Evidence suggesting an initial half dose of the vaccine being developed by drugs firm AstraZeneca and the University of Oxford is more effective than a full dose is counterintuitive, and even took the researchers by surprise.  

Why would less be better than more when it comes to triggering an immune response?

Andrew Pollard, the director of the Oxford Vaccine Group, described the findings from the Phase 3 clinical trial as "intriguing".

They showed that the vaccine had an efficacy of 62 percent among the people given two full doses a month apart.  But this rose to 90 percent for another group who received a half-dose first and then a full dose after a month.

Sarah Gilbert, professor at Oxford's Nuffield Department of Medicine, said the better result with a smaller initial dose could be because this better "mimics what happens in a real infection".

Essentially a vaccine uses a safe method to trick the immune system into believing it is dealing with a dangerous infection, triggering an immune response and an immune memory that can activate if the body comes across the real pathogen.

The Astra/Oxford vaccine employs what is known as a "viral vector", using engineered viruses to deliver genetic cargo into cells, giving them instructions on how to fight SARS-CoV-2.  The strategy uses the transporting virus as a "Trojan Horse".

The head of AstraZeneca in France, Olivier Nataf, in an interview with AFP called the development an opportunity cause it will require fewer doses to vaccinate more people. He also said the vaccine offers 100 percent protection against the occurrence of severe forms of illness and hospitalisations.  Finally, he said, there is greater simplicity in storage, transport, handling, under normal refrigeration conditions of 2 to 8 degrees. And the price of the vaccine is less than most of its rivals: around 2.50 euros ($3) per dose.