Peripartum outcomes: non-adjuvanted v. adjuvanted H1N1 vaccination

Pregnant women are at higher risk for complications from pandemic H1N1 virus infection. In CMAJ, Mahmud and colleagues1 showed a seroprevalence of 8.6% among pregnant women in Manitoba in 2009. In that year, over 1 million pregnant Canadian women received either the AS03-adjuvanted or nonadjuvanted H1N1 pandemic influenza vaccine. The safety of adjuvanted vaccine use in pregnancy has been studied,2 but has not been systematically compared with the nonadjuvanted vaccine.

We recently completed a study of pregnant women who received either the nonadjuvanted (CSL Limited) or adjuvanted (GlaxoSmithKline Pandemrix) H1N1 vaccine at prenatal hospital clinics at Mount Sinai Hospital, St. Michael’s Hospital, Sunnybrook Health Sciences Centre (all in Toronto, Ontario) or Kingston General Hospital. More details can be found at www.stmichaelshospital.com/pdf/research/mapped-tables.pdf.

The composite outcome of peripartum complications was more common in women who received the nonadjuvanted (41.7%) than adjuvanted (25.1%) vaccines (adjusted odds ratio 1.55, 95% confidence interval 1.01–2.39). Other outcomes we measured did not differ significantly (see Table 2 at www.stmichaelshospital.com/pdf/research/mapped-tables.pdf).

Our study was underpowered to detect infrequent outcomes. Because limited safety data were available at that time, the nonadjuvanted vaccine was generally recommended to women who were less than 16 weeks’ gestation, and the adjuvanted was recommended to women past that gestational age. That the nonadjuvanted H1N1 vaccine was associated with more peripartum complications is of interest. Influenza infection in the second and third trimesters of pregnancy is a relatively common event1 and there is no evidence for transplacental transmission of the virus.3 It remains to be determined if it is the adjuvanted vaccine itself, or its differential protection against H1N1, that might modulate the manner in which labour progresses.