Tests conducted in 2011 and 2012 show that the influenza vaccine effectively prevents flu infections in pregnant women, whether or not they are infected with HIV.
Vaccination also protected newborns during at least the first 24 weeks of life, as long as their mothers were HIV-free. There was no evidence of protection for children born to HIV-positive mothers, but the number of infants who fell into that category in this study was small.
"The findings conform with current practice, which is to give the flu shot to everyone," Dr. Karen Deighan, an obstetrician and gynecologist at the Loyola University Health System in Maywood, Illinois, told Reuters Health.
"This study provides greater surety in terms of the benefits of being vaccinated" even when a woman has HIV, chief author Dr. Shabir Madhi of Chris Hani-Baragwanath Hospital in Gauteng, South Africa, said in a telephone interview.
The double-blind, randomized, placebo-controlled trials were conducted with 2,237 women in South Africa - 188 of whom had HIV - who were followed with weekly monitoring until 24 weeks after they delivered their child. Sanofi Pasteur made the trivalent vaccine.
When the researchers looked at confirmed cases of influenza in all participants, the vaccine was 50.4 percent effective among HIV-free women and 48.8 effective for their children.
The effectiveness in children is important because the flu vaccine is not licensed for babies younger than six months. Yet "when you're under 6 months of age, the risk of developing the flu is 50- to 55-fold greater than in the general population," Madhi said.
Although there were far fewer HIV-positive women in the study, there was evidence of vaccine effectiveness for them as well, with 16 confirmed cases of flu in the placebo group and 7 in the vaccine group, for an efficacy rate of 57.7 percent.
"The immune response was much lower than in the (HIV-negative) women, so we expected the efficacy to be lower," Madhi said. "Instead, we found that the vaccine efficacy was the same, if not greater, in HIV-infected women, so that was a good surprise for us."
Five vaccine recipients and six placebo recipients among the babies born to HIV-infected mothers developed flu within 24 weeks of birth, a difference so small that it could have been due to chance.
Earlier studies on whether vaccinating mothers protected their babies after birth have produced conflicting results, although a study published in 2008, conducted in Bangladesh, found that infants born to vaccinated women were 63 percent less likely to develop the flu.
There were no significant side effects from the vaccine, aside from mild to moderate injection site reactions, such as tenderness and itching.
The study is the first randomized control trial of the flu vaccine in pregnant women that included a placebo group for comparison, Madhi said.
"It was a good idea to look at this," Deighan said. "Happily, it shows it will give them protection."