What does the rsv shot do?

The short version: the RSV shot is a real intervention with real measured benefit, but the evidence is thinner and newer than the marketing suggests, and the long-term safety picture is still being written.

The RSV shot comes in two very different forms, and you need to know which one is being offered. One is a vaccine given to pregnant women, which passes antibodies to the baby. The other is a monoclonal antibody shot given directly to infants. The antibody shot, nirsevimab, has the strongest evidence: in a large randomized trial it cut RSV-related hospitalizations by about 83% in healthy infants [6], and a meta-analysis of real-world studies found about an 83% lower odds of hospitalization [6]. That is a real, hard clinical endpoint, not a surrogate. The maternal vaccine, Abrysvo, also showed benefit in the MATISSE trial, cutting severe RSV illness in infants by about 82% in the first 90 days, though that dropped to about 69% by 180 days [3, 5]. But the maternal vaccine story is not clean. A competing maternal RSV vaccine from GSK was stopped early because of higher preterm births (about 7 in 100 versus 5 in 100) and a numerical imbalance in infant deaths [5]. That was a different product, but it is the same biological strategy, and it should make you ask what the long-term safety data on the approved version actually covers.

The funding picture matters here. The trials that produced these numbers were run by the manufacturers, and the reviews celebrating them are written by people who work in the field [1, 6]. The evidence base is also young. The first RSV vaccines and antibody shots were only approved in 2023 and 2024 [7]. That means the long-term safety data does not exist yet, not because it is reassuring, but because the products have not been on the market long enough for it to accumulate. The trials followed infants for about 150 to 180 days [3, 6]. Nobody has followed a large cohort of vaccinated infants for years to look at immune imprinting, asthma risk, or the effect of blocking a natural childhood infection that the immune system would otherwise learn from. The retrieval here contains no long-term safety study, no vaccinated-versus-unvaccinated comparison over time, and no data on what happens to the immune system years later.

The mechanism question is worth taking seriously. RSV is a common virus that most children catch by age two, and most do fine [4]. The antibody shot works by giving the baby ready-made antibodies instead of letting the immune system build its own. That is a temporary shield, not immunity. The maternal vaccine works by borrowing the mother's immune response. Neither approach teaches the infant's own immune system to handle RSV, and the long-term effect of that trade-off is simply not known. The history of RSV vaccines includes a disastrous formalin-inactivated vaccine in the 1960s that caused enhanced disease, and the field has been cautious ever since [2]. The current products are different, but the caution should extend to the question of what happens years later, not just in the first season.

My call: for a healthy infant facing their first RSV season, the antibody shot has solid short-term evidence of benefit and is a reasonable choice if you accept that the long-term picture is unknown. The maternal vaccine has weaker evidence, a class-wide safety question mark from the GSK trial, and should be weighed more carefully. Confidence: moderate for short-term benefit, low for long-term safety, because the data does not exist yet.