Mixing Covid vaccines offers strong immune protection – study
Having different Covid vaccines for first and second shots produces a strong immune response to the virus, according to research that will help improve the resilience of vaccine programmes around the world.
Scientists in Oxford looked at the impact of a mix-and-match approach to vaccinations where people were given either the standard two shots of Oxford/AstraZeneca or Pfizer/BioNTech vaccine, or a combination of the two.
The study found marked differences in antibody levels against the virus depending on the shots given. While two doses of Pfizer produced the highest levels of antibodies, one shot of Oxford vaccine followed by a Pfizer booster was nearly as potent.
Other combinations were not as effective. Those who had a Pfizer shot followed by an Oxford booster had antibody levels nearly seven times lower than those who had two shots of Pfizer, though this was still five times higher than the antibody levels recorded in people who had two shots of AstraZeneca.
“You’ve certainly got an extra kick to your immune system if you boost with the RNA vaccine from Pfizer rather than a booster dose of the AstraZeneca vaccine,” said Matthew Snape, the chief investigator on the Com-Cov trial and associate professor in paediatrics and vaccinology at Oxford.
The findings were released as Prof Sarah Gilbert and colleagues from Oxford received a standing ovation at Wimbledon, where they had been invited to the royal box for the first match on centre court.
Ministers are considering whether to offer different vaccines as boosters in the autumn and, based on the Com-Cov findings, following Oxford/AstraZeneca shots with a Pfizer booster may be an effective strategy. “Based on what we’re seeing here, switching over to an RNA vaccine could have some benefits in terms of antibodies,” said Snape.
The Com-Cov trial was designed to see whether mixing Covid vaccines – in the event of a supply shortage, for example – would provide as much protection against the disease as having two doses of the same vaccine.
Based on the findings in a Lancet preprint, the scientists believe that any combination of the Pfizer and Oxford vaccines provides a stronger immune response than two doses of the Oxford vaccine, meaning that if the UK or other countries need to mix vaccines, protection should still be substantial.
Immune responses were measured in 463 people a month after they were given different combinations of the vaccines four weeks apart. Further data from the study, expected next month, will look at the immune responses when the vaccines are given 12 weeks apart. In the UK, most Covid vaccines are given eight to 12 weeks apart.
The study went on to examine T cell immunity. While antibodies stick to the virus and stop it infecting cells, T cells find and destroy infected cells. Mixing the vaccines had a significant impact, with the strongest T cell activity seen in people who had the Oxford jab followed by a Pfizer booster. The T cell response was half as potent when the Pfizer jab was boosted by the Oxford vaccine, and lower still when both jabs were the same.
Another Oxford study that has yet to be peer-reviewed found that a third shot of the Oxford/AstraZeneca vaccine could work as an effective booster. A third dose given more than six months after the second led to a substantial rise in antibodies and increased the body’s T-cell ability to fight coronavirus, including its variants, scientists found.
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.
The data also shows that a prolonged interval of up to 45 weeks between the first and second dose can also be beneficial, resulting in a significantly higher antibody response 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.
“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.”