Higher Efficacy Found With Longer Interval Between Doses of Oxford COVID-19 Vaccine

February 19, 2021
Killian Meara

The interval between doses can be safely extended to 3 months given the protection a single dose offers.

A recent study conducted by investigators from the University of Oxford has found that the Oxford coronavirus disease 2019 (COVID-19) vaccine has a higher efficacy when a 3-month interval is given during the 2 doses instead of the initial 6-week interval. The new results come from a post-hoc exploratory analyses of phase 3 trial data and were published in The Lancet.

Because vaccine supplies are currently very limited, this new regimen may prove significantly beneficial.

“Vaccine supply is likely to be limited, at least in the short term, and so policy-makers must decide how best to deliver doses to achieve the greatest public health benefit," Andrew Pollard, lead author on the study said. "Where there is a limited supply, policies of initially vaccinating more people with a single dose may provide greater immediate population protection than vaccinating half the number of people with 2 doses."

The investigators behind the study combined data from randomized trials that took place in the United Kingdom, South Africa and Brazil that included a total of 17, 178 participants. All participants were 18 years of age or older and received either 2 standard doses of the Oxford vaccine or a saline placebo, with a subset in the UK receiving a lower dose of the vaccine in their first round.

Next, they conducted an analysis in order to establish the impact of 1 and 2 doses of the vaccine on reducing the overall number of cases of COVID-19 and community transmission. The single dose analysis revealed a 76% efficacy from 22 days to 3 months after administration and that the protection did not reduce over 3 months. Additionally, levels of antibodies for SARS-CoV-2 remained at similar levels for 3 months.

The interval data of 2 standard doses showed that when participants were given their doses 12 weeks apart, they had greater levels of protection than those who were given their does 6 weeks apart (81% vs 55%). The efficacy results were supported by data that showed the binding of antibody levels against SARS-CoV-2 responses were over 2 times higher in the group with a longer delay between doses.

Additional information gathered from the study led the investigators to estimate that community transmission would be reduced by up to 64% following a single dose, and that 2 doses may reduce transmission by 50%. They suggest that this means following outlines by regulators could lead to a substantial impact on community transmission by reducing the number of individuals infected with COVID-19 in the population.

“This latest analysis confirms our previous findings of the higher efficacy of a low- then standard-dose regimen. However, with additional data available, we have found that the enhanced efficacy and immunity may be partly driven by the longer interval between doses that was common in this trial group,” Merryn Voysey, an author on the study said. “This further supports the relationship we have found between vaccine interval and efficacy in those receiving two standard doses, which is the preferred regimen because there are more data to support its use, and because it is simpler to deliver a vaccine program when the same vaccine is given for both doses.”