Influenza Vaccine Protects HIV-Positive Pregnant Women from Flu Infection

The influenza vaccine effectively prevents flu infections in pregnant women, regardless of whether they are infected with human immunodeficiency virus (HIV), according to the first randomized control trial of the flu vaccine in pregnant women that included a placebo group for comparison. Vaccination also protected newborns during at least the first 24 weeks of life, as long as their mothers were HIV-free.

There are limited data on the efficacy of vaccination against confirmed influenza in pregnant women with or without HIV infection and protection of their infants. This study confirms current practice and provides clinicians with greater assurances to vaccinate all pregnant women, including those who have HIV.

“Influenza vaccine was immunogenic in HIV-uninfected and HIV-infected pregnant women and provided partial protection against confirmed influenza in both groups of women and in infants who were not exposed to HIV,” state the authors, led by Dr. Shabir Madhi of Chris Hani-Baragwanath Hospital in Gauteng, South Africa.  

The researchers conducted 2 double-blind, randomized, placebo-controlled trials of trivalent inactivated influenza vaccine in South Africa during 2011 in pregnant women infected with HIV and during 2011 and 2012 in pregnant women who were not infected. The immunogenicity, safety, and efficacy of the vaccine in pregnant women and their infants were evaluated until 24 weeks after birth. Immune responses were measured with a hemagglutination inhibition (HAI) assay, and influenza was diagnosed by means of RT-PCR assays of respiratory samples.

The study included more than 2000 pregnant women who were not infected with HIV and nearly 200 pregnant women who were infected with HIV. At 1 month after vaccination, seroconversion rates and the proportion of participants with HAI titers of 1:40 or more were higher among the vaccine recipients than among placebo recipients in both cohorts. Newborns of vaccine recipients also had higher HAI titers than newborns of placebo recipients.

The immune response was lower in the HIV-positive women compared with the HIV-negative women, and the researchers expected the vaccine efficacy to be lower as well. Surprisingly, they found that the vaccine efficacy was the same, if not greater, in HIV-infected women.

The attack rate for RT-PCR–confirmed influenza among both HIV-uninfected placebo recipients and their infants was 3.6%. The attack rates among HIV-uninfected vaccine recipients and their infants were 1.8% and 1.9%, respectively. The vaccine-efficacy rates were about 50% for both groups.

Among HIV-infected women, the attack rate for placebo recipients was 17% and the rate for vaccine recipients was 7%. The vaccine-efficacy rate for these vaccine recipients was 57.7%.

There were no significant side effects from the vaccine, aside from mild to moderate injection site reactions.

Five vaccine recipients and 6 placebo recipients among the babies born to HIV-infected mothers developed flu within 24 weeks of birth, a difference that could be due to chance. The effectiveness in children is important because the flu vaccine is not licensed for those younger than 6 months, they note.

The researchers published their results in the September 4, 2014 issue of the New England Journal of Medicine.