Astra Zeneca Oxford vaccine initial efficacy results have been released. They trialled two dosing methods, one with 1/2 strength dose then full strength a month later, the other with two full strength doses > month apart.. The first method worked better ~ 90% efficacy vs 62%.
More information here:
https://www.astrazeneca.com/content/astraz/media-centre/press-releases/2020/azd1222hlr.html
(Moderna and Pfizer vaccines are also 2 shots).