The Interaction Between Measles and DTP Vaccination

Many studies demonstrate that routine Expanded Program on Immunisation (EPI) vaccinations have "non-specific" or "heterologous" effects on child survival that cannot be accounted for by protection against the vaccine specific disease. These effects were overlooked in the past because it was assumed that the impact on survival would be proportional to reduction in the targeted infection. Large epidemiological studies in Bangladesh and Guinea-Bissau have shown that live vaccines, including measles vaccine (MV), have beneficial effects on overall mortality which cannot be explained by protection against the vaccine specific disease. Deleterious non-specific effects were shown convincingly in the high-titre MV trials in which girls, but not boys, had a two-fold increase in mortality even though the vaccine was fully protective against measles. A reinterpretation of these trials indicated that the most likely explanation was that the increased female mortality was due to inactivated vaccines provided after MV. It has further been shown in smaller observational studies that receiving diphtheria, tetanus and whole cell pertussis combined vaccine (DTwP) either with or after MV negates the beneficial effects of MV, and may lead to increased mortality. However, the increased mortality of girls in male-female twin pairs was not apparent when MV was given after the last dose of DTwP. In the present EPI schedule in The Gambia, DTwP is administered as a single dose vaccine at 2, 3 and 4 months, and MV at nine months. However, many children, particularly in low income countries, present late for their vaccines and may subsequently receive them in the wrong order. In particular DTP is often given at the wrong time point, e.g. the third DTP dose (DTP3) with MV. Given the increasing evidence that the order in which vaccines are administered is crucial, the routine practice of administering missed EPI vaccines at late presentation or boosting with DTP in year 2 may not be advisable.

The mechanism behind these effects is unknown, and whether there is an immunological basis has yet to be explored. The majority of controlled trials of vaccine efficacy have focused on vaccine specific antibody (Ab) responses to test vaccine efficacy, and few studies have investigated T cell memory induction, despite the fact that it is likely to be critical for long term protection. Indeed, the cellular response to MV seems to be crucial in protecting against severe disease and death, and cellular responses are required to protect against Bordetella pertussis infection. Live vaccines such as bacillus Calmette-Guerin (BCG) and MV have been shown to stimulate Th1 type immune responses. The immune response to DTwP, a killed vaccine, tends to be Th2 biased in early life, despite the fact that the whole cell pertussis component biases towards a Th1 response. DTwP contains an aluminium based adjuvant, aluminium hydroxide, which is excellent at promoting humoral and Th2 responses, but poor at generating good cell mediated immunity and long term T cell memory. This polarisation of cellular reactivity to live and killed vaccines may explain why administering a killed vaccine with a live vaccine abrogates the beneficial effect of the live vaccine. Conversely, administering a live vaccine after DTwP may cause a shift towards protective Th1 type immunity, thus diminishing the harmful effects of DTwP and providing a non-specific survival benefit. Interestingly, aluminium hydroxide has been shown to cause enhanced susceptibility to tuberculosis (TB) in animal models. Thus the aluminium adjuvant in DTwP may play a role in the deleterious effect of DTwP on live vaccines, possibly through an influence on the generation of T cell memory to other infections and vaccines.

Attributing the observed effects of the DTwP / MV interactions entirely to shifts in the T helper cell 1 (Th1) / Th2 profile is likely to be an over simplification, but provides a good starting point for unravelling this phenomenon. Other arms of the immune response that need to be considered are regulatory T cells (Tregs) and the Th17 inflammatory T cell lineage. The former are a heterogeneous group of naturally occurring and induced T cells, and have been shown to play a regulatory role in immunity to a number of infectious diseases, but almost nothing is known about their generation or functional role in vaccine immunogenicity in humans. Newborns are known to have high levels of functional Tregs, and these are likely to be essential in controlling the immune response to antigens encountered in early life, as well as modulating self reactive responses. Measles vaccination causes suppression of T cell responses, and measles vaccinated children upregulate Forkhead Box P3 (FOXP3) expression (Ota et al, personal communication), suggesting a role for Tregs. Thus Treg induction is likely to be an important component of the immune response to MV.

This study will investigate in detail the immunological consequences of giving DTP or MV alone or at the same time. The study will involve an intervention trial to analyse the effect of giving DTP with MV on the generation of T cell memory (effector and central), humoral responses, pro-inflammatory cytokine profile (Th1, Th2, Th17) and Treg responses to measles and recall antigens. Our detailed immunological studies will provide vital immunological data on potential mechanisms of the DTP / MV interaction. A comprehensive understanding of the immunological effects of administering either DTP alone or DTP and MV simultaneously will help us understand why these interventions might be detrimental, and may suggest the need to refine current EPI practices. It is also crucial to gain a detailed understanding of the immunological effects of possible deleterious interactions between the current EPI vaccines, before introducing new vaccines that are becoming available for testing in early life e.g. TB and malaria vaccines. Furthermore, understanding mechanisms whereby MV provides survival benefits might identify strategies that can be harnessed in future vaccine design.

All infants will be given a MV challenge at 18 months of age and immunological parameters will be analysed 1 month later. This will establish whether any vaccine group effects at 10 months off age persist until 18 months of age and influence the immune response to a new immune challenge.

Two primary hypotheses as detailed below will be tested as a starting point, but other immunological assays will be conducted. Furthermore all analyses will take sex into account since the heterologous effects of vaccines are different in males and females, with females generally being more susceptible.

Hypotheses:

1. DTP (a killed vaccine) drives primarily a Th2 response and MV (a live vaccine) drives primarily a Th1 response. A DTP driven Th2 response occurring at the time of trying to activate a Th1 response to MV will interfere with the priming of measles specific memory responses and responses to other recall antigens.
2. The extra Th2 stimulation from DTP at the time of MV will provide too many signals to allow the generation of regulatory T cells (Tregs) which are an essential component of the immunological response to MV.