Any baby born before the 37th week of pregnancy is considered to be preterm. About 12 of every 100 of births in the United States are preterm.

Maternal antibodies

Preterm babies acquire lesser quantities of antibodies through the placenta than full-term babies. Since these antibodies are present at lower levels, they do not last as long as those of full-term babies. Because preterm infants rely on their own immune systems for protection sooner than full-term babies, it is important that they receive needed vaccinations so they can protect themselves against disease.

Chronologic age

Vaccines should be given according to a baby's chronologic age — the time since delivery.

Hepatitis B vaccine

If a preterm infant is born to someone infected with hepatitis B, they should receive the hepatitis B vaccine at or shortly after birth due to the chance of being infected during delivery. If the baby weighs less than 2,000 grams, the dose should not be counted as part of the hepatitis B series, and the baby should start the three-dose series one month after birth.

If a preterm infant is born to someone known not to be infected with hepatitis B, they should get the vaccine one month after birth.

Preterm babies discharged before 1 month of age may get the vaccine at discharge as long as they are considered medically stable and have been consistently gaining weight.

In both cases, later doses should be given at least four weeks after the dose at 1 month of age. The third dose should be given at least 16 weeks after the first dose and at least eight weeks after the second dose, but not before 6 months of age.

Respiratory syncytial virus (RSV) protection

Respiratory syncytial virus, or RSV, is a seasonal virus that causes a respiratory infection in virtually all young children at some point. It is a particularly dangerous infection for premature infants because of the immaturity of their lungs. Two means of protecting preterm babies from RSV are available:

1. Pregnant people can be vaccinated against RSV. If vaccination occurs at least two weeks prior to delivery, antibodies will pass to the baby, so the baby is protected at time of birth. In the event the mother does not receive vaccine, or if delivery occurs less than two weeks after vaccination, the newborn should not be considered protected by this option.
2. If the baby is not protected at birth, they should receive Beyfortus, a monoclonal antibody. For infants born during RSV season, this should be administered at or closely after birth. For those born prior to the start of RSV season, they should receive the antibody at the start of RSV season in their area. In the event that Beyfortus is unavailable, certain high-risk premature infants will be eligible for Synagis, another monoclonal antibody product that requires multiple doses throughout RSV season.

Vaccines typically administered at 2 months

In addition to hepatitis B, 2-month-old babies require vaccinations against diphtheria, tetanus, pertussis, polio, Haemophilus influenzae type b (Hib), pneumococcus and rotavirus. Preterm infants should receive these vaccines at the chronologic age of 2 months, even if they are still hospitalized.

These vaccines should continue to be given at the appropriate chronologic ages according to the Centers for Disease Control and Prevention's vaccine schedule until each series is completed.

Other vaccines

Other vaccines should also be given according to the recommended schedule; these include vaccines for influenza, COVID-19, measles, mumps, rubella, varicella (chickenpox) and hepatitis A.

Influenza and COVID-19 vaccines are not recommended until 6 months of age. At 6 months of age, the baby may get the inactivated version of the influenza vaccine. Healthcare workers and family members in contact with a baby less than 6 months old should be immunized to lessen the baby's chance of being infected with these infections.