A novel zoonotic coronavirus was discovered in Wuhan (Hubei Province, China) mid-December 2019 and was named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The virus rapidly spread to the rest of the world, including Europe and explicitly affects the respiratory system, generating Coronavirus disease 2019 (COVID-19).

Employees of the university hospital of Brussels (UZ Brussel) presenting symptoms suggestive of COVID-19 are offered to be tested with real-time PCR on nasopharyngeal swabs. As asymptomatic infections have been described and as the PCR can be negative when taken late after onset of symptoms, serologic tests can be performed. The SARS-CoV 2003 epidemic demonstrated that serological assays were a useful diagnostic tool of non-acute infections. Although it is still uncertain whether convalescing patients have a risk of re-infection, recent data suggest that SARS-CoV-2 antibodies could protect at least for some time from subsequent viral exposures.

As the COVID-19 pandemic had devastating medical, economic and social consequences, safe and effective prophylactic vaccines were urgently needed. And thus several candidate vaccines against SARS-CoV-2 have been developed. The vaccination campaign of the health care workers of the UZ Brussel started mid January 2021. The first available vaccine was the BNT162b2 (Pfizer) vaccine. Early March 2021, in order to accelerate the vaccination of the UZ Brussel employees, the ChAdOx1 nCoV-19 (AZD12222) (Oxford, AstaZeneca) vaccination program was implemented in parallel with the BNT162b2 vaccination program In the COVEMUZ-2 study the investigators have already started to document the SARS-CoV-2 seroprevalence and seroconversion among vaccinated employees (using BNT162b2) in the UZ Brussels.

In this study, the investigators aim to prospectively document the SARS-CoV-2 seroprevalence and seroconversion among vaccinated employees (using ChAdOx1 nCoV-19) of the UZ Brussel, at three different time points, namely 6 weeks (+/- 2 weeks; T1), 6 months (+/- 1 month; T2) and 12 months (+/- 1 month; T3) after the second vaccination.

A novel zoonotic coronavirus was discovered in Wuhan (Hubei Province, China) mid-December 2019 and was named severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The virus rapidly spread to the rest of the world, including Europe and explicitly affects the respiratory system, generating Coronavirus disease 2019 (COVID-19).

Employees of the university hospital of Brussels (UZ Brussel) presenting symptoms suggestive of COVID-19 are offered to be tested with real-time PCR on nasopharyngeal swabs. As asymptomatic infections have been described and as the PCR can be negative when taken late after onset of symptoms, serologic tests can be performed. The SARS-CoV 2003 epidemic demonstrated that serological assays were a useful diagnostic tool of non-acute infections. Although it is still uncertain whether convalescing patients have a risk of re-infection, recent data suggest that SARS-CoV-2 antibodies could protect at least for some time from subsequent viral exposures.

As the COVID-19 pandemic had devastating medical, economic and social consequences, safe and effective prophylactic vaccines were urgently needed. And thus several candidate vaccines against SARS-CoV-2 have been developed. The vaccination campaign of the health care workers of the UZ Brussel started mid January 2021. The first available vaccine was the BNT162b2 (Pfizer) vaccine. Early March 2021, in order to accelerate the vaccination of the UZ Brussel employees, the ChAdOx1 nCoV-19 (AZD12222) (Oxford, AstaZeneca) vaccination program was implemented in parallel with the BNT162b2 vaccination program In the COVEMUZ-2 study the investigators have already started to document the SARS-CoV-2 seroprevalence and seroconversion among vaccinated employees (using BNT162b2) in the UZ Brussels.

In this study, the investigators aim to prospectively document the SARS-CoV-2 seroprevalence and seroconversion among vaccinated employees (using ChAdOx1 nCoV-19) of the UZ Brussel, at three different time points, namely 6 weeks (+/- 2 weeks; T1), 6 months (+/- 1 month; T2) and 12 months (+/- 1 month; T3) after the second vaccination.

Inclusion Criteria:

• Any adult employee of the UZ Brussel at T1 who has been vaccinated at the UZ Brussel with ChAdOx1 nCoV-19 vaccine between the 2nd of March and the 9th of March 2021 after participating to phase 4 of the COVEMUZ study between the 25th of January and the 12th of February and has provided a signed informed consent.

Exclusion Criteria:

• UZ Brussel employees not active during the inclusion period (T1).