]). Results for protection against reinfection were similar.

Interpretation
Prior infection and hybrid immunity both provided greater and more sustained protection against Omicron than vaccination alone. All protection estimates waned quickly against infection but remained high for hospitalisation or severe disease. Individuals with hybrid immunity had the highest magnitude and durability of protection against all outcomes, reinforcing the global imperative for vaccination.

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