The firms' phase I/II study interim results showed an immune response comparable with patients who recovered from COVID-19 in adults ages 18 to 49, but a low immune response in older adults. That's likely due to an insufficient concentration of the antigen, the companies said.

However, a challenge study in primates using an improved antigen formula demonstrated the vaccine candidate could protect against lung pathology and lead to rapid viral clearance from the nasal passages and lungs within two to four days.

The companies are planning a phase IIB study, which is slated to start in February 2021 and has support from the Biomedical Advanced Research and Development Authority, part of the U.S. Health and Human Services Office of the Assistant Secretary for Preparedness and Response. The study will compare an authorized COVID-19 vaccine with the one the firms are developing. If data are positive, a global phase III study could start in the second quarter of 2021.

Positive results from the study would lead to regulatory submissions in the second half of 2021, so the vaccine wouldn't be available until about the fourth quarter of 2021 as opposed to mid-2021 as the firms previously projected.