[nudephil (OP)]

John asks:

I am 70 and expecting to get COVID-19 vaccine, probably the Oxford/AstraZeneca one. However, the government is extending the dosage interval for the Pfizer vaccine. I understand their reasoning, but I am concerned that this can't be extrapolated to the Oxford vaccine, as a dosage error improved its efficacy significantly. Do you know if it's being proposed?

[Colin2B]

The Pfizer vaccine trials did not compare different dose spacing or compare one with two doses. So there is no evidence for the wider spacing between doses.
The Oxford-AstraZeneca trials did include different spacing between doses. It showed that a longer gap of two to three months led to a greater immune response, but the overall participant numbers were small.
They found that vaccine efficacy 14 days after a second dose was higher in the group that had more than six weeks between the two doses (65.4%) than in the group that had less than six weeks between doses (53.4%).
Press reports have focussed more on the relative size of the doses rather than the spacing. Given the shortage of vaccine the decision has been made to go with the wider spacing and give as many people as possible their first dose.
Because max immunity doesn’t occur until after the second dose it is important that those vaccinated with the first dose, and those in contact with them, should continue to take mask and distancing precautions.

[set fair]

A longer interval between first vaccination and booster virtually always gives a better immune response.

[Petrochemicals]

I think the single astra zenica is similar the published 2 dose efficacy of 70%. Probably one of the reasons the government opted for one dose.