Three complementary activities will be implemented:1) Baseline and repeat census of the catchment population; described in a separate protocol (IVI-ECOVA-03-WS1); 2) Enhanced surveillance for COVID-19 disease, and 3) AEFI-enhanced surveillance. The mass vaccination campaign will be conducted by the Government and is not part of this protocol.

Inclusion Criteria:

1. They resided in the study area at the time of mass immunization with the Sinopharm vaccine; and
2. They are at least 18 years of age during first round of mass vaccination.
3. Their onset of symptoms was <10 days prior to testing.
4. For each selected PCR test-positive case, we will select up to 3 test-negative controls from generally eligible subjects, matched to each case by age on the date of testing, sex, and calendar date of testing + 3 days.

Exclusion Criteria:

1. We will exclude subjects for selection as controls if they have had a positive test result within the following 21 days
2. Any Participant not matching the inclusion criteria mentioned above.

The study will be conducted in the Dondo District. For surveillance the catchment area will include participants from Dondo Sede and Mafambisse administrative posts. The participant will come for screening in the following health facilities; Dondo, Baptista, Mandruzi, Canhandula, Samora Machel) and in Mafambisse (Mafambisse and Bloco Nove) and at the community level, the study will involve walk-throughs for COVID-19 testing.