A single-blind, randomised, phase II UK multi-centre study to determine reactogenicity and immunogenicity of booster vaccination against ancestral and novel variants of SARS-CoV-2 (COV-BOOST)

The COV-BOOST trial is giving thousands of UK volunteers who received two doses of either the Pfizer or Oxford/AstraZeneca vaccine a third booster shot, including experimental jabs designed to target new coronavirus variants. This matters because immunity from the initial two-dose regimen wanes over time, and new variants such as Delta and Omicron can partially evade the original vaccines. While early studies tested mixing different vaccine types for the first two doses, no large-scale trial had yet established which booster—and for whom—is safest and most effective when given months after the primary course. If successful, the trial will provide the evidence needed for the NHS to roll out annual or seasonal booster programmes with confidence. It will tell policymakers which vaccine combinations produce the strongest and broadest immune response, and whether variant-specific boosters offer meaningful extra protection against emerging strains. This directly affects public health planning, vaccine procurement, and the logistics of delivering millions of booster doses each year. The results will also inform global vaccination strategies, helping other countries decide how to deploy limited booster supplies.