Influenza vaccine A/H1N1 evoked significant immune response in term and preterm children aged 6 to 23 months

Esposito S. Pediatrics. 2011;127:e1161-e1168.

A single dose of 2009 pandemic influenza A/H1N1 MF59-adjuvanted vaccine created a notable immune response against pandemic influenza A/H1N1 virus in children aged 6 to 23 months who had different gestational ages at birth.

Researchers in Italy enrolled 105 children aged 6 to 23 months in a prospective, randomized study between Nov. 9, 2009, and Jan. 16, 2010, to determine the immunogenicity, safety and tolerability of a monovalent 2009 pandemic influenza A/H1N1 vaccine in children at the highest risk for influenza-related complications, particularly preterm infants. Researchers reported choosing the MF59-adjuvanted vaccine because of evidence that younger children may have a poor response to non-adjuvanted influenza vaccines.

Participants included 35 preterm children with a gestational age of less than 32 weeks; 35 preterm children with a gestational age of 32 to 36 weeks; and 35 term children with a gestational age of 37 to 42 weeks. Exclusion criteria included known immunodeficiency; a history of seizures; a mother positive for hepatitis B surface antigen, hepatitis C virus, or HIV; a known allergy to any of the vaccine components; any treatment in the preceding 4 weeks likely to alter immune response; and the previous administration of any influenza vaccine. Children with acute respiratory tract infections were also excluded.

Each participant received two intramuscular vaccine doses (Focetria, Novartis). The first dose was given at enrollment; the second was given 4 weeks after the first (28 ± 2 days). Researchers collected serum samples for antibody measurements each before administering the first dose, before the second dose (28 ± 2 days after baseline), and again 4 weeks later (56 ± 2 days after baseline).

Of the 101 children who completed the study, irrespective of their gestational and postnatal ages, all of the children seroconverted and achieved seroprotection after two doses. After vaccination, researchers found that all participants experienced a significant titer increase (P<.05); from six to 40 times after the first dose and 13.6 to 88.9 times more after the second. Participants aged 6 to 11 months with a gestational age of less than 32 weeks had significantly lower absolute titers than any other group (P<.05) at baseline, as well as after both doses, with an increase of up to 40 times after dose one and up to 89 times after dose two.

No study groups experienced a rate of local adverse events of more than 2%, and there were no serious adverse events in any group. There were no between-group differences.

“Younger children have a crowded vaccination scheduled and any reduction in the number of vaccine administrations increases compliance with health authority recommendations,” researchers wrote. “Moreover, because children are the most important cause of the spread of influenza in the community, the use of a single does can limit the number of cases among older subjects and adults by reducing the time to full protection in the first years of life.”

Disclosure: The researchers report no relevant financial disclosures.

Kathryn M. Edwards

In earlier reports in adults and children, the addition of the adjuvant MF59 to seasonal, pandemic, and avian influenza vaccines has been shown to markedly enhance the immune responses with minor increases in local reactions. In this study Dr. Esposito in Italy expanded the evaluation of the adjuvanted pandemic influenza vaccine to young children, including infants born prematurely. The immune responses noted were robust, even after the first vaccine dose, suggesting that only one dose might be needed.

Young children have generally not responded as well to inactivated influenza vaccines as older children, and adults and have also required two doses of vaccine in the initial series. The MF59 adjuvant significantly enhances the immune responses to inactivated influenza vaccine, making it a very attractive alternative to unadjuvanted vaccine. However, continued safety studies and larger efficacy studies will need to be performed to determine whether this new vaccine will replace the old.

—Kathryn M. Edwards, MD
Infectious Diseases in Children Editorial Board Member

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