Protection afforded by post-infection SARS-CoV-2 vaccine doses: A cohort study in Shanghai.

Summary

In a population-based cohort of 199,312 previously infected and vaccinated individuals, a post-infection vaccine dose reduced Omicron reinfection risk (aHR 0.82). Protection was stronger when vaccination occurred closer to exposure (aHR 0.51 within 30 days vs 0.67 within 90 days) and generally beneficial across vaccine types and prior dose histories.

Key Findings

• Post-infection vaccination reduced reinfection risk overall (aHR 0.82; 95% CI 0.79-0.85).
• Protection was stronger when the booster was given within 30 days before the subsequent wave (aHR 0.51) versus within 90 days (aHR 0.67).
• Benefit observed across vaccine types; effect varied with prior dose history (attenuated after three prior doses: HR 0.96).

Clinical Implications

Supports offering booster doses to previously infected individuals, with consideration for timing (closer to anticipated exposure waves yields stronger protection) and broad applicability across vaccine platforms.

Limitations

• Retrospective design with potential residual confounding and testing/ascertainment bias
• Generalizability may vary across regions, variants, and testing policies

Future Directions

Prospective evaluations of booster timing strategies, variant-specific effectiveness, and durability; integration with severe outcome metrics and cost-effectiveness analyses.