India's drugs regulator on Sunday gave final approval for the emergency use of a two-dose coronavirus vaccine, COVISHIELD, developed by AstraZeneca and Oxford University. The regulator did not discuss the recommended intervals between shots.
Authorities in Britain, the first to have authorised the Covid-19 vaccine, on Wednesday recommended giving as many people as possible a first dose right away and following up with a second injection four to 12 weeks later.
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 December 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% effective - but a smaller group of volunteers received by accident half a dose followed by a full dose, and registered 90% effectiveness. 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 last month it did not know the reason for the higher efficacy rate, 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-dose regimen.
The Joint Committee on Vaccination and Immunisation has recommended that people get one dose followed by a second four to 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".
The Drugs Controller General of India, VG Somani, who heads the Central Drugs Standards Control Organisation, approved a full two-dose regimen.
In announcing the approval on January 3, Somani did not clarify the intervals between shots. Sources told Reuters the doses would be given four weeks apart.
Somani said the overall efficacy of the AstraZeneca vaccine was 70.42%, based on overseas studies on 23,745 participants aged 18 or above. Phase II/III clinical trials on 1,600 participants in India showed data "comparable with those from the overseas clinical studies".