The Public Health England’s Immunisation Against Infectious Disease (The Green book) states that it is operationally recommended that the second dose of all vaccines should be routinely scheduled between four and 12 weeks after the first dose. This will reduce confusion and allow more people to benefit from the protection provided from the first dose during the roll out phase. Longer term protection will then be provided by the second dose.
Interval between doses is longer than recommended
The Public Health England’s Immunisation Against Infectious Disease (The Green book) states that If an interval longer than the recommended interval is left between doses, the second dose should still be given (preferably using the same vaccine as was given for the first dose if possible). The course does not need to be restarted.
The same brand of vaccine is unavailable for the second dose
The Public Health England’s Immunisation Against Infectious Disease (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 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.