On Monday scientists revealed that the Oxford vaccine had an overall efficacy of 70%, but could be around 90% effective when administered as a half dose followed by a full dose a month later.

“The reason we had the half dose is serendipity,” said Mene Pangalos, executive vice-president of biopharmaceuticals research and development at AstraZeneca.

When university researchers were distributing the vaccine at the end of April, around the start of Oxford and AstraZeneca’s partnership, they noticed expected side effects such as fatigue, headaches or arm aches were milder than expected.

“So we went back and checked … and we found out that they had underpredicted the dose of the vaccine by half,” said Pangalos.