The National Travel Health Network and Centre have issued guidance on how to manage interrupted vaccination schedules.
Revision of the immune response which occurs following vaccination
A vaccine contains one or more weakened, inactivated or engineered agents (antigens) that resemble the disease-causing organism(s). In someone who has a healthy immune system, vaccination induces an immune response to the antigen in the vaccine resulting in antibody production and protection, without causing the disease. The immune system remembers the antigen using memory cells and this enables the immune system to recognise and quickly respond when exposed to natural infections.
Some vaccines can induce an immune response after one dose, however, this can be short lasting (i.e.) in the case of typhoid vaccine, some give long-lasting immunity (yellow fever). For others, a primary course of vaccine, requiring multiple doses given over a specified period is required (e.g., hepatitis B, Japanese encephalitis, oral typhoid, poliomyelitis, rabies), with a re-enforcing (‘booster’) dose recommended at a defined period following the primary course to maintain immunity (where the traveller continues to be at risk).
Clinical studies following vaccine development determine which vaccine schedule (interval to reinforcing dose) offers the best chance for immunity to develop. Recommended schedules should be adhered to wherever possible.
A course of vaccine may require multiple doses over a specified time period and each dose is important. The first vaccine in a multiple dose schedule begins to prime the immune system, however, further doses given at the recommended intervals are necessary for immunity to develop.
If the individual still requires protection, start where the course was left. Immune memory already laid down by previous dose(s) of a vaccine means that repeating doses or re-starting a course of vaccine is rarely necessary. For most vaccines, where a record exists of previous doses, start where the course was left and complete the course, observing the same interval between future doses as indicated by the manufacturer.
The effectiveness of vaccines given outside the recommended schedule may not have been evaluated in clinical studies.
Where doses are given at less than the recommended interval, the immune response may be impaired. Where possible, avoid shortening the interval unless a recognised rapid schedule is recommended by the manufacturer or the relevant chapter in immunisation against infectious disease [1, 5]. If vaccine has been administered with a reduced interval between doses, is it advisable to seek further support on a case-by-case basis.
For vaccines given according to the UK schedule, further guidance about schedule flexibility is provided in chapter 11 of immunisation against infectious disease, the ‘Green book’.
In some circumstances, such as where the effect of interrupted doses in a schedule may not be clear (i.e. oral typhoid vaccine or oral cholera vaccine – Dukoral) re-starting a course may be recommended. Additionally, where there is no or inadequate documentation of previous doses, re-starting a course of vaccine may be an option after individual risk assessment to be certain that a person is protected.