Preterm infants are at greater risk for pertussis than full-term infants, according to a population-based study from the Norwegian Institute of Public Health.
Øystein Rolandsen Riise, MD, PhD, of the institute, and his colleagues identified all live births in Norway from 1998 to 2010 from the Medical Birth Registry of Norway and studied the gestational age of 713,166 infants as an indicator of increased risk of pertussis.
Infants born at 23-27 weeks of gestational age had an incidence rate ratio of pertussis more than fourfold higher than term infants (IRR = 4.49), while those born at 32-34 weeks and 35-36 weeks each had an IRR about 1.5 time higher than term infants (Pediatr Infect Dis J. doi: 10.1097/INF.0000000000001545).
The IRR of preterm infants hospitalized with pertussis was double that of full-term infants (IRR = 1.99).
The increased risk in preterm infants could be attributed to an incomplete transfer of maternal antibodies, specifically the “abundance of IgG [that] is acquired during the last month of full-term pregnancy,” Dr. Riise and colleagues wrote.
While previous studies have found links between preterm birth and pertussis infection, those studies used birth weight instead of gestational age, which leads to possibly inaccurate results, they added.
“Although other studies have used low birth weight to identify infants with increased risk of pertussis, we would have underestimated the number of infants with increased risk by using low birth weight instead of gestational age, since many of the late preterm infants have normal birth weight,” the researchers noted.
Early vaccination is key to protecting preterm infants from pertussis, Dr. Riise and colleagues said, adding that vaccine effectiveness was similar between preterm (93%) and full-term (88.8%) infants.