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Seasonal Influenza in Pregnancy

Study Extended and Controls Added

The UKOSS Seasonal Influenza in pregnancy study has been extended and will now end on the 31st of October 2018. Please continue to report cases as previously. To enhance analysis of this study we will now request data on two control women for every case OCCURRING on or after the 1st of November 2017. You will NOT be asked to supply controls for cases that have already been reported or occurred before this date. For more information on controls please scroll down to 'Controls'.

Key points

  • Pregnant women are particularly vulnerable to severe infection, resulting in increased risk of maternal and perinatal mortality.
  • Specific groups of women are at higher risk of morbidity after 2009/H1N1 infection in pregnancy.
  • Seasonal influenza immunisation is now routinely offered to all pregnant women.
  • Further evidence on the outcomes of seasonal influenza infection in pregnancy is required.

Surveillance Period

This study will now finish in 2018
1st November 2016 – 31st October 2018


Evidence from the last influenza pandemic (2009/H1N1) showed that pregnant women were particularly vulnerable to severe infection[1][2][3][4][5], resulting in increases in both maternal and perinatal mortality[5]. Further investigations, including the 2009 UKOSS study[5], highlighted specific groups of women who were at higher risk of morbidity after 2009/H1N1 infection in pregnancy. Factors associated with admission to hospital with 2009/H1N1 in pregnancy included maternal obesity, asthma, multiparity, multiple pregnancy, black or other minority group ethnicity and smoking among women younger than 25 years[1][3][5]. Overall, these studies showed a clear increase in risk of poor maternal and pregnancy outcomes in women infected with A/H1N1v influenza. Almost half of preterm deliveries were due to early delivery for maternal compromise, indicating that the health of pregnant women, which is improved with rapid treatment with antiviral agents, is an important public health priority in future waves of this and other influenza pandemics.

Evidence that seasonal influenza vaccination during pregnancy prevents influenza in women and their children is clear. However, the impact on prevention of adverse pregnancy outcomes is less clear, and there has been a recent call for further research on seasonal influenza to assess this[6].


To use the UK Obstetric Surveillance System (UKOSS) to determine the incidence and characteristics of women hospitalised with seasonal influenza in pregnancy and the outcomes of seasonal influenza in pregnancy for mother and infant.

Research questions

  • What is the incidence of women hospitalised with seasonal influenza in pregnancy?
  • What are the outcomes of seasonal influenza in pregnancy for both mother and infant?
  • What is the influence of demographic or pregnancy characteristics on outcomes for mother and infant?
  • What is the influence of prior immunisation with seasonal influenza vaccine on outcomes for mother and infant, including an assessment of reasons for non-immunisation?
  • What is the influence of timing of delivery, particularly in relation to the use of extracorporeal membrane oxygenation (ECMO) on outcomes for mother and infant?

Case definition

All women in the UK admitted to hospital with confirmed influenza infection in pregnancy.


For every case of seasonal influenza occurring on or after the 1st of November 2017 reporters will be asked to complete two control data collection forms. These will be sent out by post along with the case data collection form. We will not be asking for control data for cases that have already been reported or occurred before the 1st of November 2017.

To identify your controls please first identify the date and time of delivery for the woman you have reported who has been hospitalised with confirmed influenza (the case). If the case woman has not delivered at the time of reporting please complete the case form as far as possible and return to the UKOSS office. Keep hold of the control forms until after the case woman's delivery. After the case woman's EDD has passed you will be sent a copy of the case data collection form for you to complete as well as a reminder to now identify and complete the control data.

Using the delivery suite/operating theatre records identify the two pregnant women delivering immediately BEFORE the woman who has had influenza (these women should NOT have had influenza). These women will act as the controls. Retrieve the hospital case notes for these control women from medical records and complete a Control data collection form for each of the women you have identified as the controls.


This study is funded by the NIHR and the Policy Research Programme in the Department of Health.

Ethics committee approval

This study has been approved by the North London REC1 (REC Ref. Number: 10/H0717/20).

Lead Investigator

Professor Marian Knight, NPEU.

Sample Data Collection Form

UKOSS Seasonal Influenza DCF


  1. a, b Critical illness due to 2009 A/H1N1 influenza in pregnant and postpartum women: population based cohort study. BMJ. 2010;340:c1279.
  2. ^ Jamieson DJ, Honein MA, Rasmussen SA, Williams JL, Swerdlow DL, Biggerstaff MS, et al. H1N1 2009 influenza virus infection during pregnancy in the USA. Lancet. 2009 Aug 8;374(9688):451-8.
  3. a, b Louie JK, Acosta M, Jamieson DJ, Honein MA. Severe 2009 H1N1 influenza in pregnant and postpartum women in California. N Engl J Med. 2010 Jan 7;362(1):27-35.
  4. ^ Siston AM, Rasmussen SA, Honein MA, Fry AM, Seib K, Callaghan WM, et al. Pandemic 2009 influenza A(H1N1) virus illness among pregnant women in the United States. JAMA. 2010 Apr 21;303(15):1517-25.
  5. a, b, c, d Yates LM, Pierce M, Stephens S, Mill AC, Spark P, Kurinczuk JJ, et al. Influenza A/H1N1v in pregnancy: Aninvestigation of the characteristics of affected women and the relationship to pregnancy outcomes for mother and infant. Health Technol Assess. 2010;14(34):109-82.
  6. ^ Savitz DA, Fell DB, Ortiz JR, Bhat N. Does influenza vaccination improve pregnancy outcome? Methodological issues and research needs. Vaccine. 2015; 33(47):6430-5.

Updated: Tuesday, 17 January 2023 18:32 (v15)