The recommended interval between doses based on the currently available data from clinical trials is 21 to 28 days. WHO suggests that for mRNA vaccines the interval can be extended up to 12 weeks in exceptional circumstances in which countries that have not yet achieved high vaccine coverage rates in the high priority groups are experiencing a high incidence of COVID-19 cases combined with vaccine supply constraints. For the Oxford-AstraZeneca vaccine in light of the observation that two-dose efficacy and immunogenicity increase with a longer interdose interval, WHO recommends an interval of 8 to 12 weeks between the doses.
There is no maximum limit to receive the second dose – as with other vaccines, to maximize protection it is best to get both doses within the recommended time interval. It is never too late to get the second dose if a delay cannot be avoided.