Mixing Covid jabs

The Oxford-led Com-Cov study is exploring the safety and efficacy of mixed-dose schedules given that they are being considered in several countries – including the UK – to fortify vaccine rollout programmes that are dependent on unstable vaccine supplies.

The trial involves 830 participants aged 50 and over, some of whom have underlying conditions. It is testing four combinations: Oxford/AstraZeneca + Oxford/AstraZeneca; Oxford/AstraZeneca + Pfizer/BioNTech; Pfizer/BioNTech + Pfizer/BioNTech; and Pfizer/BioNTech + Oxford/AstraZeneca.

Overall, both mixed-dose schedules triggered more side-effects. For instance, feverishness was reported by 34% of patients who got Oxford/AstraZeneca followed by Pfizer/BioNTech; and 41% of those given Pfizer/BioNTech followed by Oxford/AstraZeneca. Meanwhile, only 10% reported feeling feverish in the group who received both Oxford/AstraZeneca doses; and 21% reported the symptom in the group that received two Pfizer/BioNTech doses.

Similar differences were observed for chills, fatigue, headache, joint pain, malaise, and muscle ache. Overall, any adverse reactions that cropped up were short-lived and there were no other safety concerns, according to data published in the form of a letter in the journal the Lancet.

Although trial participants were aged 50 and above, real-world data suggests that younger age groups tend to have stronger reactions to vaccines, he noted.  “Reactions often relate to the stimulating of the innate immune response, so … that fundamental part of the immune response that sends an inflammatory signal – whether or not this will relate to actually improved immune response, we don’t know yet,” Snape said.

Data on the efficacy of these mixed-schedule regimens in inducing an immune response is expected in the coming weeks. Trial investigators are also evaluating the impact of dosing participants 12 weeks apart.