The safety and efficacy of the vaccine has not been evaluated on different dosing schedules as the majority of trial participants received the second dose within the window specified in the study design. There is no data to demonstrate that protection after the first dose is sustained after 21 days.

Pfizer and partner BioNTech, commenting in a joint statement earlier this week about the companies’ coronavirus vaccine and the recommended dosage, as reported by Reuters. The companies said they have no evidence that their jointly developed vaccine will continue to protect against COVID-19 if the booster shot is given later than the three-week period tested in clinical trials.