Routine immunisation of preterm infants

Incidence of adverse events in premature children following 2-month vaccination

Using a population based self-controlled case series study design we examined data on 834,740 children in the province of Ontario, Canada. We observed that when comparing to SGA10 term children (term children born in the lowest 10th percentile of weight for a given gestational age), relative incidence of emergency room visits and admission in the 3 d post 2 mo vaccination progressively decreased in near term (relative incidence ratio 0.89 (95% CI 0.74-1.07)) and very premature children [(0.67(0.49-0.93)]. When compared with all term children this decrease in risk is not statistically significant. We speculate that the immune response is reduced in premature children resulting in reduced adverse events. This is masked when comparing to all term children because the reduced birth weight of premature children results in a comparatively increased dose of vaccine. This in turn results in an increased immune response and risk of immediate adverse reactions. Future studies of immune response in premature children should examine the impact of weight at the time of immunization.

Impact of birth weight at term on rates of emergency room visits and hospital admissions following vaccination at 2 months of age

BACKGROUND

Birth weight of children born at term may theoretically be associated with risk of adverse events from immunization.

METHODS

We analyzed data on children born between April 1st 2002 and March 31st 2009 in the province of Ontario. Using the self-controlled case series design, we examined the risk of the combined endpoint of emergency room visit and hospital admission in the immediate three days post vaccination at 2 months of age compared to a control period 9-18 days after vaccination. In term children, we conducted 4 comparisons of relative incidence (RI) of events: (1) 4 lower birth weight quintiles compared to the largest quintile (2) SGA10 infants compared to non SGA10 infants, (3) low birth weight infants (<2500g) compared to non low birth weight infants and (4) SGA10 infants vaccinated before 60 days compared to those vaccinated after 60 days.

RESULTS

There was a significant trend towards increasing relative incidence of the combined endpoint with decreasing birth weight quintile (p=0.016). There was an increased relative incidence of events in SGA10 versus non SGA10 infants (RI 1.25 (95% CI 1.09-1.44)) and in SGA10 children vaccinated before 60 days of age compared to after 60 days of age (RI 1.57 (95% CI 1.14-2.18)). No significant effect was observed in low birth weight children. The impact of birth weight was primarily mediated through an increase in ER visits in the 24h following vaccination.

CONCLUSION

Lower birth weight appears to be correlated with an increased risk of emergency room visits within 24h of vaccination. The absolute risk is small and there was no impact on admissions or death.

Vaccines and preterm neonates: why, when, and with what

This review analyses what is known concerning the immune response of preterm (PTIs) and low birth weight infants (LBWIs) to widely used vaccines, the protection they receive from routine immunisation, and the safety and tolerability of the vaccines themselves. It shows why PTIs and LBWIs should be vaccinated using the same schedules as those usually recommended for full-term infants (FTIs), except in the case of hepatitis B vaccine, whose schedule should be repeated in infants who received the first dose during the first days of life when they weighed less than 2000 g because of their reduced immune response. Vaccines are immunogenic, safe and well tolerated in PTIs and LBWIs, in whom early active immunisation is particularly important because they are among the most vulnerable subjects for pediatric infectious diseases. It is therefore essential to make every effort to convince pediatricians and parents that compliance with these recommendations will not cause any clinical problems.

[Immunization of the preterm infant]

Premature infants have an increased risk of experiencing infectious diseases, some of which are vaccine preventable diseases. Maturation of immune responses begins with exposition to environmental antigens and in premature infants as fast as in term-infants. Premature infants must be vaccinated at 2 months of age, whatever the gestational age. Acellular Pertussis vaccine and pneumococcal conjugate vaccine must be given as early as possible, at two months of age. Immunization schedule in premature infants is the same as in full-term infants : three injections one month apart with a pentavalent vaccine : Diphteria, Tetanus, Poliomyelitis, Pertussis and Haemophilus type b. First injection of hepatitis B vaccine must not be taken in account when this vaccine is given at birth to infants under 2 kg birth weight. Premature infants 6 months of age or older and experiencing chronic lung disease have to be vaccinated against influenza. In all cases, surroundings have to be vaccinated. Apnea and/or bradycardia have been reported within the 48 hours following vaccination in premature infants before 32 weeks of gestational age and justify giving their first injection of vaccine under cardiorespiratory monitoring. These injections will be given before discharge as often as possible.

Special immunization considerations of the preterm infant

More changes to the American Academy of Pediatrics Recommended Immunization Schedule have occurred in the past 3 years than in the previous decade. Selection of the optimal immunization regimen is essential to forestall immunization delay. New complications to the schedule pose challenges for the care of preterm infants who are at increased risk of mortality and morbidity from vaccine-preventable diseases. This article reviews the relevant data regarding immunization of preterm infants and suggests strategies for prevention of immunization delay. Protection of preterm infants, especially for pertussis and influenza, involves not just assessing a child's vaccination status but those of other close contacts and household members.

Hypothalamic-pituitary-adrenal axis function and the cellular immune response in former preterm children

CONTEXT

Animal data suggest that adverse early experiences may affect endocrine and immune functioning in later life.

OBJECTIVE

Our objective was to assess the impact of preterm delivery on hypothalamus-pituitary-adrenal axis functioning, heart rate responses, and immune function.

PARTICIPANTS

Former preterm children [aged 8-14 yr (n = 18)], sex and age-matched full-term born control children (n = 18), data on birth weight, gestational age, birth weight for gestational age (in sd units), actual body weight, height, and body mass index were assessed.

DESIGN AND OUTCOME MEASURES

Subjects were exposed to a standardized laboratory stressor ("Trier Social Stress Test for Children"). Cortisol in saliva was determined in 10-min intervals before and after the stress test; heart rates were obtained continuously during the stress test. Additional assessment of saliva cortisol was performed: 1) on 3 consecutive days after awakening and at +10, +20, and +30 min (morning cortisol); and 2) at 0800, 1400, 1600, and 1900 h (short diurnal profile). Measurement of the delayed type hypersensitivity reaction to seven recall antigens [Multitest cellular mediated immunity (Multitest-Immignost, Biosyn, Fellbach, Germany)].

RESULTS

Exposure to the Trier Social Stress Test for Children yielded significantly increased cortisol levels [F (8, 232) = 19.86; P < 0.001] and heart rates [F (38, 988) = 10.46; P < 0.001], however, no difference between former preterms and full-terms could be observed. No between-group differences were found in the short diurnal cortisol profile. Former preterms showed significantly higher cortisol levels after awakening [F (3, 102) = 3.14; P < 0.05]. In addition, a significantly suppressed delayed type hypersensitivity response [reduced number of positive antigens (t = -2.64, P < 0.05); induration (t = -2.4, P < 0.05)] was found in former preterms.

CONCLUSION

The data suggest that preterm delivery may be associated with altered endocrine and immune functions well into late childhood.

Active immunization of premature and low birth-weight infants: a review of immunogenicity, efficacy, and tolerability

Preterm infants are at increased risk of disease and hospitalization from a number of vaccine-preventable diseases. However, these same infants have immunologic immaturities that may impact vaccine responses. Larger premature infants mount immune responses to vaccines similar to those of full-term infants, but very premature infants (<28-32 weeks' gestation at birth) may have specific defects in vaccine responsiveness. Although there are minor differences in immunogenicity, the immune responses to diphtheria, tetanus, pertussis, and polio antigens are similar enough between full-term and premature infants that clinical consequences are unlikely to result. However, the immunogenicity of Haemophilus influenzae type b conjugate vaccines varies widely among studies of premature infants, and may be affected by the choice of conjugate protein, inclusion in a combination vaccine, and by an infant's overall health. Pneumococcal conjugate vaccine is efficacious in larger premature infants, but little information is available about immunogenicity in smaller premature infants. Meningococcal group C conjugate vaccine appears immunogenic in even very premature infants, but the duration of immunity may be limited. Hepatitis B vaccine given at birth appears poorly immunogenic in infants with birth weights <1500-2000 g, with delay in the administration of the first dose yielding improved immunogenicity. Few data on influenza vaccine in premature infants are available, but infants with pulmonary disease may respond less robustly than others. Bacille Calmette Guérin vaccine appears to be most immunogenic if delayed until at least 34-35 weeks' postmenstrual age in very premature infants, although there may be non-specific advantages to its earlier administration. Premature infants may have persistently lower antibody titers than full-term infants, even years after initial immunization. Sick premature infants experience increased episodes of apnea or cardiorespiratory compromise following vaccine administration, necessitating careful monitoring. Specific factors that impair immune response, quality of the immune response, and safety and immunogenicity evaluation of new vaccines in premature infants are topics needing further research. Premature infants are at significant risk for decisions from healthcare providers that delay beginning and completing their vaccine regimens. A major challenge facing those who care for these infants is the provision of timely immunization.

Response of preterm newborns to immunization with a hexavalent diphtheria-tetanus-acellular pertussis-hepatitis B virus-inactivated polio and Haemophilus influenzae type b vaccine: first experiences and solutions to a serious and sensitive issue

OBJECTIVE

Preterm infants are at increased risk from infections and should be vaccinated at the usual chronological age. The aim of the study was to evaluate the immunogenicity and reactogenicity of a hexavalent diphtheria-tetanus-acellular pertussis-hepatitis B virus-inactivated polio and Haemophilus influenzae type b (DTPa-HBV-IPV/Hib) vaccine in preterm infants.

METHODS

In a comparative trial, 94 preterm infants between 24 and 36 weeks (mean +/- SD gestational age: 31.05 +/- 3.45 weeks; mean birth weight: 1420 +/- 600 g) and a control group of 92 full-term infants were enrolled to receive 3 doses of a DTPa-HBV-IPV/Hib vaccine at 2, 4, and 6 months. Immunogenicity was assessed in serum samples that were taken before and 4 weeks after primary vaccination. Evaluation of reactogenicity was based on diary cards.

RESULTS

All preterm (n = 93) and full-term (n = 89) infants who were included in the immunogenicity analysis had seroprotective titers to diphtheria; tetanus; and polio virus types 1, 2, and 3. The immune response to the Hib and hepatitis B components was lower in preterm than in full-term infants: 92.5% versus 97.8% and 93.4% versus 95.2%, respectively. Vaccine response rates for pertussis antigens were >98.9% in both study groups. Although most geometric mean titers were lower in preterm infants, titers were similar for pertussis, a major threat for premature infants. The vaccine was well tolerated, and there were no differences in reactogenicity between groups. Some extremely immature infants experienced transient cardiorespiratory events within the 72 hours after the first vaccination with no clinical repercussion.

CONCLUSIONS

Preterm infants who were immunized with the hexavalent DTPa-HBV-IPV/Hib vaccine at 2, 4, and 6 months displayed good immune response to all antigens. The availability of this vaccine greatly facilitates the vaccination of premature infants.

Antibody response to diphtheria-tetanus-pertussis immunization in preterm infants who receive dexamethasone for chronic lung disease

OBJECTIVE

To study the effect of dexamethasone in preterm infants with chronic lung disease (CLD) on antibody response to routine immunization against diphtheria, tetanus, and pertussis (DTP).

METHODS

Serum samples were obtained before and after immunization with DTP (Trivax-AD) from an unselected cohort of 93 preterm infants in the United Kingdom. Antibodies to diphtheria and tetanus and to 4 pertussis antigens (pertussis toxin, filamentous hemagglutinin, pertactin, and fimbrial agglutinogens 2 + 3) were measured by an enzyme-linked immunosorbent assay. Linear regression models were fitted to the natural log of antibody titers to compare the dexamethasone-treated and -untreated infants adjusting for potential risk factors.

RESULTS

Sixty-seven (72%) of 93 infants received dexamethasone. Preimmunization geometric mean titers (GMTs) were comparable in both groups for all antibodies. The rise in GMT after immunization was reduced in the dexamethasone-treated group. Final GMT was significantly lower for tetanus, diphtheria, pertussis toxin, and fimbrial agglutinogens 2 + 3 but not for filamentous hemagglutinin or pertactin. Using the minimum protective titer of 0.01 IU/mL, there was no significant reduction in protection for diphtheria and tetanus in the dexamethasone-treated infants. Using the higher reference titer of 0.1 IU/mL, there was a 16% reduction in protection for diphtheria (95% confidence interval: 3%-27%) and a 9% reduction in protection for tetanus (95% confidence interval: -7% to 20%).

CONCLUSIONS

The use of dexamethasone for CLD in preterm infants is associated with a reduction in antibody titer to routine immunization against diphtheria and tetanus. Antibody responses to 2 of 4 pertussis antigens are reduced, but the clinical significance of this observation is unclear. Protection against tetanus and diphtheria is not impaired when the lower reference value for protective antibody is used. On the basis of this study of UK preterm infants who were treated with dexamethasone for the management of CLD, we conclude that the current DTP immunization schedule is adequate and do not recommend additional booster protection against tetanus or diphtheria during early infancy. When diphtheria prevalence is increased, however, additional protection should be considered.

Seven-year follow-up of vaccine response in extremely premature infants

OBJECTIVE

To assess the immune response of 7-year-old former extremely preterm (PT) infants to routine childhood immunizations.

METHODS

Sixteen PT (<29 weeks and <1000 g) infants, followed since their primary immunizations at the recommended chronological ages, and 16 age-matched full-term (FT) control subjects were evaluated at 7 years of age. Antibodies to Haemophilus influenzae type b polyribosylribitol phosphate (Hib-PRP), tetanus, pertussis, diphtheria, polio, and hepatitis B (HBsAb) were measured.

RESULTS

The FT group had higher antidiphtheria geometric mean titers (GMT) than the PT group (1.07 vs 0.36 IU/mL). All FT and 13 of 16 PT had protective diphtheria antibody titers (>0.1 IU/mL). The tetanus GMT were 4.22 IU/mL (FT) and 1.99 IU/mL (PT). All children had protective tetanus titers (>0.01 IU/mL). Pertussis titers did not differ between FT and PT. Hib-PRP GMT were higher in FT than in PT (3.21 vs 1.41 microg/mL). All children had anti-PRP > or = 0.15 microg/mL; 12 of 16 FT and 10 of 16 PT had levels > or = 1.0 microg/mL. Polio serotype 1 and 2 GMT were similar between groups, and all children had protective titers (> or = 8). Polio serotype 3 GMT were 59 (FT) and 24 (PT) Karber units; all FT and 12 of 16 PT had protective titers. Among children who had received hepatitis B vaccine, GMT were similar in FT and PT children (120 vs 186 mIU/mL, and similar proportions of children (11 of 16 FT and 12 of 14 PT) had protective HBsAb titers (>10 mIU/mL).

CONCLUSIONS

At 7 years of age, PT children had lower antibody titers to many vaccine antigens than FT children. However, most PT children maintained antibody titers in the protective range.

Safety and immunogenicity of an acellular pertussis vaccine in premature infants

OBJECTIVES

To evaluate the safety and immunogenicity of a two-component acellular pertussis vaccine in preterm infants.

STUDY DESIGN

Fifty preterm infants (25-35 weeks of gestation; mean, 30.8 weeks) and 50 term infants as a control group received a two-component acellular pertussis vaccine irrespective of their biological age and actual weight. Adverse reactions were registered by parents on a diary card and reviewed on each visit. Antibodies against pertussis toxoid (PT) and filamentous hemagglutinin (FHA) were determined with an enzyme-linked immunosorbent assay before the first and after the third vaccination.

RESULTS

The infants of both groups showed an increase in geometric mean titers (GMT) against PT and FHA after vaccination (3 doses). There was a significant difference of antibody concentration between the preterm and the control group. The GMT for PT antibody of the preterm infants was 64. 16 U/L, and for the term infants it was 98.96 U/L. The GMT for FHA was 50.92 U/L in preterm versus 86.02 U/L in the control group. Efficacy of the immunization (more than a fourfold increase of antibody concentration in each infant) was 93.5% in the preterm group with respect to PT and 82.6% with respect to FHA. The incidence of adverse reactions was low and comparable in both study groups.

CONCLUSION

Immunization with an acellular pertussis vaccine is safe for preterm infants. The immune response is significantly lower compared with a control group of term infants, but efficacy is high.

Is zero dose oral polio vaccine effective in preterm babies?

A randomized controlled trial was done to compare seroconversion following a single dose of trivalent oral polio vaccine (TOPV) in preterm babies, vaccinated at two different post-conception ages, with that of term newborns. Sixty-two consecutive preterm babies < or = 35 weeks were randomly allocated to two groups. Group A was vaccinated 'early' at 34-35 weeks and group B 'late' at 38-40 weeks post conception. The two groups were comparable in birthweight [mean (SD) 1594 g (118) and 1599 g (126), respectively] and gestational age [mean (SD) 33.2 (1.2) and 33 (1.3) weeks, respectively]. A control group of 36 term babies (group C) were vaccinated in the 1st week of life. Polio virus antibodies were measured immediately before and 6-8 weeks after vaccination. Group A had seroconversion rates of 54.2, 12.5 and 12.5% against polio virus types 1, 2 and 3, respectively, group B had rates of 60.0, 8.0 and 16.0%, and group C rates of 53.6, 10.7 and 14.3%. Differences in the seroconversion rates in the three groups were not statistically significant. The conversion rates against types 2 and 3 are much lower than in previous studies. We conclude that preterm babies vaccinated at 34-35 weeks post conception show seroconversion rates similar to those in term newborns.

Three-year follow-up of vaccine response in extremely preterm infants

OBJECTIVE

To assess whether the adequate antibody response observed in former extremely premature infants after the primary series of immunizations is sustained after the first booster vaccines.

SUBJECTS AND METHODS

Sixteen former extremely premature (<29 weeks, <1000 g at birth) and 17 former full-term (>37 weeks) infants had sera obtained for antibody titer measurement at 3 to 4 years of age. All had received the primary series and first booster vaccines for diphtheria, pertussis, tetanus, polio, and Haemophilus influenzae type b. Twelve preterm and 14 full-term children had completed the hepatitis B vaccine series.

RESULTS

At 3 to 4 years of age, former preterm and full-term children had similar geometric mean titer (GMT) values of antibodies to tetanus, diphtheria, and pertussis. Preterm children had a lower GMT value of Haemophilus polyribosylribitol phosphate (PRP) antibody than did full-term children (0.99 vs 3.06 microg/mL). Fifty percent of preterm and 88% of full-term children had PRP antibody >1.0 microg/mL; 100% of preterm and 94% of full-term children had anti-PRP titers >0.15 microg/mL. GMT values of neutralizing antibodies to polio serotypes 1 and 2 were similar, with 94% to 100% of both groups above protective levels (>/=1:8). The difference in GMT values of polio serotype 3 approached significance (29 vs 73); fewer preterm children had protective titer values (75% vs 100%). Among children vaccinated against hepatitis B, 75% of preterm and 71% of full-term children were protected (10 mIU/mL).

CONCLUSIONS

Preterm children immunized at the recommended chronological ages displayed antibody responses similar to those for full-term children for most immunizing antigens. Responses to PRP and polio serotype 3 were less robust than those of full-term children.

Incidence of apnoea and bradycardia in preterm infants following DTPw and Hib immunization: a prospective study

OBJECTIVE

To evaluate the incidence and severity of apnoea and bradycardia in hospitalized preterm infants following immunization at 2 months of age, and identify risk factors.

METHODOLOGY

A prospective study of 98 preterm infants, of gestational age 24-31 weeks, immunized at approximately 2 months post natal age with diphtheria-tetanus-whole cell pertussis vaccine (DTPw) in the neonatal intensive care unit (NICU) at King George V Hospital Sydney. Half the infants also received Haemophilus influenzae type b conjugate vaccine (Hib) simultaneously. All infants were monitored for apnoea and bradycardia in the 24 h periods pre- and post immunization.

RESULTS

Only one infant had apnoea and/or bradycardia pre-immunization compared with 17 post immunization. For 12 infants these events were brief, self-limiting and not associated with desaturations (oxygen saturation < 90%). However, for five infants (30%) these events were associated with oxygen desaturation and two of these infants required supplemental oxygen. The group that had apnoea and/or bradycardia and the group that did not were not significantly different in terms of gestational age, birth weight and other variables. Infants who received Hib together with DTPw were less likely to have apnoea and/or bradycardia than those given DTPw alone.

CONCLUSION

When considering immunization for preterm infants, the benefits of early immunization must be balanced against the risk of apnoea and bradycardia. We recommend that the cardio-respiratory function of hospitalized infants born at less than 31 weeks gestation be monitored for 48 h post immunization.

Apnea after immunization of preterm infants

OBJECTIVE

To determine the frequency of adverse reactions, particularly the occurrence of apnea, among preterm infants after immunization with diphtheria and tetanus toxoids and whole cell pertussis vaccine adsorbed (DTP) and Haemophilus influenzae type b conjugate (HibC) vaccine in the neonatal intensive care unit.

STUDY DESIGN

After the occurrence of apnea in two preterm infants following immunization with DTP and HibC, a prospective surveillance of 97 preterm infants younger than 37 weeks of gestation who were immunized with DTP (94 also received HibC at the same time) in the neonatal intensive care unit was performed to assess the frequency of adverse reactions and in particular, the occurrence of apnea. For each infant, data were recorded for a 3-day period before and after receipt of the immunization.

RESULTS

The majority of preterm infants tolerated immunizations with DTP and HibC without ill effects. However, 12 (12%) infants experienced a recurrence of apnea, and 11 (11%) had at least a 50% increase in the number of apneic and bradycardic episodes in the 72 hours after immunization. This occurred primarily among smaller preterm infants who were immunized at a lower weight (p = 0.01), had experienced more severe apnea of prematurity (p = 0.01), and had chronic lung disease (p = 0.03).

CONCLUSION

The temporal association observed between immunization of preterm infants and a transient increase or recurrence of apnea after vaccination merits further study. Cardiorespiratory monitoring of these infants after immunization may be advisable.

Adverse events and antibody response to accelerated immunisation in term and preterm infants

A study was performed to compare adverse events and antibody response in term and preterm children vaccinated with diphtheria, tetanus, and pertussis vaccine at 2, 3, and 4 months of age. A total of 124 children were recruited and grouped according to gestational age: 37 weeks or more (n = 52), 34 to 36 weeks (n = 40), and less than 34 weeks (n = 32). Study nurses followed up children 24 hours after each vaccination to record temperature, redness, and swelling at the injection site and any systemic symptoms. Proportions of children experiencing adverse events did not differ between groups. Blood samples were obtained six weeks after the vaccination course at which time all children had protective levels of diphtheria and tetanus antitoxins. Geometric mean antibody titres (95% confidence interval) to pertussis toxin were 2754 (2042 to 3715), 5495 (4074 to 7413), and 3690 (2951 to 4677), to filamentous haemagglutinin were 541 (282 to 1023), 951 (537 to 1698), and 614 (426 to 1023), and to agglutinogens 2 and 3 were 12,106 (6918 to 21,380), 21,330 (13,183 to 34,674), and 22,387 (15,136 to 33,113) in children born at a gestational age of less than 34 weeks, 34 to 36 weeks, and 37 weeks or more respectively. These findings support the current recommendations that preterm children are vaccinated at chronological age according to the national schedule.

Incidence of apnoea and bradycardia in preterm infants following triple antigen immunization

Ninety-seven preterm infants were immunized with diphtheria-tetanus-pertussis (DTP) prior to discharge from hospital. The mean gestational age at birth was 28.1 weeks (range 24-34) and the mean age at immunization was 80.6 days (range 44-257). Nineteen (20%) infants developed apnoea or bradycardia within 24 h of immunization. The infants who developed apnoea and/or bradycardia had a younger gestational age at birth than those who did not (P = 0.03), were artificially ventilated for longer (P = 0.01) and were more likely to have a diagnosis of chronic lung disease (P = 0.006). In the majority of infants these events were not clinically significant. Two infants who developed concurrent upper respiratory tract infections required additional oxygen and one of them was treated with oral theophylline. In general, it is safe practice to immunize preterm infants with DTP unless otherwise contraindicated. However, it is recommended that cardiorespiratory function is monitored after immunization in very preterm infants who had prolonged ventilatory support and/or chronic lung disease.

Response to Haemophilus influenzae type b conjugate vaccine in chronically ill premature infants

Twenty-two premature infants with chronic lung disease (median gestational age 28 weeks) received polyribosylribitol phosphate-outer membrane protein conjugate Haemophilus vaccine at 2 and 4 months of chronologic age. The proportions with antibodies to polyribosylribitol phosphate at levels > 1 microgram/ml after doses 1 and 2 were 27% and 55%; geometric mean titers were 0.43 and 0.73 microgram/ml, significantly lower than values for term infants.

Immunisation of the preterm baby

The study was designed to quantify the preterm baby's response to routine childhood immunisations. A total of 69 preterm babies were put into two groups according to their gestational age: less than 32 weeks and between 32 and 35 weeks. Within each group the babies were randomly placed into one of three schedules for immunisation with diphtheria, tetanus, pertussis and oral poliomyelitis vaccines at 3, 4 and 5 months; 3, 4, 5 and 18 months; and 3, 4 and 10 months. Antibodies were measured before immunisation, one month after the third vaccine dose and at age 19 months. Of these children, 30 had further antibody estimations when they were between 4 and 5 years of age. One month after the third immunisation and at 19 months of age all infants, independent of gestational age, had adequate immunity to the four infections. Children sampled before and after their pre-school booster, with few exceptions, showed persistence of protective antibody and an enhanced response to the booster dose respectively. No correction needs to be made for prematurity when initiating routine immunisation in premature infants.