Factbox: Astra-Oxford vaccine: what's been said about dosage and efficacy

·2-min read
FILE PHOTO: Vials and medical syringe are seen in front of AstraZeneca logo in this illustration

(Reuters) - British authorities have recommended giving as many people as possible a first dose of the COVID-19 vaccine developed by Oxford University and AstraZeneca right away and following up with a second injection four to 12 weeks later.

This is different to the original trial plan. The differences in findings are outlined below.



AstraZeneca's late-stage trial was designed for two injections four weeks apart.

But in late-stage trial data published in the Lancet on Dec. 12., the company said most participants had delays in receiving their second shot.

The median time between two standard doses in UK volunteers was about 10 weeks, and six weeks in Brazil, it said.


In late-stage trials, two full doses were given to the majority of participants in the UK and Brazil and were shown to be 62% efficacious - but a smaller group of volunteers received by accident half a dose followed by a full dose, and registered 90% efficacy. They were in Britain and under 55 years of age.

Many regulators set 50% as the minimum efficacy rate, but Pfizer-BioNTech's and Moderna's vaccines achieved over 90% in their trials.

AstraZeneca said earlier this month it did not know the reason for the higher efficacy rate, which was the result of a miscalculation, but it was preparing further tests to confirm whether the half-dose regimen could be 90% effective.

One single dose was seen as 64% effective.



The UK Medicines and Healthcare products Regulatory Agency (MHRA) has approved a two-full-dosage regimen.

The Joint Committee on Vaccination and Immunisation has recommended that people get one dose followed by a second between four and 12 weeks later in a bid to speed up the vaccination programme.


On Wednesday, the authorities cleared up one doubt raised by the AstraZeneca-Oxford data, saying that a 90% success rate for a half-dose followed by a full dose had not stood up to analysis.

The vaccine can be 80% effective when there are three months between shots, an official involved in approving the vaccine in Britain said at a briefing, higher than the average that the developers themselves had found.

Later in the same briefing, a British scientist involved in the approval of the vaccine said one dose of the vaccine is around 70% effective after 21 days and before the second dose is given.

Wei Shen Lim, chair for COVID-19 immunisation of the Joint Committee on Vaccination and Immunisation, said the data for that finding was shared with the regulator, but was not "entirely in the public domain".

(Compiled by Josephine Mason; Editing by Jan Harvey)