Background: Multiple recent studies have shown strong first dose vaccine efficacy for both Moderna mRNA-1273 and Pfizer/BioNTech BNT 162b2, which has stimulated discussion of maximizing initial population immunity during a time of vaccine shortage by using a deferred second dose strategy for these vaccines. Methods: Our model examines the size of the effect of spacing of the second dose with 6, 12, and 24 week deferred spacing regimens relative to 3 week spacing. Results: Deferring the second dose from 3 weeks to 6 weeks, 12 weeks, and 24 weeks shows progressive benefit to population immunity for any given time period, even with significant one dose efficacy decay. The benefits are influenced by vaccine supply per capita. Conclusion: The longer the second dose is deferred the larger the benefit in initial population immunity, provided one dose efficacy does not significantly wane. Monitoring one dose efficacy duration from the UK or Quebec minimizes this risk, as the gathered data will help ensure the second dose is given at an optimal time. How this information is implemented should vary depending on the population and whether the goal is to optimally protect high risk groups or to increase total population immunity as quickly as possible. Benefits to deferring the second dose are influenced by the length of deferral, one dose efficacy, and vaccine supply per capita. The time to herd immunity could be shortened by 4 weeks with the implementation of a 12 week spacing regimen or 10 weeks with a 24 week spacing regimen.
Modelled Optimization of SARS-Cov-2 Vaccine Distribution: an Evaluation of Second Dose Deferral Spacing of 6, 12, and 24 weeks