Inactivated influenza vaccine is recommended in any stage of pregnancy, but evidence of safety in early pregnancy is limited… We sought to determine if receipt of vaccine containing pH1N1 was associated with spontaneous abortion (SAB).
We conducted a case-control study over two influenza seasons (2010-11, 2011-12) in the Vaccine Safety Datalink.
485 [cases] were eligible and confirmed by medical record review. Exposure was defined as vaccination with inactivated influenza vaccine before the SAB date; the primary exposure window was the 1-28 days before the SAB.
The overall adjusted odds ratio was 2.0 for vaccine receipt in the 28-day exposure window…
The association was modified by influenza vaccination in the prior season…
Among women who received pH1N1-containing vaccine in the previous influenza season, the aOR in the 1-28days was 7.7…
SAB was associated with influenza vaccination in the preceding 28 days.
[W]e had previously conducted an investigation of SAB and IIV during two pre-pandemic influenza seasons (2005–06 and 2006–07); the study design and implementation were nearly identical to the current study. The prior study did not find an association in the 28-day exposure window or any other exposure window.
Because influenza vaccination of pregnant women increased substantially during and after the pandemic, another difference is that more women in the current study may have received an influenza vaccine in prior years, whereas most vaccinated women in the first study probably were not previously vaccinated.
This study found that the overall odds of vaccine exposure in the 28-day exposure window was increased by a factor of two in women with SAB compared to controls.
A secondary analysis suggested that the odds among women who were also vaccinated in the previous year with pH1N1-containing vaccine was almost 8-fold (and statistically significant), while the odds among women who did not receive such a vaccine in the prior year was approximately null.