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).

This study is a monocentric interventional prospective and retrospective cohort study. After signing a written informed consent, participants will be recruited for questionnaire completion and blood sampling. Sample storage and analysis will be performed at the laboratory of microbiology of the UZ Brussel.

To document SARS-CoV-2 seroprevalence and seroconversion among employees of the UZ Brussel after mRNA vaccination for SARS-CoV-2, namely at 8 weeks after first vaccination, 6 months after first vaccination and 12 months after first 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).

UZ Brussel employees 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. During the first weeks of the vaccination campaign, the health care workers of the UZ Brussel, were invited to receive the BNT162b2 (Pfizer) vaccine.

Consequently, the investigators aim to prospectively document the SARS-CoV-2 seroprevalence and seroconversion among vaccinated employees 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 mRNA COVID-19 vaccine (Comirnaty®) between the 19th of January and the 5th of February 2021 after participating to phase 4 of the COVEMUZ study (with a maximum of 5 days difference between blood drawel and vaccination) and has provided a signed informed consent.

Exclusion Criteria:

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