In effect, participants in the trial received too little vaccine. While the too-small dose generated adequate levels of neutralizing antibodies in adults aged 18 to 49 in the trial, adults 60 and older generated lower levels of neutralizing antibodies than are seen in the blood of people who have recovered from Covid-19 infection, Ng said.

Perplexed by the findings, the company set out to figure out why the vaccine underperformed. It discovered two commercial reagents used to measure how much antigen — active vaccine — was included in each dose were giving false readings. In fact, Ng said, the concentration of the antigen “was insufficient.”

A revised formulation, tested recently in non-human primates, showed the vaccine prevented damage in the lungs, which is what triggers severe Covid disease in people, and led to rapid clearance of the virus.