In this large nationwide cohort study, vaccination with an AS03 adjuvanted pandemic A/H1N1 2009 influenza vaccine in pregnancy was not associated with an increased risk of the primary composite outcome of fetal death, or its components spontaneous abortion and stillbirth. Given the upper limit of the confidence interval, these data allowed the exclusion of a 17% increased risk of fetal death, a 74% increased risk of spontaneous abortion, and any risk of stillbirth associated with vaccination. Results were similar in healthy pregnant women and those with comorbidities.

Pandemrix is a split virion vaccine produced from the A/California/7/2009 (H1N1)v-like strain and contains an AS03 adjuvant (composed of squalene, DL-α-tocopherol, and polysorbate 80). Our results are principally applicable to this particular vaccine. Additionally, we believe that results are generalisable to non-adjuvanted vaccines produced from this virus strain (although these generally have a higher content of the antigen haemagglutinin) but not to vaccines with other adjuvants. Because of the antigenic differences, it is less clear whether our results add to the available data on safety of fetuses when using non-adjuvanted seasonal influenza vaccines, for which limited information on risk of fetal death is available. 3 8

Previous reports on the fetal safety of H1N1 vaccination, although reassuring, have been non-analytical and limited in size. The Vaccine Adverse Events Reporting System in the United States received 131 reports of pregnancy specific events in women receiving non-adjuvanted H1N1 vaccines, with spontaneous abortion (n=95) and stillbirth (n=18) representing the most common events 11 ; estimated reporting rates were considerably lower than expected rates. A review of spontaneous reports received by the European Medicines Agency identified 130 pregnancy related outcomes, including 57 abortions and 49 intrauterine deaths or stillbirths. Given the number of vaccinated women and background rates, this, in addition to data provided by the manufacturers, was not considered to indicate increased risk. 7 14 Similarly, pharmacovigilance reports from Taiwan and France found a lower than expected number of reported events. 10 13 These studies, based on data from passive surveillance systems, have limitations; importantly, although they can generate risk signals, they cannot exclude risks. Two descriptive cohort studies have been published. In a post-authorisation study of the AS03 adjuvanted H1N1 vaccine in the United Kingdom, pregnancy outcomes from 265 vaccinated pregnant women were reported 12 ; the prevalence of spontaneous abortions and stillbirths was comparable to expected rates. A French study had outcome data for 569 pregnant women receiving a non-adjuvanted H1N1 vaccine; rates of fetal death (unclear definition) and stillbirth were not higher than in other surveys. 9 Thus our cohort study confirms the findings from limited previous reports and expands on these findings by providing the first detailed comparative analysis of the risk of fetal death associated with H1N1 vaccination.

Strengths and limitations of the study

Strengths of this study include its size and its comprehensive design, with register linkage of individual level data. Information on H1N1 vaccination was obtained through a nationwide database, to which reporting was mandatory. This eliminates recall bias, ensures completeness, and improves the accuracy of information on timing of vaccination compared with self reported exposure. Registration of births is mandatory in the medical birth register; it is therefore unlikely that we missed any significant number of births. Most cases of spontaneous abortion in Denmark are likely to be managed by hospital doctors. According to national guidelines for pregnancy care, the investigation of early pregnancy bleeds includes ultrasonography as a central part, and guidelines from the Danish Society of Obstetrics and Gynecology indicate that the diagnosis of spontaneous abortion requires confirmation by ultrasonography.29 30 An ultrasound examination on this indication could only be carried out by an obstetrician-gynaecologist. We therefore believe that our hospital based strategy for the detection of spontaneous abortions was close to complete. We excluded registered spontaneous abortions with less than six completed weeks of gestation—that is, early pregnancy loss; only a limited proportion of early pregnancy losses are recognised clinically, therefore inclusion of this time period in the analyses would have introduced outcome misclassification.

A relatively small proportion (13%) of pregnant women in this nationwide Danish cohort was vaccinated against pandemic A/H1N1 2009 influenza. This rate is similar to that in some countries but contrasts with the rate in others—for example, the estimated vaccine coverage among pregnant women was 8% in Germany, 23% in France, and 40% in the United States.31 32 33 The reason for the low vaccine uptake in Denmark possibly relates to the fact that the 2009-10 season was the first time that pregnant women without comorbidities were included among the target groups for influenza vaccination.

Although we adjusted for many potential confounders, there might have been differences between vaccinated and unvaccinated women associated with both exposure and outcome that we could not measure. Of concern would be factors that could have obscured a risk of fetal death associated with vaccination. Such factors would have to be common or strongly associated with both vaccination and reduced risk of fetal death. A few factors might meet these criteria. Although vaccinated women in our study had a somewhat higher rate of comorbidities compared with unvaccinated women, a large proportion of the cohort did not have any registered comorbidities. A healthy vaccinee effect, conferred by, for example, unmeasured factors such as diet, exercise, educational level, or socioeconomic class, that masks a risk of fetal death among vaccinated women cannot be excluded. However, given our observed main estimate of 0.79, the difference in prevalence between the groups would have to be large and the association with reduced risk of fetal death strong to obscure a true increased risk associated with vaccination. Additionally, we tackled the problem of a healthy vaccinee effect partly by adjusting for smoking and body mass index, important risk factors for stillbirth,34 in a sensitivity analysis; this did not change the estimates. Because most people who contract influenza never seek healthcare, the data sources used in our study did not allow for a precise adjustment for influenza infection. However, in sensitivity analyses where we adjusted for influenza infection, defined as the registration of a hospital diagnosis for influenza or the filling of a prescription for an anti-influenza antiviral drug, the main results were unchanged. It should, however, be noted that this definition of influenza could not detect those cases of influenza infection where patients did not seek hospital care or cases where patients were not prescribed antivirals. In a scenario where a true increased risk of fetal death associated with vaccination exists, an increased risk of fetal death conferred by H1N1 infection itself among unvaccinated women might have biased results towards the null. However, given the 5% cumulative incidence of H1N1 infection in Denmark,35 we reason that confounding by infection strong enough to contradict the conclusion of this study is unlikely. Finally, the absence of a strong influence from unmeasured confounding is supported by our sensitivity analysis, which showed that when adjusting for a hypothetical confounder with high prevalence among unvaccinated women and strong association with the outcome, the confounder adjusted estimate for the association between vaccination and fetal death would not be far over 1. This sensitivity analysis indicated similar results when assuming a scenario where vaccinated women had a higher prevalence of an unmeasured confounder that was strongly protective.

Several factors need to be considered when interpreting our finding of a significantly decreased risk of stillbirth associated with vaccination against H1N1 influenza. Although H1N1 infection is associated with an increased risk of stillbirth,36 and a protective effect of vaccination therefore may seem plausible, the objective of our study was to investigate a possible increased risk of adverse events. Additionally, the analysis of stillbirth was a prespecified secondary outcome and based on a small number of cases. A chance finding therefore represents a possibility. Thus the result from this analysis should be viewed as hypothesis generating and needs investigation in other studies before any conclusions on protective effects are drawn.