- for participants who are in, or who have completed, the activ-2/a5401 trial: self-report of prior receipt of a non-mrna-based covid-19 vaccine - for participants who are in, or who have completed, the activ-2/a5401 trial: self-report of receipt of the second dose of an mrna-based covid-19 vaccine. - for participants who are in, or who have completed, the activ-2/a5401 trial: self-report of a second sars-cov-2 infection after the infection that qualified the participant for activ-2/a5401. - for non-a5401/activ-2 participants: self-report of receipt of any prior covid-19 vaccine. - for non-a5401/activ-2 participants: known prior history of any sars-cov-2-positive test (e.g., pcr test, nucleic acid amplification test (naat), antigen test, serology test). - for participants who will receive study-provided moderna mrna-1273 covid-19 vaccine: known allergy to any component of the moderna covid-19 vaccine.

- for participants who are in, or who have completed, the activ-2/a5401 trial: self-report of prior receipt of a non-mrna-based covid-19 vaccine - for participants who are in, or who have completed, the activ-2/a5401 trial: self-report of receipt of the second dose of an mrna-based covid-19 vaccine. - for participants who are in, or who have completed, the activ-2/a5401 trial: self-report of a second sars-cov-2 infection after the infection that qualified the participant for activ-2/a5401. - for non-a5401/activ-2 participants: self-report of receipt of any prior covid-19 vaccine. - for non-a5401/activ-2 participants: known prior history of any sars-cov-2-positive test (e.g., pcr test, nucleic acid amplification test (naat), antigen test, serology test). - for participants who will receive study-provided moderna mrna-1273 covid-19 vaccine: known allergy to any component of the moderna covid-19 vaccine.