* currently pregnant, planning to become pregnant during the study period, and/or breast feeding * known hypersensitivity/allergy to hydroxychloroquine or to 4-aminoquinoline compounds. * current use of hydroxychloroquine for the treatment of a medical condition. * known prolonged qt syndrome, or concomitant medications which simultaneously may prolong the qtc that cannot be temporarily suspended/replaced. these are including but not limited to class ia, ic and iii antiarrhythmics; certain antidepressants, antipsychotics, and anti-infectives; domperidone; 5-hydroxytryptamine (5-ht)3 receptor antagonists; kinase inhibitors; histone deacetylase inhibitors beta-2 adrenoceptor agonists. * known pre-existing retinopathy. * disclosure of self-administered use of hydroxychloroquine or chloroquine within 12 weeks prior to study. this window allows five half-lives of hcq (i.e. 21 days) to pass before being reintroduced to the drug. * confirmed symptomatic covid-19 at time of enrollment, i.e. symptom of covid-19 at enrollment with confirmation of sars-cov-2 infection by viral detection as performed according to local guidelines for symptomatic hcws. all participants with covid-19 symptoms at enrollment will be directed to have confirmatory testing (within the department or occupational health as per the site guidelines). participants who are negative for sars-cov-2 will be redirected to enrollment procedures; those testing positive will be excluded.

* currently pregnant, planning to become pregnant during the study period, and/or breast feeding * known hypersensitivity/allergy to hydroxychloroquine or to 4-aminoquinoline compounds. * current use of hydroxychloroquine for the treatment of a medical condition. * known prolonged qt syndrome, or concomitant medications which simultaneously may prolong the qtc that cannot be temporarily suspended/replaced. these are including but not limited to class ia, ic and iii antiarrhythmics; certain antidepressants, antipsychotics, and anti-infectives; domperidone; 5-hydroxytryptamine (5-ht)3 receptor antagonists; kinase inhibitors; histone deacetylase inhibitors beta-2 adrenoceptor agonists. * known pre-existing retinopathy. * disclosure of self-administered use of hydroxychloroquine or chloroquine within 12 weeks prior to study. this window allows five half-lives of hcq (i.e. 21 days) to pass before being reintroduced to the drug. * confirmed symptomatic covid-19 at time of enrollment, i.e. symptom of covid-19 at enrollment with confirmation of sars-cov-2 infection by viral detection as performed according to local guidelines for symptomatic hcws. all participants with covid-19 symptoms at enrollment will be directed to have confirmatory testing (within the department or occupational health as per the site guidelines). participants who are negative for sars-cov-2 will be redirected to enrollment procedures; those testing positive will be excluded.

- currently pregnant, planning to become pregnant during the study period, and/or breast feeding - known hypersensitivity/allergy to hydroxychloroquine or to 4-aminoquinoline compounds. - current use of hydroxychloroquine for the treatment of a medical condition. - known prolonged qt syndrome, or concomitant medications which simultaneously may prolong the qtc that cannot be temporarily suspended/replaced. these are including but not limited to class ia, ic and iii antiarrhythmics; certain antidepressants, antipsychotics, and anti-infectives; domperidone; 5-hydroxytryptamine (5-ht)3 receptor antagonists; kinase inhibitors; histone deacetylase inhibitors beta-2 adrenoceptor agonists. - known pre-existing retinopathy. - disclosure of self-administered use of hydroxychloroquine or chloroquine within 12 weeks prior to study. this window allows five half-lives of hcq (i.e. 21 days) to pass before being reintroduced to the drug. - confirmed symptomatic covid-19 at time of enrollment, i.e. symptom of covid-19 at enrollment with confirmation of sars-cov-2 infection by viral detection as performed according to local guidelines for symptomatic hcws. all participants with covid-19 symptoms at enrollment will be directed to have confirmatory testing (within the department or occupational health as per the site guidelines). participants who are negative for sars-cov-2 will be redirected to enrollment procedures; those testing positive will be excluded.

- currently pregnant, planning to become pregnant during the study period, and/or breast feeding - known hypersensitivity/allergy to hydroxychloroquine or to 4-aminoquinoline compounds. - current use of hydroxychloroquine for the treatment of a medical condition. - known prolonged qt syndrome, or concomitant medications which simultaneously may prolong the qtc that cannot be temporarily suspended/replaced. these are including but not limited to class ia, ic and iii antiarrhythmics; certain antidepressants, antipsychotics, and anti-infectives; domperidone; 5-hydroxytryptamine (5-ht)3 receptor antagonists; kinase inhibitors; histone deacetylase inhibitors beta-2 adrenoceptor agonists. - known pre-existing retinopathy. - disclosure of self-administered use of hydroxychloroquine or chloroquine within 12 weeks prior to study. this window allows five half-lives of hcq (i.e. 21 days) to pass before being reintroduced to the drug. - confirmed symptomatic covid-19 at time of enrollment, i.e. symptom of covid-19 at enrollment with confirmation of sars-cov-2 infection by viral detection as performed according to local guidelines for symptomatic hcws. all participants with covid-19 symptoms at enrollment will be directed to have confirmatory testing (within the department or occupational health as per the site guidelines). participants who are negative for sars-cov-2 will be redirected to enrollment procedures; those testing positive will be excluded.