Young infants are most vulnerable to severe disease and even death when infected with Bordetella Pertussis. The current vaccines and vaccination programs do not guarantee protection of neonates. During the last weeks of pregnancy, maternal IgG antibodies are transferred actively to the fetus. Administration of a pertussis containing vaccine during pregnancy offers protection through high titers of maternal antibodies transferred to the child. Since transplacental transport is immature, infants who are born prior to 37 weeks of gestation, might be vulnerable to pertussis infection even though maternal vaccination was administered, but specific data are lacking. The primary aim of this observational study is to measure whether vaccination during pregnancy offers protection to preterm born infants through higher titers of maternal antibodies, despite immature transplacental transport. Four cohorts of mother-infant pairs will be recruited: term versus preterm born infants, born from either vaccinated women or not vaccinated women. These mother-infant pairs are recruited according to the vaccination status of the mother and to the gestational age at delivery. Pertussis specific antibody titers (anti-Pertussis Toxin, anti-Filamentous haemagglutinin, anti-Pertactin titers) will be monitored in blood samples of the mothers at delivery to measure the possible influence of both gestational age and maternal vaccination status. In order to measure the decline of maternal antibodies in the first weeks of life, blood will be taken from cords as well as from infants at 8 weeks of age, before the first infant pertussis vaccine is administered.

Pertussis antibodies to the same antigens will be measured in all infants after a primary series of acellular pertussis vaccines administered at 8,12 and 16 weeks of age and before and after a booster dose in the second year of life.

In addition, cellular mediated immune responses will be evaluated in a subgroup of infants before and after a primary series of infants vaccines. A last goal is to measure whether vaccination during pregnancy could offer additional maternal antibodies through breast milk. Again a comparison is made between preterm and term born infants, born from either vaccinated or unvaccinated women. The amount of lactoferrin and pertussis toxin specific IgA in breast milk samples will be measured in samples taken at birth (colostrum), and at several time points afterwards as long as breastfeeding is continued.

Inclusion Criteria for participating women:

• Pregnancy
• Signed informed consent
• Intend to be available for follow-up visits and phone call access through 16 months following delivery
• Willing to have infant immunized with hexavalent vaccine according to the recommended Belgian schedule

Exclusion Criteria for participating women:

• Significant mental illness (e.g. schizophrenia, psychosis, major depression)
• Serious underlying immunological condition (e.g. immunosuppressive disease or therapy, human immunodeficiency virus (HIV) infection)
• Anything in the opinion of the investigator that would prevent volunteers from completing the study or put the volunteer at risk
• Receipt of a blood product or experimental medicine within 4 weeks prior to delivery
• Multiple pregnancies

Exclusion Criteria for children:

• No signed informed consent from both parents
• Severe reactions to any vaccine
• Serious underlying medical condition (e.g., genetic disorder (eg Down syndrome), immunosuppressive disease or therapy, human immunodeficiency virus (HIV) infection, lung/heart disease, liver/kidney disease, chronic or recurrent infections)
• Children suffering from primary humoral immune disorders (B cell related): severe X linked agammaglobulinaemia, CVID (Common variable immunodeficiency, late onset agammaglobulnaemia) and SAD (specific antibody deficiency); suffering from primary cellular immune deficiencies (T cell related): SCID (Severe combined immune deficiency syndrome), CID, hyper IGM syndrome, di George's syndrome and others; suffering from disorders in phagocytosis and chemotaxis (CGD, Schwach Diamond syndrome) and disorders from the complement cascade
• In addition children with oncologic disorders will be excluded. All these children can receive the inactivated pertussis vaccines, but will respond different from the normal population to vaccination.
• Anything in the opinion of the investigator that would prevent volunteers from completing the study or put the volunteer at risk.

Four cohorts of women-infant pairs will be recruited before or directly after delivery.

A cohort of preterm born infants will be compared to term born infants, and both will be compared within groups of vaccinated women (adult acellular pertussis containing vaccine) or unvaccinated women.