To evaluate the immune response to the ChAdOx1 nCoV-19 vaccine by measuring the titers of antibody against the SARS-CoV-2 spike protein receptor binding domain (RBD) among healthcare workers of My Duc Hospital and investigate potential associations of vaccine protection against infection in this population. The study's data can suggest the groundwork for the development of predictive models for post-vaccination protection in the Vietnamese population as well as for establishing vaccination strategies to control disease outbreaks in the future.

Severe acute respiratory syndrome caused by coronavirus 2 (SARS-CoV-2) emerged in December 2019 from Wuhan, China. This pandemic quickly spread to many countries and became a global health challenge with profound impacts on human health as well as on economic and social life. As of September 12, 2021, according to the statistic registry of the Ministry of Health of Vietnam, the total number of SARS-CoV-2 infections is 624,547 cases, of which the number of deaths is up to 15,660 cases.

Besides social distancing, masking, and handwashing, vaccine is one of the most efficient ways to prevent COVID-19. Most of the vaccines currently available only provide one precursor or vector of protein S, the anti-nucleocapsid (anti-N) antibodies are not fully developed and only anti-S antibodies could be detected (which target spike protein sub-domains, S1 and RBD). On the other hand, due to the biological role of nucleocapsid and the fact that it is protected from antibodies by viral or cellular membranes, nucleocapsid protein (NP) antibodies are unlikely to directly neutralize SARS-CoV-2. For the above-mentioned reasons, screening for anti-S or anti-RBD antibodies is commonly used to investigate the presence of humoral immune markers following a Covid infection or after vaccination.

In the event of vaccination, the duration of antibody persistence is yet unknown. Large cohort studies showed that 90-99% of individuals infected with the SARS-CoV-2 virus developed detectable neutralizing antibodies within 2-4 weeks following infection and immune responses remain robust and protective against reinfection for at least 6-8 months later. In Vietnam, studies on antibody levels after exposure to SARS-CoV-2 or after vaccination are receiving more and more attention. Mai et al. conducted a study to assess the kinetics of antibody titers of eleven COVID-19 infected individuals and found out that the titers of RBD-targeting NAbs decayed by 18-30 weeks after the infection. Chau et al (2021) investigated the vaccine-induced production of neutralizing antibodies after injecting ChAdOx1 nCoV-19 AZD1222 vaccine (Oxford-AstraZeneca) on 554 Vietnamese healthcare workers. This study demonstrated that the concentration of neutralizing antibodies increased after each dose. However, three months after the first dose, neutralizing antibody levels reduced significantly, while the seroconversion rate slightly declined from 98.1% to 94.7% at day 14 after the second dose, but this study did not report neutralizing antibody levels.

Some studies indicated that the efficacy of the Covid-19 vaccine might decrease over time, as demonstrated by the decline of blood antibodies level, thus suggest providing additional booster dose in the hope of increasing protection. However, more information about antibody titers after vaccination is still needed, especially for the Vietnamese population. Therefore, This study is conducted to evaluate the immune response created by the ChAdOx1 nCoV-19 vaccination by measuring the titers of antibodies against the SARS-CoV-2 spike protein receptor binding domain (RBD) among healthcare workers of My Duc Hospital and to investigate potential associations of vaccine protection and infection in this population.

Inclusion Criteria:

• Above 18 years old
• Being vaccinated with at least one dose of ChAdOx1 nCoV-19
• Accepted to take part in the study

Exclusion Criteria:

• Currently being diagnosed with Covid-19

Healthcare workers of My Duc hospital who were vaccinated with at least one dose of ChAdOx1 nCoV-19.