Impact of SARS-CoV-2 inactivated vaccine on symptoms following omicron variant breakthrough infection.

The SARS-CoV-2 Omicron variant and its sublineages continue to circulate widely. Clinical outcomes with this variant differ among individuals, primarily influenced by host immunity. Previous studies have explored the relationship between immune responses and severe diseases in infected or convalescent patients. However, the impact of vaccine-induced immune responses on disease severity, especially in cases of mild infection following breakthrough infection, remains unclear. This is primarily due to the lack of assessment of immune status in vaccinated individuals before infection. In this study, we aimed to elucidate the causality between virus-specific cellular and humoral immune responses and the severity of symptoms in breakthrough infected patients from a long-term follow-up post-vaccination cohort. A questionnaire survey was conducted to collect general symptoms upon breakthrough infection with the Omicron variants. Plasma levels of specific antibodies (neutralizing antibodies, anti-S IgG, and anti-N IgG) and T cell responses induced by inactivated SARS-CoV-2 vaccine were evaluated. The findings revealed that individuals with milder symptoms, particularly lower peak fever temperatures, exhibited higher antibody levels and enhanced T cell activation and responses prior to infection. This suggests that cellular and humoral immunity induced by inactivated vaccines may provide protection against severe clinical symptoms following breakthrough infection.