Pregnant women who were vaccinated against pandemic influenza in 2009 were not at increased risk of experiencing fetal death, though pregnant women who contracted influenza had an increased risk of fetal death.
During the swine influenza pandemic of that year, there were anecdotal reports of miscarriages and stillbirths occurring shortly after vaccination, so the Norwegian Institute of Public Health initiated a study to investigate if there was an association between pandemic influenza vaccination and the fetal deaths.
Pregnant women are at increased risk of severe influenza infection and previous studies have shown that pregnant women who were hospitalized with influenza had increased risk of fetal death and the researchers were interested in whether pandemic influenza also increased the risk of fetal death among pregnant women who were not admitted to hospitals.
During the influenza A(H1N1) pandemic of 2009, the entire Norwegian population was offered the pandemic vaccine Pandemrix. Approximately 45 per cent were vaccinated. More than half (54 per cent) of those pregnant during the pandemic took the vaccine.
The study included more than 117,000 births in Norway occurring before, during and after the pandemic. Nearly 26,000 women were vaccinated against pandemic influenza while pregnant.
"Norway is one of very few countries that have the opportunity to study these issues because of our excellent health registers. The registers include information about pregnancy and pandemic vaccination," says Dr. Camilla Stoltenberg, Director General of the Norwegian Institute of Public Health.
When analyzing the data, the researchers took into account the timing of vaccination or influenza in pregnancy and the duration of each pregnancy.
- Pregnant women who were vaccinated did not have increased risk of foetal death.
- Pregnant women who were vaccinated had a considerably reduced risk of influenza.
- Pregnant women who were diagnosed with influenza during the pandemic had almost doubled risk of foetal death.
The Medical Birth Registry of Norway collects information about fetal death after week 12 of pregnancy, so the researchers could not examine early losses. Fetal death is rare in Norway (about 5 per 1000 births). Although this study found an increased risk after influenza, few pregnancies end in fetal death.
The results from this large study are in line with smaller studies from Denmark and Canada that also indicate that the pandemic vaccine was not associated with increased risk of fetal death.
"It is reassuring that such a large and comprehensive study did not find any evidence that vaccination increased the risk of foetal death," says Stoltenberg. "The results suggest that influenza during pregnancy can be detrimental for the foetus, even if the mother is not seriously ill and admitted to hospital."
"We recommend that pregnant women are vaccinated against influenza. In addition to protecting the mother, the vaccine protects the child in the first months after birth, at a time when the child is too young to be vaccinated."
Published in the New England Journal of Medicine.