A study published in May showed that combining these vaccines is safe, though mixed regimens did lead to greater side effects. In this paper, the researchers provide data on the immune response in 463 people who got their shots four weeks apart. The team is also testing a 12-week interval. All of the regimens seemed to prompt strong immune responses. The combination regimens prompted stronger responses than a double dose of Oxford-AstraZeneca. In fact, the group that received Oxford-AstraZeneca followed by Pfizer-BioNTech had antibody levels nine-fold higher than the group that got a double dose of Oxford-AstraZeneca. But a double dose of Pfizer-BioNTech outperformed every other regimen, including the mixed regimen that offered Pfizer-BioNTech first.
The caveats: In this study, two doses of the Oxford-AstraZeneca vaccine elicited the lowest antibody levels. But Snape points out that study tested a four-week interval between doses. That’s not the way the vaccine is being administered. Doses are typically spaced 8 to 12 weeks apart. And studies suggest that even longer intervals might work better. In a new preprint researchers found the highest antibody levels with a delay of 45 weeks. That may be why the standard Oxford-AstraZeneca regimen appeared to elicit the weakest response.
What’s next: The team hopes to present data on the groups that had a 12-week gap between doses in the next month or so. And they’ll also be assessing the duration of the response and examining combinations involving two more vaccines: Moderna and Novavax. These data could help health officials decide which vaccines should be offered as a booster, if one is needed. “Based on what we’ve seen here, there is a suggestion that switching over to an RNA vaccine could actually have some benefits in terms of antibodies,” Snape says.