Newborns develop a Th1-type immune response to Mycobacterium bovis bacillus Calmette-Guérin vaccination

Data obtained in animals indicate that neonatal immune responses are biased toward Th2. This could reduce the efficacy of vaccines against viral and mycobacterial diseases. The ability of human newborns to develop a Th1 immune response upon immunization has not been studied. Since the vaccine Mycobacterium bovis bacillus Calmette-Guérin (BCG) triggers a Th1-type response in adults, we investigated whether it induces a similar response in newborns and whether age at vaccination influences immunogenicity. We found that BCG vaccination at birth induces a memory Th1-type response of similar magnitude to that when given later in life. This study demonstrates that human newborns can be immunized against pathogens controlled by a Th1 immune response.

Immunization with a pneumococcal capsular polysaccharide vaccine during pregnancy

The feasibility of preventing invasive pneumococcal infections during the first few months of life by immunization during pregnancy has been investigated. One hundred and fifty Gambian women were immunized with either a 23-valent pneumococcal polysaccharide vaccine or a meningococcal polysaccharide vaccine during the last trimester of pregnancy. Pregnant women showed a good antibody response to five of the six pneumococcal polysaccharides tested (types 1, 3, 5, 6, 14 and 19) but not to type 6 polysaccharide. Mean cord blood/maternal blood IgG antibody ratios varied from 24% (type 1) to 49% (type 3) and differed substantially between individual mother/infant pairs. Pneumococcal antibody levels were higher at birth in infants of women immunized with pneumococcal polysaccharide vaccine than in control infants. However, these antibodies disappeared rapidly during the first few months of life and it is uncertain how much clinical protection against pneumococcal infection maternal immunization would have provided.

Pilot trial of a pentavalent pneumococcal polysaccharide/protein conjugate vaccine in Gambian infants

BACKGROUND

Invasive pneumococcal disease is a major cause of mortality and morbidity in young children in developing countries. Pneumococcal polysaccharide/protein conjugate vaccines, which are likely to be immunogenic in the very young, offer a potential way for preventing these infections. Therefore a pilot safety and immunogenicity study of a five-valent conjugate vaccine has been undertaken in an area of rural Africa where invasive pneumococcal disease is prevalent.

METHODS

Thirty Gambian infants were vaccinated with 3 doses of a five-valent pneumococcal conjugate vaccine containing 5 micrograms of type 6B, 14, 18, 19F and 23F polysaccharides conjugated to the diphtheria toxin mutant protein CRM197 at the ages of 2, 3 and 4 months; 30 infants received 2 doses at the ages of 2 and 4 months and 30 infants who received three doses of a Haemophilus influenzae type b vaccine acted as controls. Local and systemic reactions were recorded after vaccination and antibody titers were measured by an enzyme-linked immunosorbent assay.

RESULTS

No serious local or systemic reactions to vaccination were recorded. Antibody responses to each component of the vaccine were demonstrated. One month after immunization with three doses of vaccine, antibody titers were 3 to 11 times higher than before vaccination (postvaccination titers ranged from 2.49 micrograms/ml for type 19 polysaccharide to 7.59 micrograms/ml for type 14). Elevated titers were well-maintained during the subsequent 4 months. Three doses of vaccine induced higher titers than did two doses. Antibody titers increased 2- to 3-fold over the period of immunization in children who received H. influenzae type b vaccine.

CONCLUSIONS

A five-valent pneumococcal conjugate vaccine proved safe and immunogenic in Gambian infants. However, a vaccine containing a larger number of serotypes will be necessary to achieve a maximal clinical impact.

Meningococcal antibody titres in infants of women immunised with meningococcal polysaccharide vaccine during pregnancy

Seventy five Gambian women were immunised with a single dose of a group A+group C meningococcal polysaccharide vaccine during the last trimester of pregnancy. IgG antibody titres were measured in mothers and in their infants by an enzyme-linked immunosorbent assay (ELISA). All women had a good response to vaccination and maternal antibodies were high at the time of delivery (23.2 micrograms/ml for group A antibodies and 14.3 micrograms/ml for group C antibodies). However, only a proportion of this antibody crossed the placenta; cord blood:maternal antibody ratios were 30% for group A antibody and 44% for group C antibody, respectively. Considerable variability in cord blood:maternal blood ratios was seen between individuals. This could not be related to age, parity, or ethnic group. Mean group A and group C cord blood:maternal blood ratios were lower in women with serological evidence of syphilis than in seronegative women, and diminished transfer of group A antibody was noted in women with active malarial infection of the placenta. Antibody titres declined rapidly in infants and by the age of 3-4 months these had reached control values. Maternal immunisation may give infants some protection against group A and group C meningococcal disease but only during the first few months of life.

A trial of a group A plus group C meningococcal polysaccharide-protein conjugate vaccine in African infants

The safety and immunogenicity of a group A plus group C meningococcal polysaccharide-CRM197 conjugate vaccine was evaluated in 304 8- to 10-week-old Gambian infants. Infants were immunized with one, two, or three doses of conjugate vaccine or with two doses of a meningococcal A plus C polysaccharide vaccine. The conjugate vaccine produced few systemic side effects, and local reactions were similar to those produced by the polysaccharide vaccine. Postvaccination group A meningococcal polysaccharide antibody levels, measured by ELISA, increased progressively after one, two, or three doses of conjugate vaccine. However, one dose of conjugate vaccine given at the age of 6 months induced a higher group C meningococcal antibody response than did two doses of conjugate vaccine given at 2 and 6 months. Two doses of conjugate vaccine induced higher levels of antibody than did two doses of polysaccharide vaccine. Thus, this new meningococcal conjugate vaccine proved to be safe and immunogenic.

Decline in meningococcal antibody levels in African children 5 years after vaccination and the lack of an effect of booster immunization

Antibodies to group A meningococcal polysaccharide were measured by hemagglutination (HA) and by ELISA in sera obtained from Gambian children before vaccination and 3 weeks, 2 years, and 5 years after vaccination with a group A + group C meningococcal capsular polysaccharide vaccine. Children were 1-4 years old at the time of vaccination. Most showed a good initial response to vaccination, including those aged 1-2 years. However, antibody titers declined progressively during follow-up, and 5 years after vaccination, antibody titers measured by both HA and ELISA had returned to prevaccination levels. This decline was not influenced significantly by a booster dose of vaccine given 2 years after initial immunization. Administration of malaria chemoprophylaxis reduced the rate at which antibody levels fell after initial immunization. Sustained protection of children against group A meningococcal disease will require the development of vaccines that are immunogenic in infants and that can induce T cell memory.