RSV protection in older adults looks weaker in the second season

Design

systematic review of randomised trials and observational studies with risk-of-bias assessment

Population

75 eligible studies on US-licensed RSV vaccines and monoclonal antibodies across older adults, pregnancy and infancy

Primary outcome

effectiveness against RSV-related hospitalisation and laboratory-confirmed illness, and safety

Effect

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

This review identified 75 studies on US-licensed RSV vaccines and monoclonal antibodies: 12 randomised trials, 33 comparative observational studies, 16 without a comparator and 14 epidemiological descriptions.

In older adults, vaccination was associated with 83.3% effectiveness against RSV-related hospitalisation (95% CI 42.9 to 96.9). But durability is where the new information sits: one study found effectiveness against hospitalisation and laboratory-confirmed illness significantly lower at 12 to 18 months than in the first season after vaccination, while another comparing one season with two found no significant difference. Maternal vaccination at 32 to 36 weeks gave 51.0% (−29.6 to 83.3) to 70.0% (37.0 to 86.0) against infant hospitalisation. Nirsevimab effectiveness against RSV hospitalisation within 6 to 12 months ranged from 63.6% (26.9 to 81.9) to 93.0% (83.0 to 97.0), varying with timing of administration.

On safety, no Guillain-Barré syndrome cases appeared across three randomised trials, though one self-controlled case series reported a possible increase in the 42 days after vaccination in older adults. No comparative study linked vaccination in pregnancy to preterm birth or other adverse pregnancy outcomes.

What changes is the counselling. An older adult vaccinated two seasons ago should not be assumed to still be protected, and the timing of nirsevimab relative to the season start appears to matter for how well it works — which makes it a scheduling question rather than simply a prescribing one.

• Do not assume an older adult vaccinated in a previous season remains protected
• Time nirsevimab against the local season start; effectiveness varied with timing
• Reassure pregnant patients that comparative studies found no link with preterm birth
• Mention the Guillain-Barré signal honestly as a possible association from one case-series design, not an established risk

Why it matters

A single RSV vaccination may not be the one-off protection it has been presented as.

Don't overread it

The waning signal comes from one study, with another finding no significant difference — this raises the question rather than settling it.

The statistics, in plain English

The older-adult interval of 42.9% to 96.9% is very wide — protection is real but its size is poorly pinned down. The maternal estimate of 51.0% has an interval running from −29.6% to 83.3%, which includes the possibility of no benefit; the 70.0% estimate, with an interval of 37.0 to 86.0, is the more informative one. A self-controlled case series compares a person with themselves across time periods, which controls fixed confounders well but is sensitive to how the risk window is chosen.

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