Whether Pfizer’s maternal respiratory syncytial virus (RSV) vaccine candidate increases the risk of preterm birth dominated discussions at a meeting last week of the committee advising the US Food and Drug Administration (FDA).1 While all members of the Vaccines and Related Biological Products Advisory Committee (VRBPAC) voted positively on efficacy, four out of 14 voted that the data were not adequate to support safety.
As The BMJ revealed earlier this month,2 Pfizer’s clinical trials showed increased levels of preterm births among vaccinated women, and GSK, which was working on a similar vaccine, halted vaccination in its phase 3 trials after observing an increase in neonatal deaths and preterm births.34
Paul Offit, professor of paediatrics at the Children’s Hospital of Philadelphia and VRBPAC member who voted “no” on safety, said at the meeting that he had “no problem” with efficacy. On safety, however, “the GSK experience” troubled him.5 “If preterm births are in any way a consequence of this vaccine it would be tragic.”
In Pfizer’s phase 2 trial, there were three preterm births (2.6%) in the placebo group v six (5.3%) in the vaccine group. In the phase 3 trial, which studied preterm births as an “adverse event of special interest,” 5.7% of babies were born preterm among vaccinated women v 4.7% in the placebo group. There was also an imbalance in babies with low birth weight (5.1% in the vaccine group v 4.4% in the placebo group).6 None of the differences were statistically significant.
It’s insignificant that the statistically insignificant differences all go in the same direction.
