Oxford/AstraZeneca’s COVID-19 vaccine reduces transmission & benefits from extended dose interval

Extended dose interval of Oxford COVID-19 vaccine

Date: 3rd February 2021

Many countries around the world have implemented a phased roll out of COVID-19 vaccines and are heavily relying on them as a route out of lock down.  However, whilst some have extended the dosing interval for vaccines to maximise the number of vulnerable groups receiving their first dose, this approach has be criticised by others as up until now the supportive data was lacking.  A further query as to whether the vaccines could reduce transmission also remained a key element in any potential easing of restrictions.

Now, AstraZeneca and Oxford University have announced that their COVID-19 vaccine, ChAdOx1 nCoV-19 or AZ1222, gives 76% protection starting after the first dose and up to 90 days, which increased to 82% with a 12-week or more dose interval.  Furthermore, ChAdOx1 nCoV-19, can reduce the transmission of SARS-CoV-2 by 67% after the first dose.

Researchers at the University of Oxford have published, in Preprints with The Lancet, the primary analysis of their Phase III clinical trial from the UK and Brazil and a PhaseI/II study in the UK and South Africa from their ongoing trials of the ChAdOx1 nCoV-19 vaccine.

The results were based on 17,177 participants with 332 symptomatic cases.  The vaccine was given in two doses, and had an efficacy of 76% after the first standard dose from 22 to 90 days, with no signs of the protection waning in this 3 months. They also saw an increased efficacy with a longer inter-dose interval of 12 weeks or more, rising to 82.4% from 54.9% – when the second dose was given at an interval of less than six weeks.

Antibody levels were maintained up 60 days after the first dose was administered, and immunogenicity data showed that binding antibody responses were >2-fold higher after a dosing interval of 12 or more weeks compared with an interval of <6 weeks, further supporting a longer dosing interval.

Importantly, there were no hospitalisations in the vaccine group after the initial 21 day exclusion period – whilst the immune response is activated –  but there were 15 in the control group.

The team also found the first indications that the vaccine could reduce transmission of the virus.   Swabs were taken from UK volunteers and a 67% reduction in transmission was recorded after the first dose of the vaccine.

Conclusions and further work

These data provide an important verification of the original data that supported the emergency use authorisation by more than 25 regulators around the world.  It also provides evidence that a 12-week prime-boost interval is an optimal approach for the ChAdOx1 nCoV-19 roll out.  However, there is still a high level of efficacy after 22 days from a single dose.

The team are hoping to publish their data on the new SARS-Cov-2 variants in the coming days.  Although they are expecting, as with the Novavax vaccine that they will be broadly similar.

AstraZeneca is also seeking Emergency Use Listing from the World Health Organization for an accelerated pathway to vaccine availability in low-income countries.


For more information please see the press release from Oxford University or AstraZeneca

Voysey et al., Single Dose Administration, And The Influence Of The Timing Of The Booster Dose On Immunogenicity and Efficacy Of ChAdOx1 nCoV-19 (AZD1222) Vaccine: Available at SSRN: https://ssrn.com/abstract=3777268