Table 2.
Rate Ratios for the Association Between the Proportion of Positive Influenza Specimens and the Primary Study Outcomes During Early and Late Pregnancy Exposure Windows, Ontario, Canada 2003–2012
| Outcome and Exposure Window | Unadjusted RRa | 95% CI | Adjusted RRa,b | 95% CI |
|---|---|---|---|---|
| Preterm birth at <37 weeksc | ||||
| Final week of gestation | 1.00 | 0.99, 1.01 | 1.01 | 1.00, 1.02 |
| Final month of gestation | 0.99 | 0.98, 1.00 | 1.01 | 0.99, 1.02 |
| First month of gestation | 1.00 | 0.99, 1.01 | 1.00 | 0.99, 1.02 |
| Preterm birth at <32 weeksd | ||||
| Final week of gestation | 0.98 | 0.96, 1.00 | 0.98 | 0.97, 1.00 |
| Final month of gestation | 0.98 | 0.96, 1.01 | 0.99 | 0.97, 1.01 |
| First month of gestation | 1.00 | 0.98, 1.03 | 1.00 | 0.98, 1.03 |
| Stillbirthe | ||||
| Final week of gestation | 0.98 | 0.96, 1.01 | 0.98 | 0.96, 1.01 |
| Final month of gestation | 0.99 | 0.97, 1.02 | 0.99 | 0.96, 1.02 |
| Perinatal deathf | ||||
| Final week of gestation | 1.00 | 0.98, 1.02 | 1.00 | 0.98, 1.02 |
| Final month of gestation | 1.01 | 0.98, 1.03 | 1.01 | 0.99, 1.03 |
Abbreviations: CI, confidence interval; RR, rate ratio.
a Estimates represent the relative change in the rate for a 10–percentage point increase in the proportion of positive influenza specimens.
b Adjusted for long-term and seasonal trends via cubic splines and for the proportion of positive respiratory syncytial virus specimens in corresponding time period.
c For late-pregnancy exposure windows, the number of preterm births (<37 weeks’ gestation) per influenza surveillance week was aggregated by the date of birth and offset by the number of gestations at risk during the corresponding week (i.e., ongoing gestations at 20–36 weeks). For the early-pregnancy exposure window, the number of preterm births (<37 weeks’ gestation) per influenza surveillance week was aggregated by the estimated date of conception and offset by the total number of conceptions during the corresponding week that resulted in livebirths.
d For late-pregnancy exposure windows, the number of preterm births (<32 weeks’ gestation) per influenza surveillance week was aggregated by the date of birth and offset by the number of gestations at risk during the corresponding week (i.e., ongoing gestations at 20–31 weeks). For the early-pregnancy exposure window, the number of preterm births (<32 weeks’ gestation) per influenza surveillance week was aggregated by the estimated date of conception and offset by the total number of conceptions during the corresponding week that resulted in livebirths.
e For late-pregnancy exposure windows, the number of stillbirths per influenza surveillance week was aggregated by the date of birth and offset by the number of gestations at risk during the corresponding week (i.e., ongoing gestations at ≥20 weeks). Outcomes could not be assessed during the early-pregnancy exposure window.
f For late-pregnancy exposure windows, the number of perinatal deaths per influenza surveillance week was aggregated by the date of birth and offset by the number of gestations at risk during the corresponding week (i.e., ongoing gestations at ≥20 weeks). Outcomes could not be assessed during the early-pregnancy exposure window.