Toronto: Prioritising widespread inoculation with available first doses may reduce new infections, hospitalisations, and deaths, according to a study that suggests delaying the second dose could be an effective public health strategy.
The emergence of novel, more contagious SARS-CoV-2 variants has led to a public health debate on whether to vaccinate more individuals with the first dose of available vaccines and delay the second dose, or to prioritise completion of the two-dose series based on tested schedules in clinical trials.
The study, led by Seyed Moghadas at York University in Toronto, Canada, found delaying the second dose for 9-15 weeks after the first dose avoided more hospitalisations, infections, and deaths compared to following the recommended schedules for Moderna and Pfizer-BioNTech vaccines.
"When racing against a burgeoning outbreak, our results show that prioritising vaccine coverage with rapid distribution of the first dose would be critical to mitigating adverse outcomes and allow the healthcare system to also address non-Covid-19 medical needs of the population," said lead author Moghadas.
The findings are detailed in the open access journal PLOS Biology.
However, the study has several limitations, including a lack of clinical evidence quantifying the durability of the vaccines when administered under different schedules, the team noted.
Researchers thus assumed that protection levels of the first dose were stable if second doses were delayed, and that the protection level after delaying second doses was identical to overall protection after two doses when vaccinating according to schedule.
Further studies are needed to pinpoint the optimal time between doses for each type of vaccine.
"We still do not have the full picture of vaccine effectiveness as new and more contagious variants spread. Efficacy of vaccines against these variants is an additional factor that would need to be considered in determining the outcomes of on-time versus delayed second dose and interval between doses," Moghadas said.
In order to compare the epidemiological impact of each vaccination strategy, researchers built a mathematical model that simulated both Covid-19 transmission and various delayed second dose vaccination schedules.