The large UK studies need diverse groups of volunteers to check if the jabs will work for all populations.
But of the 270,000 already recruited, only 7% are people belonging to ethnic minorities, who are at higher risk of complications if they develop Covid-19.
Elderly people are also vulnerable - and researchers want more over-65s to volunteer for vaccine trials too.
These groups would be among the first offered a vaccine if one becomes available.
Scientists hope to recruit 500,000 volunteers in total for the studies, including the Oxford trial recently resumed.
Oxford Vaccine Group principal investigator Dr Maheshi Ramasamy: "We know that people from black, Asian and minority ethnic backgrounds are disproportionately affected by Covid in terms of severe disease and mortality.
"So when we do have a vaccine that we roll out to the general population, it's really important that we can demonstrate to people from these communities that we have evidence that the vaccine works."
The UK has secured access to six different possible vaccines, which fall into four categories:
* adenoviral vaccines (Oxford/AstraZeneca, Janssen)
* inactivated whole virus vaccines (Valneva)
* protein adjuvant vaccines (GSK/Sanofi, Novavax)
Equalities Minister Kemi Badenoch, who is herself volunteering, said: "The UK is leading the world in the search for a Covid-19 vaccine.
"At home, we have to ensure every community trusts a future vaccine to be sage and that it works across the entire population.
"But with less than 0.5% of people on the NHS Vaccine Registry from a black background, we have a lot more work to do.
"That is why I am urging more people from the ethnic-minority backgrounds to join me in signing up to the NHS Vaccine Registry and taking part in a trial.
"Together, we can be part of the national effort to end this pandemic for good."
Kate Bingham, who chairs the government's Vaccine Taskforce, said: "The only way to check how well a coronavirus vaccine works is to carry out large-scale clinical trials involving thousands of people.
"Researchers need data from different communities and different people to improve understanding of the vaccines.
"The only way to get this is through large clinical trials."
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