AstraZeneca-Oxford adenovirus-vectored COVID-19 vaccine
The AstraZeneca-Oxford adenovirus-vectored COVID-19 vaccine seems to perform better when the dosing interval is longer — and might lower the risk for asymptomatic SARS-CoV-2 transmission — according to phase 3 trial data. U.K. health regulators authorized the vaccine as a two-dose regimen, 4 to 12 weeks apart, in December 2020.
In the trial, over 17,000 adults in the U.K., Brazil, and South Africa were randomized to receive two doses of the COVID-19 vaccine or a control vaccine. In the U.K., some participants inadvertently received a low dose for their first shot.
Roughly 2% of participants developed symptomatic COVID-19, with an overall vaccine efficacy of 67% after the second dose. In a subanalysis based on the interval between doses, vaccine efficacy rose with increasing dosing interval, from 55% among those who received their second dose less than 6 weeks after their first, to over 80% among those who had at least 12 weeks between doses.
In addition, in analyses after a single standard dose, the vaccine showed 76% efficacy against symptomatic infection between 22 and 90 days after vaccination.
In the U.K., participants also self-administered nose and throat swabs each week to assess the effects on asymptomatic infection. For these analyses, the vaccine showed 22% efficacy against asymptomatic infection, although a significant benefit was limited to participants who received a low dose as their initial shot.
The researchers conclude, “Vaccinating a large proportion of the population with a single dose, with a second dose given after a 3 month period is an effective strategy for reducing disease, and may be the optimal for rollout of a pandemic vaccine when supplies are limited in the short term.”