But a second, related study found that Canadians aged 6 months or older who received the H1N1 vaccine were at a slightly higher risk for developing Guillain-Barre syndrome during the two months following the shot.
Guillain-Barre syndrome is a nervous system disorder. It can causes muscle weakness, loss of reflexes, and numbness or tingling in the arms, legs, face, and other parts of the body. Most people recover without any lasting damage. In severe cases, however, GBS can cause paralysis and death.
According to the study, about two people developed GBS for every 1 million who received the vaccine.
The swine flu vaccine of 1976 was also linked to an increased risk of GBS, with one case of GBS per 100,000 people receiving the vaccine.
The flu itself increases risk of GBS as well.
H1N1 Shot Not Linked to Major Birth Defects, Preterm Birth
In the Danish study, researchers analyzed rates of major birth defects, preterm birth, and restricted growth in the uterus for infants in Denmark delivered between Nov. 2, 2009, and Sept. 30, 2010. Of 53,432 infants, 13.1% were exposed to the vaccine during pregnancy.
Infants whose mothers received the vaccine were not at any greater risk for any of these outcomes. This was true regardless of which trimester in pregnancy their moms received the shot, but fewer women got vaccinated in the first trimester.
"We conclude that H1N1-vaccinated pregnancies are not at increased risk of these adverse events," says study author Anders Hviid, an epidemiologist at Statens Serum Institut in Copenhagen, Denmark. "Our study contributes to the information available on the safety of influenza vaccination in pregnancy, [but] it is by no means a complete evaluation of vaccination in pregnancy."
During the H1N1 flu pandemic of 2009, pregnant women were urged to get vaccinated against this new swine flu strain with an all-new shot. An estimated 2.4 million women were vaccinated against the H1N1 flu during pregnancy in the United States alone, according to information in the new study.
The H1N1 vaccine in the new studies contained an adjuvant, or added chemical, to increase the body's immune response to the vaccine. Adjuvants were not given in the U.S. Neither the U.S. pandemic H1N1 vaccine nor the U.S. seasonal flu vaccines contain adjuvants.
But that doesn't mean the findings are irrelevant, says Mark C. Steinhoff, MD. He is the director of the Children's Global Health Center at Cincinnati Children's Hospital Medical Center. Steinhoff co-wrote an editorial accompanying the two new studies.
"In the event of a future pandemic, we will know that this adjuvant is safe," he says.
Pregnant Women Should Get Flu Shot
The most important message is that pregnant women need to get the flu shot to protect their own health and that of their infant. Pregnancy can affect the immune system.
"The probability of getting hospitalized with the flu when you are 20-something and pregnant is seven times higher than when you are 20-something and not pregnant," Steinhoff says. "Your baby pays the price, too."
Infants may be protected by their moms' antibodies against the flu for the first six months of life, a period of time when they are susceptible to the flu and severe complications.
Jill Rabin, MD, always reminds pregnant women to get their yearly flu shots. She is the chief of ambulatory care, obstetrics, and gynecology, and head of urogynecology at Long Island Jewish Medical Center in New Hyde Park, N.Y.
"Getting the flu during pregnancy is much more dangerous to the mother and baby than the vaccine," she says. "Pregnant women should be the first in line, as the benefits outweigh the risks, and the risks can be fatal."
The two studies appear in the July 11 issue of the Journal of the American Medical Association.