Administering first doses of a Covid-19 vaccine to more individuals instead of withholding available supply for use as a second dose may reduce the number of new cases in the United States, according to a study published in the Annals of Internal Medicine on Monday.
The US government currently withholds 50% of the available vaccine supply, distributing to states and other jurisdictions weeks later to be administered as the second in a two-dose series. By reducing the amount withheld to 10% for the first three weeks and supplying a steady dose of 6 million doses per week, the US could avoid up to 29% more cases over eight weeks, the study found.
The researchers modeled various scenarios, with variables including vaccine supply, protection provided by the first dose and waning efficacy of first dose if the second dose is delayed. Only in the unlikely worst-case scenario of a collapse in supply and minimal protection among individuals who have received the first dose would the model suggest that withholding 50% of available supply provide better protection.
“We find that under most plausible scenarios, a more balanced approach that withholds fewer doses during early distribution in order to vaccinate more people as soon as possible could substantially increase the benefits of vaccines, while enabling most recipients to receive second doses on schedule,” write the study’s authors, who were supported by the Canadian Institutes for Health Research and the Centers for Disease Control and Prevention.
Both vaccines authorized for emergency use in the US -- made by Pfizer/BioNTech and Moderna -- are authorized only as a two-dose series, though efficacy estimates allow for partial protection after the first dose. The study modeled scenarios using characteristics of the Pfizer/BioNTech vaccine.
The study assesses the US strategy for vaccine allocation but does not consider delays in vaccine administration at the state level.