May 14, 2021

Early data suggest mixing COVID-19 vaccines increases reports of side effects

Preliminary results from a trial testing a heterologous prime-boost approach to COVID-19 vaccination indicate that volunteers who received one dose of ChAdOx1 nCoV-19 (ChAd) COVID-19 vaccine (Vaxzevria, AstraZeneca) followed by BNT162b2 (BNT) COVID-19 vaccine (Comirnaty, Pfizer-BioNTech), or vice versa, were more likely to develop mild-to-moderate reactions, compared to those who got the same vaccine for both prime and boost.

Findings from the Com-COV trial, which were published in The Lancet as correspondence, focused only on initial reactogenicity and safety data. "It is reassuring that all reactogenicity symptoms were short-lived, and there were no concerns from the limited haematology and biochemistry data available," said Matthew D Snape and colleagues from the Oxford Vaccine Group, University of Oxford, Oxford, UK.

The data consisted of self-reported solicited local and systemic symptoms collected in the 7 days after both prime and boost vaccination in a total of 830 participants aged 50 years and older. Volunteers were randomised into cohorts to test combinations of ChAdOx1 nCoV-19 and BNT162b2 given as two doses, either four weeks or 12 weeks apart. The groups consisted of those who received ChAd-ChAd; ChAd-BNT; BNT-BNT; and BNT-ChAd.

Both heterologous vaccine schedules triggered greater systemic reactogenicity following the boost dose than their homologous counterparts. Feverishness was reported by 34% of participants given ChAd-BNT, compared with 10% for those who got ChAd for both prime and boost (difference 24%, 95% CI 13–35%). Among those given BNT-ChAd, the rate of feverishness was 41%, compared with 21% for people who got BNT both times (difference 21%, 95% CI 8–33%).

Researchers said similar increases were observed for chills, fatigue, headache, joint pain, malaise and muscle ache, with most of this increase in reactions occurring in the 48 hours after immunisation. They noted that paracetamol use in the 48 hours post-boost mirrored the reactogenicity pattern.

Meanwhile, there were no hospitalisations due to solicited symptoms, and no cases of thrombocytopenia in any group at day 7 post-boost. Haematology and biochemistry profiles were also similar between mixed and non-mixed vaccine schedules.

Researchers pointed out that as these data were obtained in older participants, "reactogenicity might be higher in younger age groups, for whom a mixed vaccination schedule is being advocated in Germany, France, Sweden, Norway, and Denmark." Those recommendations target younger people who have already received ChAd as a first dose, in light of concerns about thrombotic thrombocytopenia seen with AstraZeneca's vaccine.

"Pending availability of a more complete safety dataset and immunogenicity results for heterologous prime-boost schedules (to be reported shortly), these data suggest that the two heterologous vaccine schedules in this trial might have some short-term disadvantages," the authors said, noting that these could be mitigated with routine prophylactic use of paracetamol. Meanwhile, ongoing studies evaluating heterologous prime-boost schedules, incorporating vaccines by Moderna and Novavax, will be "crucial to informing the appropriateness of mixed COVID-19 vaccine schedules," they added.

SOURCE: The Lancet
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