Table 2.
Crude and adjusted hazard ratios for time-based pregnancy and birth outcomes by maternal influenza vaccination status at two obstetric hospitals in South Australia 2015–2018.
| Variables | Total | Unvaccinated N (%) | Vaccinated N (%) | Crude HR* (95% CI) | p-value | Adjusted † aHR (95% CI) | p-value |
|---|---|---|---|---|---|---|---|
| Pre-delivery hospitalisation due to influenza like illness ‡ | 95/1253 (7•5) | 60/650 (9•2) | 35/603 (5•8) | 0•58 (0•37, 0•91) | 0•018 | 0•61 (0•39, 0•97) | 0•038 |
| Spontaneous abortion § | 34/1253 (2•7) | 31/650 (4•7) | 3/603 (0•5) | 0•66 (0•20, 2•19) | 0•507 | 0•42 (0•12, 1•45) | 0•171 |
| Gestational hypertension || | 81/1205 (6•7) | 41/606 (6•7) | 40/599 (6•6) | 0•80 (0•49, 1•31) | 0•391 | 0•78 (0•47, 1•29) | 0•343 |
| Pre-eclampsia || | 111/1205 (9•2) | 58/606 (9•5) | 53/599 (8•8) | 0•85 (0•58, 1•26) | 0•445 | 0•84 (0•54, 1•27) | 0•417 |
| Severe pre-eclampsia || | 28/1204 (2•3) | 14/606 (2•3) | 14 /598 (2•3) | 0•86 (0•37, 1•96) | 0•725 | 0•65 (0•26, 1•64) | 0•368 |
| Gestational diabetes ¶ | 190/1207 (15•7) | 85/608 (13•9) | 105/599 (17•5) | 1•33 (0•95, 1•84) | 0•088 | 1•16 (0•82, 1•66) | 0•383 |
| Preterm premature rupture of the membranes ⁎⁎ | 47/1207 (3•8) | 27/608 (4•4) | 20/599 (3•3) | 0•82 (0•43, 1•56) | 0•561 | 0•85 (0•44, 1•63) | 0•634 |
| Preterm birth ⁎⁎ | 89/1207 (7•3) | 49/608 (8•0) | 40/599 (6•6) | 0•94 (0•60, 1•47) | 0•802 | 0•94 (0•59, 1•49) | 0•817 |
| Spontaneous preterm birth ⁎⁎ | 59/1207 (4•8) | 36/608(5•9) | 23/599 (3•8) | 0•71 (0•40, 1•26) | 0•253 | 0•74 (0•41, 1•33) | 0•323 |
| LBW (<2500 g) †† | 80/1205 (6•6) | 49/606 (8•0) | 31/599 (5•1) | 0•70 (0•42, 1•14) | 0•158 | 0•71 (0•43, 1•19) | 0•202 |
| LBW at term (<2500 g)††,‡‡ | 29/1116 (2•6) | 20/557 (3•5) | 9 /559 (1•6) | 0•43 (0•18, 0•99) | 0•048 | 0•38 (0•16, 0•89) | 0•027 |
| SGA †† | 144/1207 (11•9) | 83 /608(13•6) | 61/599 (10•1) | 0•77 (0•54, 1•09) | 0•152 | 0•84 (0•58, 1•20) | 0•346 |
| Difference in means (vaccinated- unvaccinated) | Difference in adjusted means (vaccinated -unvaccinated) | ||||||
| Mean birth weight ††, g (95% CI) | 3334•9 ± 557 | 3301•8 ± 610 | 3368•4 ± 498 | 63•7 (0•09, 127•0) | 0•050 | 58•8 (- 4•2, 121•7) | 0•067 |
| Mean gestational age at delivery, weeks (95% CI) | 39•2 ± 2•0 | 39•1 ± 2•3 | 39•4 ± 1•6 | 0•26 (0•04, 0•49) | 0•019 | 0•27 (0•04, 0•49) | 0•019 |
CI=confidence interval. HR=hazard ratios. LBW=low birthweight. SGA=small for gestational age.
HR results compared outcome variable in vaccinated group to reference (unvaccinated).
Adjustments were made for maternal age, race/ethnicity, education, household income, gravidity, intake of alcohol and recreational drugs, smoking, pre-pregnancy body mass index (continuous), use of multivitamin supplements, Edinburgh Postnatal. Depression Scale (EPDS), The State-Trait Anxiety Inventory (STAI), Perceived Stress Scale (PSS-10), physical activity, infertility treatment, asthma and estimated season of delivery.
Women admitted to hospital with influenza/ respiratory tract infection were censored at their admission date.
The time metric for spontaneous abortion analysis was the first week of gestation up to the event (week of last available pregnancy data or week 20 of gestation; whichever occurred first).
For hypertensive disorders analysis, women who were vaccinated at or after the gestational age at diagnosis (≥ 20 weeks' gestation) and pregnancies ending prior to 20 weeks of gestation were censored.
Women who were vaccinated at or after the gestational age at diagnosis of gestational diabetes mellitus (median gestational age at screening was 27•8 (IQR, 26•5–29) weeks) were censored.
Women vaccinated at 37 weeks’ or later were censored because they were no longer at risk of preterm birth.
Additionally adjusted for infant's sex.
Low birthweight at term (<2500 g and ≥ 37 completed weeks’ gestation at birth).