One dose of influenza A (H1N1) vaccine may protect children

Despite current two-dose influenza A (H1N1) vaccine recommendations for children in the United States, Australian infants who received a single dose of unadjuvanted, monovalent 2009 H1N1 vaccine achieved levels of antibody high enough to confer protection against the virus, new findings suggest.

Researchers observed antibody titers of 1:40 or greater in 92.5% of infants aged 6 months and older who received one 15-mg dose (n=174) and in 97.7% of those who received one 30-mg dose (n=172).

Seroconversion occurred in 86.8% in the 15-mcg group (95% CI, 80.9%-91%) and in 94.2% of those in the 30-mcg group (95% CI, 89.6%-96.8%). The vaccine was well tolerated and had a safety profile similar to that of seasonal influenza.

The immune responses among children in this study were similar to those observed in other recently published studies involving adults, according to the researchers, who noted that a single 15-mcg dose may be sufficient to protect children.

The Australian study involved 370 healthy children infants and children aged 6 months to 9 years, who were randomly assigned to receive a two-dose regimen of either vaccine formulation administered intramuscularly, 21 days apart. All participants achieved antibody titers of 1:40 or greater after receiving the second dose.

“The conservative view will be that we should stick with two doses, but one dose is almost certainly adequate, at least with the vaccine that we have studied (CSL Limited),” study researcher Terry Nolan, PhD, head of the Melbourne School of Population Health and Department of Public Health at the University of Melbourne, told Infectious Disease News.

In an accompanying editorial, Anthony E. Fiore, MD, MPH, medical epidemiologist in the CDC’s Influenza Division and Kathleen M. Neuzil, MD, MPH, of the University of Washington School of Medicine in Seattle, issued some caveats about the study findings. “[T]he findings cannot be generalized with confidence to all children, epidemiological circumstances in every country, or different vaccine formulations.”

Two doses of H1N1 vaccine remain necessary as only 7.5-mcg formulations are licensed for this age group in the U.S., they wrote.

Fiore and Neuzil noted that antibody titers were still about 30% lower than those in adults given the same dose, and that some children in the study may have experienced previous subclinical infections, as more children had a protective level of antibody prior to vaccination than expected (9% to 14% of those younger than 3 years and 28% to 30% of those aged 3 to 9 years). – by Nicole Blazek

Nolan T. JAMA. 2010; doi: 10.1001/jama.2009.1911.