RSV vaccines have a strong overall safety profile, with side effects that are mostly mild and short-lived. In clinical trials involving tens of thousands of participants, serious adverse events occurred at similar rates in vaccinated and unvaccinated groups. That said, a few rare safety signals have emerged that are worth understanding, particularly a small risk of a neurological condition in older adults and an unresolved question about preterm birth in pregnant people.
Three RSV vaccines are available for adults 60 and older (Arexvy by GSK, Abrysvo by Pfizer, and mResvia by Moderna), and one (Abrysvo) is approved for use during pregnancy. A separate product, nirsevimab (Beyfortus), protects infants directly. Each has its own safety data.
Common Side Effects in Older Adults
The side effects of all three RSV vaccines for adults look a lot like what you’d expect from a flu shot. Pain at the injection site is the most frequent complaint, followed by fatigue, headache, and muscle or joint pain. These reactions typically show up within a day or two and resolve on their own.
The GSK vaccine (Arexvy) tends to cause more noticeable reactions than the other two, likely because it contains an immune-boosting ingredient called an adjuvant. In clinical trials, 61% of Arexvy recipients reported injection site pain compared to 9% of those who got a placebo. Fatigue hit 34% of the vaccine group versus 16% of the placebo group. About 3.8% of Arexvy recipients experienced a severe reaction (meaning it interfered with daily activity), compared to 0.9% in the placebo group. The same adjuvant is used in the shingles vaccine Shingrix, which is known for causing similar short-term discomfort.
If you’re planning to get an RSV vaccine alongside a flu shot or COVID booster, the combination appears safe. Studies of the Moderna RSV vaccine given at the same time as influenza or COVID vaccines found no increase in serious adverse events, and the pattern of side effects was similar to getting each vaccine alone.
The Guillain-Barré Syndrome Signal
The most closely watched safety concern is a possible link to Guillain-Barré syndrome (GBS), a rare condition where the immune system attacks the nerves, causing weakness and sometimes temporary paralysis. In clinical trials, a small number of GBS cases appeared in the weeks following vaccination with the GSK and Pfizer vaccines.
The numbers are very small. The FDA estimated the risk at roughly 10 extra cases per 1 million vaccinated adults aged 60 and older. Post-market surveillance data from the CDC has confirmed GBS remains rare, with fewer than 10 cases per million doses. Because so few cases occurred in trials, researchers still can’t say definitively whether the vaccines caused them or whether they happened by coincidence.
Notably, the Moderna vaccine (mResvia), which uses mRNA technology rather than a protein-based approach, had zero reported GBS cases within 42 days of vaccination in its clinical trials. This is one reason some people and their doctors may prefer it, though longer-term monitoring is still ongoing.
Atrial Fibrillation: A Minor Signal
Both the GSK and Pfizer vaccines showed a slight imbalance in atrial fibrillation (an irregular heart rhythm) within 30 days of vaccination. In both trials, 10 cases occurred in the vaccine group compared to 4 in the placebo group. Most of these people already had a history of atrial fibrillation. The numbers are too small to draw firm conclusions, but the CDC continues to monitor this signal through its safety surveillance systems.
Safety During Pregnancy
The Pfizer vaccine (Abrysvo) is the only RSV vaccine approved for use during pregnancy, given between weeks 32 and 36 of gestation. It works by passing protective antibodies to the baby before birth. The safety picture here is more nuanced than for older adults.
The primary concern is a possible increase in preterm birth. In the large Phase 3 trial (called MATISSE), 5.6% of babies born to vaccinated mothers arrived before 37 weeks, compared to 4.7% in the placebo group. That difference did not reach statistical significance, meaning it could be due to chance, but it was consistent enough to warrant caution. A smaller Phase 2 trial showed a similar trend, with preterm birth rates of 5.2% in the vaccine group versus 2.6% in the placebo group.
Real-world data has been somewhat reassuring. The CDC’s Vaccine Safety Datalink tracked over 10,000 pregnant people who received the RSV vaccine during the 2023-2024 season. The preterm birth rate among vaccinated individuals was 4.1%, which falls within the historically expected range of 3.1% to 6.1%. Reports to the VAERS passive surveillance system included 37 cases of premature delivery out of 121 total reports, though VAERS collects all reported events regardless of whether the vaccine caused them.
No maternal deaths were reported in post-market surveillance. Serious adverse events in infants were essentially identical between vaccine and placebo groups in clinical trials, occurring in 17.5% of both groups. No infant serious adverse events were considered related to the vaccine by investigators, though the FDA could not rule out a connection in one case involving extreme prematurity.
Nirsevimab for Infants
Nirsevimab (Beyfortus) is not technically a vaccine. It’s a single injection of a protective antibody given directly to infants, rather than training the immune system to make its own. Its safety profile is notably clean. In clinical trials, side effects were uncommon: injection site reactions occurred in just 0.3% of treated infants (compared to 0% with placebo), and rash appeared in 0.9% versus 0.6% with placebo.
Severe allergic reactions are possible, as with any injected medication, but they are extremely rare. Signs to watch for include facial swelling, difficulty breathing, or a bluish tint to the skin. These would require immediate medical attention.
Who Should Not Get an RSV Vaccine
The main contraindication for all RSV vaccines is a history of severe allergic reaction to any component of the vaccine or to a previous dose. If you’ve had GBS in the past, your doctor will weigh the risks and benefits carefully, particularly with the GSK and Pfizer products given the small signal described above.
For pregnant people, the timing window matters. The vaccine is recommended only between 32 and 36 weeks of gestation and only during RSV season (typically September through January in most of the United States). Getting it earlier in pregnancy has not been studied as thoroughly, and the preterm birth signal makes precise timing important. If your baby will receive nirsevimab after birth, you generally do not also need the maternal vaccine, as both serve the same purpose of protecting the newborn.