Young children and pregnant women are among the groups at highest risk of serious complications from H1N1 and the results supported the government's current vaccine dosing recommendations. However delays in H1N1 vaccine production have limited the distribution of vaccine to these groups.
Anne Schuchat, director of the Centers for Disease Control and Prevention's National Center for Immunization and Respiratory Disease said Monday that 30 million doses of the H1N1 vaccine have been delivered to the government.
"Over time, we expect that supply will start to increase and eventually catch up with the tremendous demand that we are seeing now," Schuchat said.
The study results released Monday should reassure pregnant women of the effectiveness of the vaccine, said NIAID Director Anthony Fauci.
"The immune responses seen in these healthy pregnant women are comparable to those seen in healthy adults at the same time point after a single vaccination, and the vaccine has been well tolerated," he said.
The early analysis of 50 pregnant women participating in the trial found that of the 25 women who received a single 15-microgram dose of the vaccine, an immune response was generated that would protect 92 percent of the women.
The response in the 25 women who received a single 30-microgram dose of the vaccine was likely to be protective in 96 percent of the women.
Howard Strassner, chairman of Obstetrics and Gynecology at Rush University Medical Center in Chicago, said the results are reassuring to medical professionals but may not sway pregnant women in their choice to get the vaccine or not because most patient concerns are about the potential effect of the vaccine on the fetus. The CDC has said repeatedly that both the seasonal and H1N1 vaccine are safe for pregnant women and their babies.
Strassner said there has been a better understanding of the risks of not getting a flu vaccine this year and providers have been emphasizing the importance of pregnant women and other members of the household being vaccinated so they don't pass on H1N1 to the newborn.
"There's been a recommendation for years that pregnant women should receive influenza vaccinations, but that response is usually very low," he said. "Because the H1N1 vaccine is different we are finding a much improved acceptance rate for seasonal flu and for H1N1 when it's available. The risks are so much higher."
Strassner said Rush has already seen some pregnant patients with severe complications from H1N1, much worse than what has been seen in past years from seasonal flu.
At least 100 pregnant women have been hospitalized in intensive care because of H1N1 this year, according to the Centers for Disease Control and Prevention.
Interim results of a separate child-focused trial supporting the recommendation that children under 9-years-old get two doses of the vaccine showed that 25 percent of children 6 months to 35 months old had a robust response to one 15-microgram dose of the vaccine, while 55 percent of children 3 to 9 years did.
But after receiving a second dose, 100 percent of children between the ages of 6 months and 35 months had a good immune response and 94 percent of children 3 to 9.
The results come after the World Health Organization's Strategic Advisory Group of Experts on Immunization recommended last week that "priority be given to the administration of one dose of vaccine to as many children as possible," in light of shortages of the vaccine.
But Fauci stressed the importance of getting the follow-up dose.
"We would like to get children as fully protected as we possibly can," said Fauci, so the "optimal thing is to get children within the group that requires two doses to get both."