Indonesia was the fifth country with highest number of preterm infants after India, China, Nigeria and Pakistan; and the ninth country of most prevalence preterm birth 15.5 per 100 live birth in 2012. 
Immunization is often delayed in preterm infants due to medical problems. On the other hand, preterm infants are prone to infectious diseases due to immature immune system and have lower maternal antibody compared to term infants. Therefore, the preterm infants should be protected by immunization. Stable preterm infants who do not have serious infection, metabolic, renal, cardio-respiratory or neurologgic problem and show  improvement and steady weight gain can be immunized, 

Studies shown evidences that preterm infants could produce sufficient antibody to vaccines compared to term infants with equivalent chronological age, not corrected age, except for hepatitis B vaccine which is influenced by low birth weight. Preterm infants with birth weight less than 2000 g born from HBsAg-negative mother, hepatitis B immunization could be postponed until body weight reached 2000 g or until 1 month of age. Preterm infants born to HBsAg-positive mother, should have hepatitis B vaccine and hepatitis B immuneglobuline within 12 hours at separate extremities.  
  
Preterm infants can tolerate most of immunization as term infants. Nevertheles extremely low birth weight infants may suffer adverse event following immunization such as apneu attack with/without bradicardia was reported following diphtheria, tetanus and whole-cell pertussis vaccine. Risk factors include apneu during 24 hours before immunization, younger gestational age, body weight less than 2000 g during immunization which becomes indication to monitor infants until 48 hours after immunization.