Characteristics of the 24 cohort studies and 5 case–control studies that compared seasonal influenza vaccination (TIIV and or QIIV) to no vaccination
| Study Country of conduct; design; funding | Recruitment period | No of women (infants) | Interventions (cohort studies)/exposure window (case–control studies) | Safety endpoints |
| Cohort studies | ||||
| McMorrow et al, 202086 South Africa; RC; Non-industry | Women who sought antenatal care between 14 May 2015–16 August 2015 and 1 April 2016–25 August 2016 | 4084 (4084) | Seasonal vaccine (TIIV) No vaccine | Stillbirths, PTB, LBW, SGA |
| Mohammed et al, 202087 Australia; PC; No funding | First antenatal appointment between March 2015 and December 2017 | 1253 (1201) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA, SAB, CA |
| Ohfuji et al, 202091 Japan; PC; Non-industry | Attended antenatal clinic between October and December 2013, prior to the 2013/2014 influenza season | 10 330 (NR) | Seasonal vaccine (TIIV) No vaccine | SAB, stillbirths, PTB, LBW, CA |
| Speake et al, 2020104 Australia; RC; Non-industry | Date of conception estimated to have occurred between December and July, from April 2012 to July 2015 | 70 838 (70 838) | Seasonal vaccine (TIIV and QIIV) No vaccine | Stillbirths, PTB, SGA |
| McHugh et al, 201984 Australia; RC; Non-industry | Pregnant between 1 December 2003 and 31 December 2006 or between 1 January 2009 and 31 June 2011 | 576 (697) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA |
| McHugh et al, 201985 Australia; PC, post hoc analysis; Non-industry | Within 55 days of birth from 1 April 2012 to 31 December 2015; recruitment maximised during influenza seasons (April–October) | 8827 (8,827) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA |
| Singh et al, 2019103 India; RC; NR | Pregnant and vaccinated or refused vaccination between January 2016 and March 2018 | 346 (348) | Seasonal vaccine (TIIV) No vaccine | Maternal SAEs, PTB |
| Ohfuji et al, 201892 Japan; PC; Non-industry | Pregnant between September and December 2013 | 3044 (3044) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, CA |
| Arriola et al, 201758 Nicaragua; PC/RC; Non-industry | Births from 21 July 2014 to 4 December 2014 | 3268 (3268) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA |
| McHugh et al, 201783 Australia; Nested RC; Non-industry | Births from 1 April 2012 to 31 December 2014 | 7126 (7126) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW |
| Zerbo et al, 2017108 USA; RC; Non-industry | Births from 1 January 2010 to 31 December 2015 | 145 869 (145 869) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA |
| Chambers et al, 201664 USA and Canada; PC; Mixed | Pregnant between 2010 and the end of 2014 | 1730 (1707) | Seasonal vaccine (TIIV and QIIV) No vaccine | SAB, stillbirths, PTB, SGA |
| Olsen et al, 201693 Laos; PC; Non-industry | Births from April 2014 to February 2015 | 5103 (5103) | Seasonal vaccine (TIIV by Afluria) No vaccine | PTB, SGA |
| Regan et al, 201697 Australia; RC; Non-industry | Births from 1 April 2012 to 31 December 2013 | 58 008 (57 660) | Seasonal vaccine (TIIV) No vaccine | Stillbirths |
| Ahrens et al, 201457 USA; RC (control group from a case–control study); Non-industry | Pregnant and vaccinated or matched by LMP to vaccinated group from 2006 to 2010 | 1619 (1619) | Seasonal vaccine (TIIV) No vaccine | PTB, SGA |
| Legge et al, 201477 Canada; RC; No funding | Births from 1 November 2010 to 31 March 2012 | 12 223 (11 293) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA |
| Nordin et al, 201490 USA; RC; Non-industry and Insurance | Pregnant during influenza seasons between 2004/2005 and 2008/2009 | 150 089 (150 089) | Seasonal vaccine (TIIV) No vaccine | PTB, SGA |
| Adedinsewo et al, 201355 USA; RC; Non-industry and Insurance | Births from 1 January 2005 to 31 December 2008 | 5422 (NR) | Seasonal vaccine (TIIV) No vaccine | PTB, SGA |
| Nordin et al, 201389 USA; RC; Non-industry and Insurance | Pregnant between 1 June 2002 and 31 July 2009 and vaccinated or matched | 223 898 (NA) | Seasonal vaccine (TIIV) No vaccine | Maternal SAEs |
| Dodds et al, 201268 Canada; RC; Non-industry | Births from 1 April 2006 to 31 October 2009 | 9647 (9647) | Seasonal vaccine (TIIV) No vaccine | PTB, LBW, SGA |
| Sheffield et al, 2012102 USA; PC; NR | Pregnant between October and March in 2003–2008 | 84 843 (85 783) | Seasonal vaccine (TIIV) No vaccine | CA, stillbirths, PTB, LBW |
| Omer et al, 201194 USA; RC; Non-industry | Births from 1 June 2004 to 30 September 2006 | 4168 (NR) | Seasonal vaccine (TIIV) No vaccine | PTB, SGA |
| Munoz et al, 200588 USA; RC; Non-industry | Pregnant during influenza seasons from 1 July 1998 to 30 June 2003 | 1051 (1051) | Seasonal vaccine (TIIV by Aventis Pasteur or Wyeth) No vaccine | Maternal SAEs, PTB, CA |
| Black et al, 200461 USA; RC; NR | Births between November and February from November 1997 to February 2002 | 49 585 (48 639) | Seasonal vaccine (TIIV) No vaccine | PTB |
| Case–control studies | ||||
| Panagiotakopopulos et al, 2020116 USA; RCC; Non-industry | Pregnancies ending in live or stillbirth from 1 January 2012 to 30 September 2015 | 3975 | Seasonal vaccine (TIIV, with or without Tdap), 14 days after last menstrual period through 7 days before index date | Stillbirths |
| Donahue et al., 2019111 USA; RCC; Non-industry | Pregnant during the 2012–2013, 2013–2014 or 2014–2015 influenza seasons | 2472 | Seasonal vaccine (TIIV) 1–28 days after conception, >28 days after conception and 1–28 days before SAB | SAB |
| Donahue et al, 2017110 USA; RCC; Non-industry | During the 2010–2011 or 2011–2012 influenza seasons | 970 | Seasonal vaccine (TIIV) 1–28 days before SAB | SAB |
| Louik et al, 2016115 USA; RCC; Mixed | During the 2011–2012, 2012–2013 or 2013–2014 influenza vaccine seasons | 1910 | Seasonal vaccine (TIIV or QIIV) during first trimester | CA |
| Irving et al, 2013113 USA; RCC; Non-industry | During the 2005–2006 or 2006–2007 influenza seasons | 486 | Seasonal vaccine (TIIV) any time during pregnancy and 1–28 days before SAB | SAB |
CA, congenital anomalies; LBW, low birth weight; NR, not reported; PC, prospective cohort; PTB, preterm birth; QIIV, quadrivalent inactivated influenza vaccine; RC, retrospective cohort; RCC, retrospective case–control; SAB, spontaneous abortion; SAE, serious adverse events; SGA, small-for-gestational-age birth; Tdap, tetanus toxoid, reduced diphtheria toxoid and acellular pertussis; TIIV, trivalent inactivated influenza vaccine.