The preliminary safety results may be correlated with stronger vaccination efficacy and broader protection. However, this analysis is based on self-reported descriptive symptoms and is not powered for statistical significance, said an analyst
Preliminary safety results from the Oxford-led phase-II Com-COV trial investigating immune responses in patients given heterologous prime-boost vaccine schedules indicate that mixing vaccines leads to higher reactogenicity and more frequent side effects, according to GlobalData.
Nancy Jaser, PharmD/MBA, Pharma Analyst, GlobalData, said, “The preliminary safety results may be correlated with stronger vaccination efficacy and broader protection. However, this analysis is based on self-reported descriptive symptoms and is not powered for statistical significance. The trial enrolled 830 participants aged 50 years and older, which may limit extrapolation of the results to other age groups.
“It is important to keep in mind that severe side effects, such as thrombosis, have not been identified, even in large phase-III trials because they are so rare; there is a chance that other adverse events won’t be observed until the mixed vaccine schedule is actually implemented.”
Com-COV investigators have noted that the higher reactogenicity observed when mixing vaccines is especially important when planning immunisation schedules of healthcare workers, as it may increase their need for time off work.
Jaser continued, “Unfortunately, the majority of healthcare workers are under 50 years of age and this study will not be able to capture the impact of heterologous vaccination in this population.”
A similar phase-II study in Spain, called Combivacs, recently announced positive preliminary data that slightly contradicts the Com-COV results, showing that the heterologous vaccine schedule is effective and safe.
Combivacs enrolled 670 participants between the ages of 18 and 59 who have already received one dose of AstraZeneca’s vaccine. The interim analysis consisted of 450 participants who were given the Pfizer vaccine as their second dose. Side effects such as headaches and muscle pain were reported in 1.7 per cent of this group vs 34 per cent reported from the Com-COV arm receiving the same heterologous schedule. More data is necessary to determine the reason for these differences.
If further data for mixing vaccines proves higher reactogenicity, it is likely that heterologous schedules will also lead to more severe reactions in younger populations. More data will be required to assess how severe the reactions can be, and if greater reactogenicity could be coupled to broader and/or longer protection from contracting COVID-19.
Jaser added, “Given the rare adverse events associated with Oxford-AstraZeneca’s COVID-19 vaccine, many countries are advising individuals initially primed with this vaccine to receive an alternate vaccine as their second dose. Com-COV and Com-COV2 will likely uncover the effects of mixing vaccines in adults 50 years and older. However, data in younger populations is now crucial as vaccine shortages continue to delay global pandemic recovery. Mixing vaccines, if determined safe and effective, can greatly enhance vaccine rollouts in countries where there are concerns over safety or shortages of the priming vaccine, helping ensure that these regions remain on a steady path towards herd immunity.”
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