Abstract
Objective: To determine if children born preterm were at increased risk
of influenza hospitalization up to age five. Methods: National registry
data on all children born in Norway between 2008 and 2011 was used in
Cox regression models to estimate adjusted hazard ratios (aHR) for
influenza hospitalizations up to age five in children born preterm
(<37 pregnancy weeks). HRs were also estimated separately for
very preterm (<32 weeks), early term (37-38 weeks), and
post-term (≥42 weeks) children. Results: Among 238 628 children born in
Norway from January 2008 to December 2011, 15 086 (6.3%) were born
preterm. There were 754 (0.3%) children hospitalized with influenza
before age five. The rate of hospitalizations in children born preterm
was 1.4 per 10 000 person-years (95% confidence interval [CI]:
1.1-1.7), and 0.6 per 10 000 person-years (95% CI: 0.5-0.6) in children
born at term (≥37 weeks). Children born preterm had a higher risk of
influenza hospitalization before age 5: aHR 2.33 (95% CI: 1.85-2.93).
The risk increased with decreasing gestational age and was highest among
those born very preterm; aHR 4.07 (95% CI: 2.63-6.31). Compared to
children born at 40-41 weeks, children born early term also had an
elevated risk of influenza hospitalization; aHR (37 weeks) 1.89 (95%
CI: 1.43-2.40), aHR (38 weeks) 1.43 (95% CI: 1.15-1.78). Conclusion:
Children born preterm had a higher risk of influenza hospitalizations
before age five. An elevated risk was also present among children born
at an early term. Children born preterm would benefit from influenza
vaccinations.