How safe is RSV vaccine

The short version: RSV vaccines carry a real, measurable risk of Guillain-Barré syndrome in older adults, and a preterm birth signal in pregnant women, while the evidence for benefit is based on short-term trials funded by the manufacturers.

The most concrete safety finding comes from a national study in England that tracked every case of Guillain-Barré syndrome (GBS) after the bivalent RSV vaccine (Abrysvo, Pfizer) in adults aged 74-79 [3]. The study found about 23 extra cases of GBS per million doses in that age group. The risk was highest in the first six weeks after the shot, with a relative incidence of 3.34 compared to later time periods [3]. This is not a theoretical concern. It is a measured signal from a real-world surveillance system that looked at every treated case in the country.

For pregnant women, a 2025 analysis of VAERS reports found that preterm birth was the most frequently reported pregnancy-specific adverse event after the RSVpreF vaccine, with a disproportionate reporting signal [1]. Passive surveillance like VAERS misses most events, so this signal is a warning, not a full count. The manufacturer-funded trials reported efficacy numbers for infant protection, but the safety data on pregnancy outcomes remains thin and comes from a system designed to underreport [1, 4].

The efficacy numbers in the trials look impressive on paper. The adjuvanted GSK vaccine (Arexvy) showed about 83% efficacy against RSV lower respiratory tract disease in the first season, dropping to 56% in the second [8]. The Pfizer vaccine showed similar waning [8]. But these are surrogate endpoints from manufacturer-funded trials, not hard outcomes like all-cause hospitalization or death. The real-world effectiveness from observational studies is lower, around 75-80% [8]. No trial compared vaccinated to truly unvaccinated groups over a long period.

My call: for a healthy older adult, the GBS risk is real and the benefit is modest protection against a respiratory infection that most people recover from. For pregnant women, the preterm birth signal is enough to pause. The evidence does not support universal use. Confidence: moderate — the GBS signal is well-measured, but the long-term safety data and head-to-head comparisons against unvaccinated groups do not exist.