Latest clinical trial supports UK’s decision to delay second COVID vaccine dose

Researchers in the UK, South Africa and Brazil have reported the latest efficacy results from phase III trials of the Oxford–AstraZeneca vaccine currently being rolled out in the UK to protect against coronavirus disease 2019 (COVID-19).

Researchers from the Oxford Vaccine Group and collaborators say the findings support the current regimen of administering two standard doses 12 weeks apart – a policy that has recently been a topic of substantial debate.

Merryn Voysey and colleagues say: “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 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.”

Furthermore, the trial findings suggest that a single vaccine dose is highly effective for the first 90 days, thereby further supporting the current policy.

However, since patient follow-up is limited to the time period described here, it is not clear how long protection from a single dose might last, say the researchers.

“For this reason, a second dose of vaccine is recommended,” they write.

An initial version of the research paper is available on the Preprints with The Lancet* server, while the article undergoes peer review.

The current situation with vaccine rollout

The high morbidity and mortality rates associated with the COVID-19 pandemic have led to unprecedented efforts to develop effective vaccines against the causative agent severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Vaccination programs are now being rolled out at the population level in several countries, focusing on high-risk groups such as the elderly, people with underlying health conditions, and frontline workers.

The Oxford–AstraZeneca ChAdOx1 nCoV-19 (AZD1222) vaccine received emergency use authorization from the Medicines and Healthcare products Regulatory Agency (MHRA) based on interim efficacy results (70.4%) pooled from Brazil and the UK until 4th November 2020.

The MHRA approval was received based on a regimen of two standard doses administered 4 to 12 weeks apart – a policy that has since been approved for use in many other countries.

“Following regulatory approval, a key question for policymakers to plan the optimal approach to rollout is the optimal dose interval,” says Voysey and colleagues.

The planned rollout of the AZD1222 vaccine in the UK involves vaccinating high-risk categories with a first dose that is given immediately, and a second dose given 12 weeks later - a policy that has been the topic of much debate.

The World Health Organization has recently recommended a maximum 6-week interval between the two doses, say the researchers.

What have the latest investigations involved?

Now, Voysey and the team have reported the findings from phase III efficacy trials conducted in the United Kingdom and Brazil, and phase I/II clinical trials conducted in the UK and South Africa.

The results follow a further month of data collection (cut-off date 7th December 2020) among a population of 17,177 adult participants who tested negative for SARS-CoV-2 at baseline.

Participants were randomly assigned to receive two standard doses (SD) of the AZD1222 vaccine (5x1010 viral particles) or a control vaccine (saline placebo). In the UK trial, a subset of participants received a lower dose (LD) of the first vaccine (2.2x1010 viral particles) or a control vaccine.

Results are broken down by the interval between the two doses and the efficacy of a single vaccine dose is also investigated.

What are the latest findings?

Over the study period, 619 people tested positive for SARS-CoV-2 by nucleic acid amplification test (NAAT). Of those, 332 had developed symptomatic infection after more than 14 days following a second vaccination.

The overall vaccine efficacy of more than 14 days post-second dose was 66.7%, with an efficacy of 74.0% observed in the SD/SD group and 57.4% in the LD/SD group.

Efficacy following a single standard dose was 59% day 22 post-vaccination and 86% at day 90 post-vaccination. Furthermore, antibody levels were maintained during this 3-month period, with only minimal waning observed by day 90.

Efficacy was higher with a longer prime-boost interval

Interestingly, after a second dose in the SD/SD group, the team found that vaccine efficacy was higher among those with a longer interval between first and second dose.

Efficacy reached 82.4% among those who received a booster after an interval of 12 weeks or more, whereas it only reached 54.9% among those with a prime-boost interval of less than 6 weeks.

The findings support the current rollout regimen in the UK

The researchers say the analysis provides strong evidence to support the efficacy of two standard vaccine doses – the regimen currently approved by the MHRA and other regulators.

The study also shows that higher vaccine efficacy is obtained with a longer prime-boost interval and that a single dose is highly effective during the first 90 days, thereby providing further support for current policy, says the team.

However, “it is not clear how long protection might last with a single dose as follow up is currently limited to the time periods described here, and, for this reason, a second dose of vaccine is recommended,” warns Voysey and colleagues.

“However, where there is a limited supply of vaccine, a policy of initially vaccinating a larger cohort with a single dose may provide better overall population protection than vaccinating half the number of individuals with two doses in the short term,” concludes the team.

*Important Notice

Preprints with The Lancet publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

Journal reference:
Sally Robertson

Written by

Sally Robertson

Sally first developed an interest in medical communications when she took on the role of Journal Development Editor for BioMed Central (BMC), after having graduated with a degree in biomedical science from Greenwich University.


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