In clinical trials, the timing of administration of COVID-19 Vaccine AstraZeneca ranged from approximately 4 weeks up to 26 weeks. In a post-hoc analysis, vaccine efficacy following the second dose of COVID-19 Vaccine AstraZeneca progressively increased with a longer interval between doses and appeared to be greatest when the interval was ≥12 weeks.
Short-term efficacy from 3 weeks after the first dose, before the second dose and up to 12 weeks post vaccination was about 73% (95% CI: 48.79–85.76).39Also refer to the Vaccine information section for further information. Short term efficacy from 22 days until 90 days after a single dose was 76% (95% CI: 59·3–85·9). The duration of protection after a single dose has not yet been established, and a second dose is recommended for optimal protection.
Shortening the interval from 12 weeks to no less than 4 weeks between doses is acceptable and may be appropriate in certain circumstances, for example, imminent travel or anticipated risk of COVID-19 exposure. If a dose has been inadvertently given before the minimum 4-week interval, it is not currently recommended that the vaccine dose be repeated.
If the second dose of COVID-19 Vaccine AstraZeneca is administered later than the recommended interval, no further doses are required.