The Vivaldi 2 study, which will test care home residents as well as care workers for Covid immunity, will be three times bigger than the original ongoing Vivaldi study.
It intends to give a detailed picture of coronavirus infection in care homes in England.
In the interest of giving the broadest study of English care homes, 340 facilities will now take part.
This will equate to 14,000 residents and staff being tested quarterly for immunity.
Researchers at University College London (UCL) said it will inform their understanding of immune response and the impact of Covid-19 on care homes.
Minister for Care Helen Whately said: “Expanding this brilliant study, with the support of UCL, is another step towards improving our understanding of the virus.
“Testing people’s antibody reaction to Covid-19 is crucial in helping us to control the spread of the virus, particularly amongst people who are vulnerable.
“The more we know about this virus and are able to control it, the safer it will be for people in care homes.”
Researchers hope to answer a number of questions, including whether individuals can be infected twice, how quickly neutralising antibodies wane and whether the immune response in the elderly is the same in younger generations.
Meanwhile, Laura Shallcross of UCL Institute of Health Informatics, said:
“Expanding the Vivaldi study will tell us how many people living and working in care homes have been infected with COVID-19, and whether being infected once protects against future infections.
“This study will help us protect the most vulnerable members of society from this devastating infection.”