This study will address specific questions concerning the additional protection of the AZ COVID-19 vaccine as a second booster dose in preventing severe COVID-19 outcomes (hospitalisation, ICU admission etc) and will provide data to inform COVID-19 vaccine booster dose recommendations.

In Brazil, second COVID-19 boosters became available to specific risk groups in early 2022. By June 2022, immunocompromised individuals aged 12 years and above, individuals aged 50 years and above, and healthcare workers were eligible for a second booster. The vaccines available for second booster doses in Brazil are produced by AstraZeneca (AZ), Janssen, Pfizer and Sinovac.

While several studies have estimated absolute vaccine effectiveness (aVE) or relative vaccine effectiveness (rVE) of second boosters of mRNA COVID-19 vaccines, only a single study has, to date, estimated aVE of AZ vaccine as a second booster dose, and no studies have estimated rVE for AZ vaccine as a second booster dose. As the pandemic progresses, more individuals are choosing to become vaccinated (and boosted) against COVID-19 and thus there is greater need to evaluate the additional protection offered by a second booster dose amongst individuals previously administered a first booster. This is particularly important in the context of waning protection of a first booster dose.

The primary objective is to estimate relative vaccine effectiveness (rVE) against COVID-19 hospitalization of the AZ COVID-19 vaccine as a second booster dose relative to a first booster dose only, of any COVID-19 vaccine, administered at least four months ago.

This study will also evaluate rVE of other vaccines used in Brazil as a second booster doses. The study will further evaluate rVE of all vaccines (including the AZ-COVID-19 vaccine) as a first booster dose relative to a complete primary series only, of any COVID-19 vaccine administered at least four months ago."

Inclusion Criteria:

• Hospitalized for SARS between 01 January 2022 and 31 December 2022 (Population A) or hospitalised for SARS between 01 August 2021 and 31 December 2022 (Population B).
• Eligible for a second COVID-19 vaccine booster dose at the time of SARS hospitalization (i.e., aged 50+ or aged 18+ and either a HCW or with an IC) for second booster objectives [Population A] OR eligible for a first COVID-19 vaccine booster dose at the time of SARS hospitalisation (i.e., aged 18+) for first booster objectives [Population B].
• Results available for a RT-PCR and/or antigen test for SARS-CoV-2 conducted on a sample taken up to 14 days after symptom onset.
• Individuals have required matching variables (e.g. age and sex).

Exclusion Criteria:

• Hospitalized for COVID-19 within 3 months prior to date of current hospital admission for SARS
• Nosocomially-acquired infection (where SARS case is flagged as nosocomial and/or where symptom onset date is after hospitalisation date)

Population A is for evaluating second booster doses and will consist of individuals aged 18 years and over with immunocompromising conditions, healthcare workers aged 18 years and over, and individuals aged 50 years and over.

Population B is for evaluating first booster doses and will consist of all individuals aged 18 and over.