How did a dosing error lead to Oxford vaccine’s 90% effectiveness?

Oxford University’s COVID-19 vaccine reached 90% efficacy by accident after some trial participants mistakenly got half dose in the first shot.

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.

Researchers administered the second, full dose booster shot at the scheduled time for these participants, who experienced milder side effects than expected. The efficacy was 62% in the group who received two full doses as per the original plan

While they were administering doses to trial participants in Britain. They soon noticed expected side effects such as fatigue, headaches or arm aches were milder than expected, he said.

So, they went back and checked  and found that they had underpredicted the dose of the vaccine by half

The company decided to continue with the half dose and administer the full dose booster shot at the scheduled time.

Scientists said they still could not fully explain why the half dose gave better protection, but said it may be that it triggers the immune system differently.

Prof Sarah Gilbert from Oxford University, who led the research, said: “It could be that by giving a small amount of the vaccine to start with and following up with a big amount, that’s a better way of kicking the immune system into action and giving us the strongest immune response and the most effective immune response.”