Low dose of AstraZeneca may be key to success We’re sorry, this service is currently unavailable. Please try again later. Dismiss January 16, 2021 — 12.04am Normal text size Credit: Your writer mentions the mystery of the 90 per cent efficacy rate for those people who accidently received a low dose in the AstraZeneca trial, followed by a standard dose ("Gambling on the right vaccine", January 15). No matter why it happens, why not adopt this as standard immunisation practice rather than implement a procedure with only 65 per cent efficacy? Diana Wyndham, North Sydney Reluctance to adopt the AstraZeneca vaccine compared with Pfizer vaccine centres around the efficacy disparity - 65 per cent versus 90 per cent in the latter (''Comparing virus shots is tricky, especially at speed'', January 15). Although not yet fully understood, a half dose of the AstraZeneca given first by mistake followed by a full dose produced 90 per cent efficacy in phase three of the trial. Why wouldn't we go with that strategy given that this is an issue about efficacy, not safety?