Will... The original guidance on Astra was 4 weeks but trials here in the UK showed that increasing the period between the doses improved overall effectiveness. Doses of both Astra Zenica and Pfizer, the two in use here, are both given at 11-12 weeks interval on the basis that most of the protection comes from the first dose so, with limited supplies, it is better to get a single dose into more people than two doses into fewer people.