RSV immunisation: strong first-season protection, and an open question about the second

Design

systematic review with Cochrane risk-of-bias assessment; four databases searched August 2025 to June 2026

Population

75 studies of US-licensed RSV vaccines and monoclonal antibodies across older adults, pregnancy, infants and immunocompromised groups

Primary outcome

effectiveness against RSV hospitalisation and laboratory-confirmed illness, plus adverse events of special interest

Effect

older adults VE 83.3% (95% CI 42.9-96.9); maternal 51.0-70.0%; nirsevimab 63.6-93.0% against infant hospitalisation

The companion review covered 75 studies of US-licensed RSV vaccines and monoclonal antibodies — 12 randomised trials, 33 comparative observational studies, 16 without comparators and 14 epidemiological descriptions. In older adults, vaccination was associated with an 83.3% reduction in RSV-related hospitalisation (95% CI 42.9 to 96.9). Maternal vaccination between 32 and 36 weeks gave effectiveness against infant RSV hospitalisation ranging from 51.0% (95% CI -29.6 to 83.3) to 70.0% (95% CI 37.0 to 86.0). Nirsevimab in infants ranged from 63.6% (95% CI 26.9 to 81.9) to 93.0% (95% CI 83.0 to 97.0) against hospitalisation at 6 to 12 months, varying with timing of administration.

Durability is where the review adds something. One study found effectiveness in older adults significantly higher in the first season than at 12 to 18 months; another comparing one season against two found no significant difference. That is an unresolved disagreement, not a settled decay curve, and it bears directly on whether and when a second dose is worth giving.

Safety reads reassuringly with one flag kept visible. No Guillain-Barre syndrome occurred across three randomised trials, but a self-controlled case series in older adults suggested a possible raised risk within 42 days of vaccination. A case-series design of that kind is good at detecting a temporal clustering and poor at establishing whether it is causal, and the randomised evidence does not support it — which is exactly the state of uncertainty to describe honestly when consenting an older adult. On pregnancy, no comparative study found an association with preterm birth or other adverse pregnancy outcomes.

• Timing drives nirsevimab effectiveness; align administration with the local RSV season rather than a fixed calendar date
• Do not promise sustained protection into a second season — the evidence disagrees with itself
• Mention the Guillain-Barre signal to older adults as unconfirmed, from one observational design and absent from trials
• The wide maternal-vaccination range (51.0% to 70.0%, one interval crossing zero) reflects few studies, not variable biology
• RSV product availability and cost differ markedly in India; confirm what is actually obtainable before recommending

The statistics, in plain English

The 83.3% estimate carries an interval from 42.9% to 96.9%, so the effect is clearly real but its size is loosely pinned — a common pattern when a highly effective intervention is studied in relatively few hospitalised cases. One maternal-vaccination estimate ranges from -29.6% to 83.3%, meaning that study alone could not exclude harm; it is the consistency with the other estimate that makes the overall picture credible. A self-controlled case series compares a person with themselves across time windows, which removes confounding by fixed patient characteristics but is vulnerable to anything that changes seasonally.

Read the rest in the app

You have read your two free briefings this month. The app carries all 27 specialties, every morning, free — and this finding is waiting in it.

Get Daily Doctor — free

Scan to keep reading on your phone. No account needed to start.