A CDC-funded study found that people who had been vaccinated early in pregnancy with an influenza vaccine containing the pandemic H1N1 (H1N1pdm09) component and who also had been vaccinated the prior season with an H1N1pdm09-containing influenza vaccine had an increased risk of spontaneous abortion (miscarriage) in the 28 days after vaccination. This study did not quantify the risk of miscarriage and did not prove that influenza vaccine was the cause of the miscarriage. Earlier studies have not found a link between influenza vaccination and miscarriage. A larger follow-up study also funded by CDC which included 3 more years of data found no association between early miscarriage and influenza vaccination regardless of previous influenza season vaccination. These results are reassuring regarding the safety of influenza vaccination during pregnancy.
CDC, ACIP, and the American College of Obstetricians and Gynecologists (ACOG) all recommend influenza vaccination during any trimester of pregnancy. Influenza poses a danger to pregnant people and the vaccine can prevent influenza in pregnant people and their infants.