The MHRA Information for Healthcare Professionals states that the vaccine is administered intramuscularly, preferably into the deltoid muscle. The vaccination course consists of two separate doses administered between 4 and 12 weeks after the first dose.
Interval between doses is longer than recommended
If it has been more than the recommended interval between the two doses (maximum interval not quantified), Public Health England advise that the second dose should still be given. The course does not need to be restarted.
Interval between doses is less than recommended
If the second dose has been given at less than the recommended 28 day interval, but at least 21 days after the first dose, Public Health England advise that the dose does not need to be repeated.
If the second dose is given less than 21 days after the first, it should be discounted and another dose (a third dose) should be given at least 28 days after the second dose (i.e. the dose that was given too early). If this situation arises, the third dose cannot be supplied under the Patient Group Direction (PGD) or the National Protocol for this vaccine. Instead a prescription or a Patient Specific Direction (PSD) would be required to enable the administration of a third dose. You can find more information about PSDs on our website.
The same brand of vaccine is unavailable for the second dose
The MHRA Information for Healthcare Professionals states that there are no data available on the use of COVID-19 Vaccine AstraZeneca in persons that have previously received a full or partial vaccine series with another COVID-19 vaccine. It is recommended that individuals who receive a first dose of COVID-19 Vaccine AstraZeneca complete the vaccination course with COVID-19 Vaccine AstraZeneca.
The Public Health England’s Immunisation Against Infectious Diseases (The Green Book) states that there is no evidence on the interchangeability of the COVID-19 vaccines although studies are underway. Therefore, every effort should be made to determine which vaccine the individual received and to complete with the same vaccine. For individuals who started the schedule and who attend for vaccination at a site where the same vaccine is not available, or if the first product received is unknown, it is reasonable to offer one dose of the locally available product to complete the schedule. This option is preferred if the individual is likely to be at immediate high risk or is considered unlikely to attend again. In these circumstances, as both the vaccines are based on the spike protein, it is likely the second dose will help to boost the response to the first dose. For this reason, until additional information becomes available, further doses would not then be required.