No comparison with a messenger RNA vaccine, and one trial in about 3000 adults which did not show any added clinical benefit compared to covid-19 vaccine ChAdOxl-S (a viral vector vaccine). As of early 2023, the available messenger RNA (mRNA) vaccines have proven efficacy in reducing serious forms of covid-19 in adults (at least against the variants circulating up to summer 2022), and with acceptable adverse effects. The viral vector vaccines (including covid-19 vaccine ChAdOxl-S) carry a risk of rare but serious thrombotic events. The clinical evaluation of vaccines based on the Spike protein + one adjuvant is less extensive, particularly against the Omicron variant (1-4).
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