This is a prospective randomized controlled trial study to explore whether the combination of Chinese herbal medicine and Vitamin C is effective and safe to prevent COVID-19 transmission among health care workers in Hong Kong Mobile Cabin Hospital governed by the Hong Kong Hospital Authority who have been caring for patients with COVID-19. A total of 652 adults will be enrolled. Eligible subjects who provide written informed consent will be assessed for inclusion/exclusion criteria.

All participants are asymptomatic and test negative for SARS-CoV-2 at the study's commencement. Subjects with major medical illness, renal insufficiency and hypersensitivity to Chinese herbal medicine will be excluded. All participants will receive Vitamin C (VC) supplementation and 28 packets of the herbal medicine free of charge and are advised to consume daily for 14 days. After written informed consent, the subjects will be included and randomly allocated to either CHM+VC group or VC treatment.

Inclusion Criteria:

1. Health care workers in Hong Kong Mobile Cabin Hospital;
2. Asymptomatic and test negative for SARS-CoV-2;
3. Aged ≥18;
4. Voluntarily signing a written informed consent form;

Exclusion Criteria:

1. With major medical illness or renal insufficiency;
2. With hypersensitivity to Chinese herbal medicine;
3. Pregnancy or lactation.

All participant data will be de-identified.