Scientists must develop the next generation of COVID-19 vaccines now, if the world is to meet the challenge of SARS-CoV-2 variants and reduce vaccine inequity by increasing global supply. This can be done only if comparator COVID-19 vaccines — those that have already been approved — are available to support clinical trials. Such comparator vaccines are almost impossible to secure; governments, developers and manufacturers must find a solution to unlock supplies.

So far, COVID-19 vaccines have received approval on the basis of data from unvaccinated participants in placebo-controlled efficacy trials. These trials become increasingly difficult to carry out as the number of people who are immunized rises. Comparator vaccines, essentially replacing placebos, are therefore needed for trials that assess whether new candidate vaccines provide comparable levels of protection, including against emerging variants.

The number of comparator-vaccine doses needed to support clinical trials is small. However, contracts between manufacturers and governments for approved vaccines restrict their use to improving public health. This must change if vital COVID-19 vaccine research and development are to progress.

Competing Interests

M.S. holds shares in Sanofi, a vaccine company, and is also an inventor for patents for non-COVID vaccines.

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