Data from four clinical trials posted today by researchers from the University of Oxford, AstraZeneca, and partners showed a single dose of their COVID-19 vaccine to be 76% effective from 22 to 90 days following vaccination, while a two-dose regimen proved more effective the longer the interval between doses.

The vaccine, recently renamed COVID-19 Vaccine AstraZeneca and formerly called AZD1222, also showed potential for reducing transmission of the virus.

According to the data—posted in Preprints with The Lancet and under review for formal publication in The Lancet—the vaccine’s two-dose efficacy rose from 54.9% when doses were spaced less than six weeks apart, to 82.4% when the interval between doses was 12 or more weeks.

That finding is supported by additional immunogenicity data that showed binding antibody responses in volunteers ages 18-55 years that were more than two-fold higher after an interval of 12 or more weeks, compared with an interval of less than 6 weeks. For those volunteers, the geometric mean titer ratio (GMR) was 2.19, with GMR ranging from 2.12 to 2.26.

“Another great result on COVID vaccine!” tweeted Akiko Iwasaki, PhD, Waldemar Von Zedtwitz Professor of Immunobiology and Molecular, Cellular and Developmental Biology at Yale School of Medicine and a Howard Hughes Medical Institute investigator.

The vaccine was also shown to build immune system protection from COVID-19 22 days post-vaccination, with the protective effect lasting 90 days post-vaccination, with an efficacy rate of 76%. A model-based analysis showed that the protection did not wane during the three-month period.

In addition, researchers found a 67% reduction in positive swabs obtained from volunteers in the U.K. arms of the trials following the first dose—demonstrating the vaccine’s potential to reduce transmission of SARS-CoV-2.

“ChAdOx1 nCoV-19 vaccination programs aimed at vaccinating a large proportion of the population with a single dose, with a second dose given after a three 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,” the research team concluded in the preprint, using the vaccine’s original name.

As a result, the results appear to validate the U.K. strategy of addressing limited supply of the vaccine by delaying the second dose of the vaccine so that more people could receive the first.

“An Important verification”

“These new data provide an important verification of the interim data that was used by more than 25 regulators including the MHRA [the U.K. Medicines and Healthcare products Regulatory Agency] and EMA [European Medicines Agency] to grant the vaccine emergency use authorization,” stated Professor Andrew Pollard, MBBS, PhD, chief investigator of the Oxford Vaccine Trial.

Pollard added that the data also supported a policy recommendation of the U.K. Joint Committee on Vaccination and Immunization (JCVI) for a 12-week interval between first and second doses of the vaccine.

“There were grounds to think a 12-week gap for the Oxford/AstraZeneca vaccine would be effective but great to see the data,” tweeted Anita Charlesworth, Director of Research and the REAL Centre (Research and Economic Analysis for the Long term) at The Health Foundation, the U.K.’s second largest endowed foundation focusing on health.

The AstraZeneca/Oxford COVID-19 vaccine is based on an adenovirus vaccine vector and the COVID-19 spike protein. After vaccination, the surface spike protein of the coronavirus is produced, which primes the immune system to attack the coronavirus if it later infects the body.

The vaccine is among 21 “Front Runner” leading candidates among the more than 300 COVID-19 therapeutics under study in GEN’s updated and just-launched “COVID-19 Drug & Vaccine Candidate Tracker.”

Data posted today was derived from 17,177 people who tested negative for COVID-19 infection at the start of Phase III trials in the U.K. and Brazil, as well as Phase I/II trials in the U.K. and South Africa—where variant strains of SARS-CoV-2 have surfaced in recent weeks. The cutoff date for the data was December 7, 2020.

As of that date, researchers found a total 332 cases of primary symptomatic COVID-19, up from 131 previously reported.

Of volunteers tested in the efficacy analysis, 8,948 were in the U.K., 6,753 in Brazil, and 1,476 in South Africa.

The trials from which the analysis was conducted were Phase III studies in the U.K. (NCT04400838) and Brazil (ISRCTN89951424), as well as Phase I/II trials in the U.K. (NCT04324606) and South Africa (NCT04444674).