How Does the Flu Vaccine Effect Preterm Vs. Full-Term Infants?

Infection with influenza tends to be more severe in preterm (PT) infants, and this population is at a much higher risk for related complications. Physicians often associate this risk with PT infants’ tendency to “experience rapid serum decay in antibody responses” to influenza infections and level of severity of symptoms. To address this correlation, a research team headed by Carl D’Angio, MD, a physician in the Department of Pediatrics at the University of Rochester Medical Center in Rochester, New York, compare flu vaccine response in PT versus FT infants.

In 2011, a group of doctors based in Calcutta published a similar study that examined the efficacy of vaccinating PT babies for measles given that these infants may have lower levels of antimeasles antibodies than their FT counterparts. In that study, the doctors determined that PT infants should receive their first measles vaccination at 5 months instead of 9 months — which is how it’s administered in FT babies – in order to balance out this inequality. Few studies have evaluated how this strategy would work for the flu vaccine, which is administered once annually (after 6 months of age) and not on an age-based schedule.

In their study, Dr. D’Angelo and her colleagues set out to “investigate the relationships among the frequencies of influenza-specific antibody secreting cells (ASC) and ASC subsets (including LLPC) and antibody responses to influenza vaccines” among PT and FT infants. The study included 11 PT infants with a mean gestational age of 27.3 weeks at birth and 11 FT babies with a mean gestational age of 39.2 weeks at birth. All infants received their first flu vaccination between six and 17 months of age via 2 doses of inactivated, trivalent IV or quadrivalent IV during 2012-2013 and 2013-2014 flu seasons. The first round of vaccination occurred on day 0 of the study and the second on day 28. The researchers collected blood samples from the infants on days 0, 10, 35, and 56 days, and finally again at 9 months of age.

Not surprisingly, the PT infants weighed less at their first vaccination than did the FT infants, but the postnatal ages among infants in both groups were nearly identical, with PT infants receiving their first vaccination at a mean age of 8.1 months and FT infants receiving their first vaccinations at a mean age of 8.0 months. This is likely due to Centers for Disease Control and Prevention (CDC) guidelines specifying that, “in the majority of cases, infants born prematurely, regardless of birth weight, should be vaccinated at the same chronological age and according to the same schedule and precautions as full-term infants and children,” with the exception of the hepatitis B vaccine. The same guidelines recommend against modifying dosages in most cases, and state, “Decreased seroconversion rates might occur among certain premature infants with low birth weights (i.e. <2,000 grams) [in Hep B vaccination specifically, but] by chronological age one month, all premature infants, regardless of initial birth weight or gestational age, are as likely to respond adequately as older and larger infants.”

In order to measure levels of antibodies in the infants after vaccination, the group used hemagglutination inhibition (HAI) measures at 0 days, 56 days, and 9 months, vaccine-specific ASC numbers at 10 and 35 days, and ASC subsets at 0, 10, and 35 days. “PT infants had post-vaccine HAI titers greater than or equal to those of the FT infants at 56 days and nine months after the initial immunization,” said Dr. D’Angio, adding that ASC numbers were higher in PT infants than FT ones as 35 days and positively correlated with HAI numbers at 56 days. “We also concluded that there were no statistical differences between PT and FT infants [except that] ASC subset numbers at 35 days were marginally higher in PT than in FT infants,” he said, adding, “Influenza-specific ASC numbers in both FT and PT infants correlated with peak antibody titers, but we could not detect an ASC subset that correlated with the durability of antibody response.”