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Influenza is an acute viral infection of the respiratory tract. Of the three types of influenza virus (A, B and C) influenza A and B are responsible for most clinical illness. Influenza is characterised by the sudden onset of fever, chills, headache, myalgia and extreme fatigue. For otherwise healthy people, influenza is an unpleasant but generally self-limiting illness. However, for certain groups - including pregnant women - there is a risk of more serious illness. Influenza during pregnancy may also be associated with perinatal mortality, prematurity, smaller neonatal size and lower birth weight.
All pregnant women are recommended to have the flu vaccine, irrespective of their stage of pregnancy. Inactivated vaccine can be administered safely during any trimester, and there has been no study to date that shows any increase in the risk of harm to the unborn baby or in maternal complications. On the contrary, flu vaccination during pregnancy may reduce the likelihood of preterm birth and smaller infant size at birth, and it also offers some protection to the baby in its first few months of life. Ideally, vaccine should be given before flu starts to circulate, but it is important that vaccination is offered to any woman who becomes pregnant during the flu season. The perfect time for this to take place is at the woman's first midwife appointment, but if the midwife is not an independent prescriber, the woman should be encouraged to attend her GP surgery for vaccination.
Public Health England (2015). Immunisation against infectious disease. Influenza: the green book, chapter 19. Available at: https://www.gov.uk/government/publications/influenza-the-green-book-chapter-19