The membrane cells of chicken eggs (the egg itself is of course one cell). When I last looked at this, during the swine flu pandemic, companies typically get 3

The membrane cells of chicken eggs (the egg itself is of course one cell). When I last looked at this, during the swine flu pandemic, companies typically get 3 doses of antigen per egg. And this is a very slow process for new strains, first a seed culture has to be created by serious specialists that will both grow well in eggs (can be hard for avian flu...) and expresses the correct surface antigens. Much better approaches like this company's https://en.wikipedia.org/wiki/Protein_Sciences https://en.wikipedia.org/wiki/Protein_Sciences are desperately needed if a killer flu is someday brewed (this tends to happen in an animal that is simultaneously infected with two strains of flu, so it's not mutation per se).