Table 3.
Crude and adjusted hazard ratios for pre-delivery hospitalisation due to influenza like illness and key adverse birth outcomes stratified by trimester of influenza vaccination and influenza activity.
| Variables | Unvaccinated N (%) | Vaccinated N (%) | Crude HR* (95% CI) | p-value | Adjusted† aHR (95% CI) | p-value |
|---|---|---|---|---|---|---|
| Pre-delivery hospitalisation due to influenza like illness 1st trimester 2nd trimester 3rd trimester Low influenza activity High influenza activity |
60/650 (9•2) | 35/603 (5•8) | 0•58 (0•37, 0•91) 0•43 (0•18, 0•99) 0•09 (0•01, 0•68) 0•70 (0•43, 1•13) 0•58 (0•33, 0•99) 0•47 (0•26, 0•85) |
0•018 0•049 0•019 0•149 0•049 0•013 |
0•61 (0•39, 0•97) 0•46 (0•19, 1•09) 0•09 (0•01, 0•72) 0•73 (0•44, 1•22) 0•60 (0•35, 1•05) 0•53 (0•29, 0•96) |
0•038 0•080 0•023 0•244 0•079 0•039 |
| Preterm birth‡ 1st trimester 2nd trimester 3rd trimester Low influenza activity High influenza activity |
49/608 (8•0) | 40/599 (6•6) | 0•94 (0•60, 1•47) 0•46 (0•18, 1•16) 0•91 (0•43, 1•93) 0•79 (0•47, 1•33) 0•61 (0•33, 1•12) 0•85 (0•50, 1•45) |
0•802 0•111 0•811 0•384 0•112 0•571 |
0•94 (0•59, 1•49) 0•50 (0•19, 1•28) 0•90 (0•41, 1•94) 0•75 (0•43, 1•29) 0•61 (0•33, 1•12) 0•89 (0•52, 1•53) |
0•817 0•151 0•793 0•304 0•113 0•696 |
| Spontaneous preterm birth ‡ 1st trimester 2nd trimester 3rd trimester Low influenza activity High influenza activity |
36/608(5•9) | 23/599 (3•8) | 0•71 (0•40, 1•26) 0•37 (0•11, 1•22) 0•61 (0•21, 1•73) 0•61 (0•31, 1•18) 0•53 (0•25, 1•11) 0•58 (0•28, 1•17) |
0•253 0•104 0•361 0•147 0•096 0•131 |
0•74 (0•41, 1•33) 0•41 (0•12, 1•37) 0•61 (0•21, 1•76) 0•63 (0•32, 1•25) 0•55 (0•26, 1•17) 0•60 (0•29, 1•26) |
0•323 0•149 0•364 0•193 0•123 0•180 |
| LBW (<2500 g)§ 1st trimester 2nd trimester 3rd trimester Low influenza activity High influenza activity |
49/606 (8•0) | 31/599 (5•1) | 0•70 (0•42, 1•14) 0•56 (0•24, 1•35) 0•47 (0•16, 1•30) 0•57 (0•32, 1•03) 0•66 (0•37, 1•19) 0•43 (0•22, 0•86) |
0•158 0•206 0•161 0•065 0•171 0•018 |
0•71 (0•43, 1•19) 0•54 (0•23, 1•29) 0•41 (0•14, 1•16) 0•63 (0•34, 1•15) 0•63 (0•34, 1•14) 0•46 (0•23, 0•94) |
0•202 0•168 0•096 0•138 0•170 0•033 |
| LBW at term (<2500 g)§,||,¶ Low influenza activity High influenza activity |
20/557 (3•5) | 9 /559 (1•6) | 0•43 (0•18,1•09) 0•61 (0•24, 1•54) 0•20 (0•05, 0•89) |
0•048 0•303 0•035 |
0•38 (0•16, 0•89) 0•48 (0•18, 1•36) 0•20 (0•04, 0•87) |
0•027 0•132 0•032 |
| SGA§ 1st trimester 2nd trimester 3rd trimester Low influenza activity High influenza activity |
83 /608 (13•6) | 61/599 (10•1) | 0•77 (0•55, 1•09) 1•15 (0•71, 1•86) 0•79 (0•42, 1•49) 0•57 (0•36, 0•89) 0•88 (0•59, 1•32) 0•60 (0•37, 0•95) |
0•152 0•549 0•476 0•014 0•596 0•030 |
0•84 (0•58, 1•20) 1•22 (0•74, 2•02) 0•79 (0•41, 1•50) 0•61 (0•38, 0•98) 0•92 (0•61, 1•39) 0•65 (0•40, 1•04) |
0•346 0•347 0•483 0•044 0•708 0•079 |
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 vaccinated at 37 weeks’ or later were censored because they were no longer at risk of having a preterm birth.
Additionally adjusted for infant's sex.
Low birthweight at term (<2500 g and ≥ 37 completed weeks’ gestation at birth).
Analysis by trimester of influenza vaccination was not performed because a small number of mothers who delivered LBW at term babies received the vaccine prior to their third trimester (n = 1 during 1st trimester, n = 1 during 2nd trimester).