Here is the design of a clinical trial:

Official Title: PROTECT RCT (PRevention of COVID-19 With Oral Vitamin D Supplemental Therapy in Essential healthCare Teams

Included conditions:
- COVID-19

Study Armgroups:
- {'label': 'Placebo', 'type': 'PLACEBO_COMPARATOR', 'description': '10 placebo tablets taken orally at baseline, followed by 1 placebo tablet once a week for 16 weeks\n\nNote that the study may be prolonged according to the overall infection rate monitored monthly.', 'interventionNames': ['Dietary Supplement: Placebo']}
- {'label': 'Vitamin D3', 'type': 'EXPERIMENTAL', 'description': '10 tablets containing 10,000 IU (total : 100,000 IU) of Vitamin D3 taken orally at baseline, followed by 10,000 IU once a week for 16 weeks.\n\nNote that the study may be prolonged according to the overall infection rate monitored monthly.', 'interventionNames': ['Dietary Supplement: Vitamin D']}

Interventions:
- {'type': 'DIETARY_SUPPLEMENT', 'name': 'Placebo', 'description': 'Weekly oral dose of placebo', 'armGroupLabels': ['Placebo']}
- {'type': 'DIETARY_SUPPLEMENT', 'name': 'Vitamin D', 'description': 'Weekly oral dose of Vitamin D', 'armGroupLabels': ['Vitamin D3'], 'otherNames': ['cholecalciferol']}

Primary Outcomes:
- {'measure': 'Change in incidence of laboratory-confirmed COVID-19 infection', 'description': 'documented by salivary or NP samples obtained clinically for screening or diagnostic purposes throughout the study period, self-obtained salivary samples at endpoint, analysed by RT-qPCR or COVID-19 seroconversion at endpoints', 'timeFrame': '16 weeks'}

Please estimate the sample size based on the assumption: 
1:1 treatment allocation, 80% power, 15% dropout rate, interim analysis at 75% participant completion of 12 weeks, trial termination for efficacy if P(OR<1)>0.95, and potential extension to 24 weeks based on infection rate progress.
Sample size calculation
        Given uncertainties in infection progression, a Bayesian adaptive design was used where the posterior probability of effectiveness, that is, P(OR<1|data) was the basis of inference and decision making.34 Assuming an expected OR of 0.80 and 1:1 treatment allocation, a total net sample size of 2100 was required to identify a 20% reduction in the risk of COVID-19 in the vitamin D versus control group, with 80% power with the design described above. Considering a dropout rate of 15%, 2414 participants were targeted. An interim analysis was planned when 75% of participants would have reached week 12, at which time the following assessments were to be made: the progression over time in the incidence of infection (slope of the curve of infection) was to be updated and if the probability of effectiveness exceeded 0.95 (p(OR<1)>0.95), the trial would have been terminated for efficacy at the interim point (12 weeks); otherwise, the study would have continued to 16-week follow-up. Simulation results showed that, with the net sample size of 2100 (assuming an expected OR of 0.80 and 1:1 treatment allocation), there was about a 55% chance that the trial would be terminated for efficacy at the interim analysis.35 The overall infection rate was monitored on a monthly basis: note that the study could have been extended to 24 weeks based on the progress of the infection rate, if required.