Importance of epidemic severity and vaccine mode of action and availability for delaying the second vaccine dose

Abstract

Following initial optimism regarding the potential for rapid vaccination, delays and shortages in vaccine supplies have occurred in many countries. Various strategies to counter this gloomy reality and speed up vaccination have been set forth, of which the most popular approach has been to delay the second vaccine dose for a longer period than originally recommended by the manufacturers. Controversy has surrounded this strategy, and overly simplistic models have been developed to shed light on this issue. Here we use three different epidemic models, all accounting for the actual COVID-19 epidemic in the Czech Republic, including the rise and eventual prevalence of the B.1.1.7 variant of SARS-CoV-2 virus and real vaccination rollout strategy, to explore when delaying the second vaccine dose from 21 days to 42 days is advantageous. Using the numbers of COVID-19-related deaths as a quantity for comparing various model scenarios, we find that vaccine mode of action at the beginning of the infection course (preventing contagion and symptom appearance), mild epidemic and sufficient vaccine supply rate call for the original inter-delay scenario of 21 days regardless of vaccine efficacy. On the contrary, for vaccine mode of action at the end of infection course (preventing severe symptoms and death), severe epidemic and low vaccine supply rate, the 42-day inter-dose period is preferable, at any plausible vaccine efficacy.