Is the production of a Covid-19 vaccine using transformed Pasteurella plausible?

While a viable method for the production of a Covid-19 vaccine is of huge interest at the moment, and it is important to adopt the experience and lessons from prophylactic vaccination in veterinary medicine, we feel that some aspects of the underlying science put forward in these letters require supporting evidence and further clarification to demonstrate they are plausible. Are the authors suggesting that orally administered inactivated P multocida cells expressing severe acute respiratory syndrome coronavirus 2 proteins will not be digested in the human stomach and will induce virus-neutralising systemic antibodies? [...]in the field vaccination of 9000 birds that Myint and Jones reported to be a success, was there a controlled trial comparing unvaccinated birds and birds vaccinated with untransformed P multocida with those vaccinated with P multocida expressing viral antigens?