Single dose of Oxford Astrazeneca vaccine 76% effective for 12 weeks

A single shot of Oxford University’s coronavirus vaccine is 76 per cent effective at preventing symptomatic illness and may have a ‘substantial effect’ on transmission, research suggests.

In a huge boost to the UK’s immunisation drive, analysis of the jab trials found the first dose was extremely successful in preventing people from falling ill within the 12-week time window between getting a second dose.

When the second dose is administered after three months, the jab’s efficacy is bumped up to 82.4 per cent, according to the study, which has been submitted to The Lancet for publication.

The results, from more than 17,000 trial volunteers, suggest Britain’s vaccination gamble to delay its dosing regimen has paid off.

In a bid to get wider vaccine coverage quicker, regulators pivoted from their original plan to give people their second dose after 21 days when the Oxford University/AstraZeneca jab was approved in late December.

They pushed back the second dose for 12 weeks in the hope that giving partial protection to as many vulnerable people as possible would drive down hospital admissions.

The strategy has helped make Britain a world-leader in vaccinations, with 9.6million people now injected with at least a single dose of either Pfizer’s or AstraZeneca’s jab.

Meanwhile, analysis of PCR positive swabs carried out on nearly 7,000 patients in the UK arm of Oxford’s trial suggests the vaccine may reduce transmission by 67 per cent.

Health Secretary Matt Hancock described the findings as ‘hugely encouraging’, adding: ‘It further reinforces our confidence that vaccines are capable of reducing transmission and protecting people from this awful disease.’

Since numerous vaccines have been shown to block symptomatic Covid, officials have turned their attention to how effective jabs are at stopping spread altogether.

Vaccines that can both prevent serious illness and make a dent in the virus’ ability to spread will help drive down Britain’s epidemic much quicker.

Professor Andrew Pollard, chief investigator of the Oxford Vaccine Trial, and co-author of the latest analysis, said: ‘These new data provide an important verification of the interim data that was used by more than 25 regulators including the MHRA [the UK’s Medicines and Healthcare products Regulatory Agency] and the EMA [European Medicines Agency] to grant the vaccine emergency use authorisation.

‘It also supports the policy recommendation made by the Joint Committee on Vaccination and Immunisation (JCVI) for a 12-week prime-boost interval, as they look for the optimal approach to roll out, and reassures us that people are protected from 22 days after a single dose of the vaccine.’

As part of the latest analysis, Oxford researchers went back through their vaccine trial data – which aimed to test how well the two-dose regimen worked – to see how much protection a single shot provides, and for how long.

The research looked at 17,177 participants from three trials in the UK, Brazil and South Africa who were monitored weekly throughout the months-long studies

They found the vaccine had an efficacy of 54.9 per cent when it was administered in two doses within six weeks of each other.

This rose to 82.4 per cent when the second dose was pushed back 12 weeks or more. Scientists are still not certain as to why the longer space between doses provides more protection.

One of the prevailing theories is that giving a second dose too soon can interrupt the immune system before it’s had the chance to fully process the first one.

Creating the initial antibodies and memory cells after the first vaccine can take weeks, which is also why there is a lag before protection is achieved.

The finding is significant because Government vaccine advisers have recommended that the second dose can be delayed up to 12 weeks in a bid to get more people protected quickly.

Britain came under international criticism, including from the World Health Organization (WHO), for the move because the original vaccine trials did not specifically look at this dosing regimen.

Another significant finding in the Oxford study is that the vaccine is likely to significantly reduce transmission of the virus.

PCR swabs showed there was a 67 per cent reduction in positive tests in those who had been vaccinated – in another sign the UK’s inoculation gmable has paid off.

A positive PCR would signal that even someone who is vaccinated and immune to the disease is carrying fragments of the virus in their nose or throat which they could pass to others.

Reducing Covid’s spread is critical for country’s to achieve ‘herd immunity’, when so many people are immune that a disease peters out.

In the coming days, the Oxford researchers  hope to also report data regarding the new variants, and expect the findings to be broadly similar to those already reported by fellow vaccine developers.

Studies of approved jabs by Pfizer and Moderna have shown that the vaccines can stimulate a strong enough immune response to neutralise the South African and Brazilian variants.

But there are still question marks about how protective a single dose of either jabs will be effective against the worrying mutant strains.

Results from laboratory tests by Cambridge University published today found that a single shot of the Pfizer jab might not stimulate a strong enough immune response to kill the South African strain in over-80s.

Antibody levels only appeared to be protective after a second dose — but the researchers admitted the jab is still likely to be ‘less effective’ when dealing with the E484K mutation on the South African variant’s spike protein. 

Experts believe the mutation — which is also present in the Brazilian Covid variants — helps the strain ‘hide’ from the body’s natural defences. 

The news is particularly worrying as the UK Government has this week warned that cases of the South Africa variant are cropping up in people who haven’t travelled there, and the same E484K mutation is also appearing on unrelated cases caused by the Kent variant and even older versions of the virus.