A third shot of the Oxford/AstraZeneca vaccine could be an effective booster jab without the need for tweaks, research suggests.
An Oxford University study found that giving people a third dose more than six months after their second led to a substantial rise in antibodies and increased the body’s T-cell ability to fight coronavirus, including its variants.
Prof Sir Andrew Pollard, the head of the Oxford Vaccine Group, said it was not yet known whether people would need a booster shot in autumn but the new data showed the existing vaccine could be effective.
He said real-world data from Public Health England (PHE) had already shown that two doses offered good protection against hospital admission and death from the Alpha and Delta variants.
The same study showed that coronavirus antibodies induced by the Oxford/AstraZeneca vaccine remain elevated for at least a year after the first dose, in a boost for countries where vaccine supply is limited and authorities are struggling to keep pace with their vaccine deployments.
The data, presented in a paper that is still to be peer-reviewed, shows that although antibody levels erode with a longer gap between the first and second doses – they remain above the baseline. Data also shows that a prolonged interval of up to 45 weeks between the first and second dose can be beneficial, resulting in a significantly higher antibody response after the second dose.
The recommended gap is between four to 12 weeks, depending on geography, but vaccine supply hiccups have resulted in some people not receiving the vaccine in this timeframe.
Although antibody levels alone do not predict vaccine effectiveness, they are a key component of our immune response.
Previous research has shown protection against symptomatic Covid is maintained after a single dose for at least three months, despite some waning of antibody levels. So this fresh data – based on a small group of 30 participants who got late doses – provides further reassurance that a delay in second dose will not compromise the level of protection obtained after the second dose.
“We have countries facing future waves of disease at this moment with a largely unvaccinated population. That is a situation where getting the first dose into more people – as soon as possible – is the most urgent priority, and certainly before third doses are given,” said Pollard, director of the Oxford Vaccine Group, department of paediatrics at the University of Oxford.
“We should be trying to make sure all those vulnerable people – the older adults, those with other health conditions, and all around the world are protected.”
The research was initially designed such that the timing of the second dose was varied to observe the persistence of immune responses after the first dose. Then, in 90 participants who had received the second dose, the researchers added on a third dose to assess the immune response.
They found that a third dose of the AstraZeneca vaccine – given more than six months after the second dose – lead to a substantial rise in antibodies and a strong boost to the immune response against the virus, including against the variants Alpha, Beta and Delta.
Data from Public Health England has shown that two doses of the vaccines being deployed in the UK are enough to prevent more than 90% of hospitalisation admissions, noted Pollard.
“Whether three doses would add another few per cent on to that is difficult to say. But I don’t think we have any evidence that that’s the case at the moment,” he said. “Boosters are much more about if protection gets lost over time – and we don’t know that – but if it does, could you boost? And the answer to that … is yes you could.”
Prof Danny Altmann, an expert in immunology of infectious disease at Imperial College London, who was not involved in the research, said the data comes at a time when there has been much concern over evolution of vaccine protocols racing ahead of the evidence base from controlled studies.
This paper addresses some key questions, he noted. “Is an extended interval between the first and second dose a plus or a minus for vaccine response? Is there really any further benefit from a third dose, or does the response plateau after two? Does repeated vaccination lead to suppressed responses through inhibitor antibodies to the vector?”
“The answers on each count are encouraging. A big question now is whether these answers will extrapolate to the mRNA vaccines.”