Modelled Optimization of SARS-Cov-2 Vaccine Distribution: an Evaluation of Second Dose Deferral Spacing of 6, 12, and 24 weeks

Abstract

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.

Modelling the impact of extending dose intervals for COVID-19 vaccines in Canada

“…17,19 Although our findings reflect population-level effects, it is important to consider the impact of extending the dose interval in subgroups for whom the vaccine may be less effective, such as immunosuppressed individuals. 25,26 The influence of first-dose effectiveness and duration of protection have similarly been highlighted by other vaccine models that compared extended dosing intervals to no delay for mRNA-vaccination strategies, 27,28 as well as vaccine models that compared the use of different proportions of the vaccine supply for extended dosing strategies. 29 Extended intervals up to 24 weeks were preferred given high first dose effectiveness against disease (Moghadas et al: 80% 28 ; Jurgens and Lackner: 46.5% 27 ), or given limited waning (greater than 18-week duration of protection when first dose effectiveness was low 28 ; up to ~10% waning per month 27 ).…”

“…25,26 The influence of first-dose effectiveness and duration of protection have similarly been highlighted by other vaccine models that compared extended dosing intervals to no delay for mRNA-vaccination strategies, 27,28 as well as vaccine models that compared the use of different proportions of the vaccine supply for extended dosing strategies. 29 Extended intervals up to 24 weeks were preferred given high first dose effectiveness against disease (Moghadas et al: 80% 28 ; Jurgens and Lackner: 46.5% 27 ), or given limited waning (greater than 18-week duration of protection when first dose effectiveness was low 28 ; up to ~10% waning per month 27 ). Using a greater proportion of the supply for extended dosing strategies was preferred even with extreme waning assumptions (e.g., protection drops to zero within 6 weeks of not receiving second dose).…”