Dear Editor, The coronavirus disease 2019 (COVID-19) pandemic that caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has affected millions of people and is an ongoing issue globally. The application of effective and safe vaccines is critical to achieving control of the pandemic. 1 Data from clinical trials have shown that several vaccines can induce humoral and cellular immune responses against SARS-CoV-2. 2-4 However, given the long time required for vaccine production, difficulty in vaccine distribution and public skepticism about their safety and benefits, the vaccine coverage rate is growing slowly worldwide. Therefore, to make full use of available resources and maximize the efficacy of vaccination is critically important in fighting against COVID-19.

Currently, there are limited data on the immune responses to SARS-CoV-2 vaccination regarding the potential impact of timing of vaccination in a 24-hour circadian rhythm cycle. We know that the immune system is influenced by circadian rhythm, and immune cells vary in cell number and immune function in the blood in a circadian fashion. 5 In certain cases, antibody response to flu vaccines is stronger when vaccinated in the morning than in the afternoon. 6 However, whether circadian rhythm affects immune response to SARS-CoV-2 vaccines is not known. We have therefore investigated the immunological response to SARS-CoV-2 vaccine delivered in the morning vs afternoon among healthcare workers (HCWs). The receptor binding domain (RBD) of SARS-CoV-2 spike protein-based chemiluminescent immunoassay (CLIA) was utilized for neutralizing antibody (NAb) measurement, and flow cytometry for immune phenotype monitoring and antigenspecific memory B cell measurement. To investigate the potential impact of vaccine delivery timing on immune responses, 7 a prospective cohort study design was adopted. Sixty-three HCWs from the First Affiliated Hospital of Sun Yat-sen University (FAH-SYSU) in Guangzhou, China, received vaccination in the morning (9 am-11 am) or afternoon (15 pm-17 pm) according to the assignment of relevant hospital staffs (Supplementary information, Table S1). All participants received an inactivated SARS-CoV-2 vaccine (BBIBP-CorV, Sinopharm, Beijing) on day 0 and day 28. The two doses of vaccine were given at the same time of a day. Blood samples were collected on day 0, day 14, day 21, day 28 and day 56. The study was approved by the research committee of FAH-SYSU and registered to Chinese Clinical Trial Registry (ChiCTR2100042222). Written informed consents were obtained from all participants. During the study, no serious adverse events related to the vaccination were recorded (Supplementary information, Table S1).