Premature infants whose clinical condition is satisfactory should be immunized with age-appropriate doses of vaccine at the same chronological age and according to the same schedule as full-term infants, regardless of birth weight. In premature infants, maternally derived antibody is present at lower titres and for a shorter duration than in mature infants. As well, the severity of vaccine-preventable illnesses may be greater in this population. Therefore, immunization of premature infants should not be delayed.
Antibody response to immunization is generally a function of chronologic age and not of maturity. Although studies demonstrate conflicting results, premature infants may have lower antibody responses than full-term controls to several vaccinations. Despite this, vaccine efficacy remains high. Several recent studies have demonstrated that healthy premature infants generally tolerate immunizations well, with low rates of adverse events that are similar to those of full-term infants. These studies have evaluated the pentavalent combination vaccine products, hepatitis B vaccine and the newer conjugated pneumococcal and meningococcal vaccines.
Premature and very low birthweight infants (i.e., 1500 g) still hospitalized at the time of immunization, however, may experience a transient increase or recurrence of apnea and bradycardia following vaccination. This subsides within 48 hours and does not alter the overall clinical progress of the child. The risk of these events is greater among infants with ongoing cardiorespiratory issues at the time of vaccination, but such events can also occur in those who are clinically stable. Given these findings, it is recommended that hospitalized premature infants have continuous cardiac and respiratory monitoring for 48 hours after their first immunization.
The response to hepatitis B vaccine may be diminished in infants with birth weights < 2000 g. Routine immunization of infants of mothers known to be negative for hepatitis B surface antigen (HBsAg) should be delayed until the infant reaches 2000 g or 1 month of age. Premature infants born to women who are HBsAg positive should, however, still receive hepatitis B immune globulin (HBIg) within 12 hours of birth and the appropriate dose of vaccine starting at birth. These infants require a fourth dose of hepatitis B vaccine (please refer to the Hepatitis B Vaccine chapter for more information).
If the mother's status is unknown, the vaccine should be given in accordance with the recommendations for the infant of an HBsAg-positive mother. The maternal status should be determined within 12 hours, and if the mother is HBsAg positive the infant should also receive HBIg.
All children < 2 years of age are now considered to be at high risk of significant morbidity and mortality from influenza and should be immunized starting at 6 months of age. This includes infants born prematurely. Household contacts of all infants < 23 months of age, including those < 6 months of age, who are too young to receive influenza immunization themselves, should also be immunized to prevent household transmission to the infant (please refer to the Influenza Vaccine chapter for more information).
Infants born at 32 weeks and 0 days' gestation or earlier who are ≤ 6 months of age (with or without bronchopulmonary dysplasia [BPD]) at the start of the RSV season, infants born between 32 and 35 weeks' gestation in isolated communities where hospital care is not readily accessible, as well as children ≤ 24 months of age with BPD who required oxygen and/or medical therapy for that illness within the 6 months preceding the RSV season and children < 2 years of age with hemodynamically significant cyanotic or acyanotic congenital heart disease should be considered for monoclonal anti-RSV antibody, palivizumab, to decrease the likelihood of serious RSV infection requiring hospitalization, and supplemental oxygen therapy (please refer to the Passive Immunizing Agents chapter, for more information).