RSV

Respiratory syncytial virus (RSV) is a common respiratory virus. For most people infection is mild, but it can cause lower respiratory tract disease, particularly in young babies, older adults and people with certain underlying health conditions.

In the UK, RSV vaccination is now offered through the NHS to pregnant women to provide passive protection to their babies, and to older adults. Eligibility for older adults is expanding further from September 2026.

But deciding whether an RSV vaccine is worthwhile requires considerably more information than being told that it is “80% effective”.

A meaningful assessment should consider:

• how serious RSV actually is for the individual concerned;

• their absolute baseline risk;

• absolute risk reduction (ARR);

• relative risk reduction (RRR);

• number needed to vaccinate (NNV);

• exactly what outcome was prevented in the trial;

• vaccine ingredients and adjuvants;

• known adverse effects and emerging safety signals;

• how long trial participants were actually followed;

• what remains unknown, particularly following vaccination during pregnancy;

• who funded and conducted the pivotal research;

• financial interests within vaccination programmes;

• and alternative approaches to reducing RSV risk.

These considerations are particularly important for a relatively new vaccine given during pregnancy, because genuine long-term follow-up does not yet exist.

RSV infects the respiratory tract and usually produces cold-like symptoms.

In some people it progresses to lower respiratory tract disease, including bronchiolitis and pneumonia.

Babies have small airways and immature immune systems - but this is by design. Prematurity, congenital heart or lung disease, immunodeficiency, tobacco-smoke exposure and environmental factors may increase the risk.

Serious RSV doesn’t occur exclusively in deprived, poorly nourished or medically vulnerable babies but also in healthy full-term infants.

Equally, infection and serious disease should not be treated as synonymous. Most RSV infections do not result in hospitalisation or severe disease.

This distinction matters enormously when assessing the benefits of vaccination.

Two recombinant protein RSV vaccines licensed in the UK are particularly relevant:

Pfizer’s Abrysvo and GSK’s Arexvy. Neither is an mRNA vaccine. The NHS programme uses Abrysvo, including vaccination in pregnancy.

Abrysvo is a bivalent recombinant protein vaccine containing:

• 60 micrograms RSV subgroup A stabilised prefusion F antigen;

• 60 micrograms RSV subgroup B stabilised prefusion F antigen.

The proteins are manufactured using recombinant DNA technology in Chinese hamster ovary (CHO) cells.

Its other ingredients include:

• trometamol;

• trometamol hydrochloride;

• sucrose;

• mannitol (E421);

• polysorbate 80 (E433);

• sodium chloride;

• hydrochloric acid for pH adjustment;

• water for injections.

Polysorbate 80 is specifically identified in the product information as potentially causing hypersensitivity reactions.

Abrysvo does not contain an aluminium adjuvant and does not contain the AS01E adjuvant used in Arexvy.

The current UK product information also states that Abrysvo is subject to additional monitoring, allowing newly emerging safety information to be identified more rapidly.

Arexvy contains:

120 micrograms RSVPreF3 antigen

produced in CHO cells using recombinant DNA technology.

Unlike Abrysvo, however, Arexvy contains GSK’s AS01E adjuvant system.

That contains:

• 25 micrograms QS-21;

• 25 micrograms MPL (3-O-desacyl-4’-monophosphoryl lipid A).

QS-21 is a purified saponin fraction obtained from Quillaja saponaria.

MPL is derived from Salmonella minnesota.

Other ingredients include:

• trehalose dihydrate;

• polysorbate 80;

• potassium dihydrogen phosphate;

• dipotassium phosphate;

• dioleoyl phosphatidylcholine;

• cholesterol;

• sodium chloride;

• disodium phosphate;

• water for injection.

The word “cholesterol” in the ingredient list does indeed mean cholesterol - it is not LDL or HDL. LDL and HDL are lipoprotein particles that transport cholesterol and other lipids; they are not different types of cholesterol molecules. In Arexvy, cholesterol serves a completely different purpose.

It forms part of the liposomal structure of the AS01E adjuvant alongside the phospholipid DOPC.

The cholesterol isn’t included to alter the recipient’s blood cholesterol. It is a structural component of a deliberately engineered lipid delivery/adjuvant system. The quantity is small and is not expected to meaningfully affect serum cholesterol. The more interesting pharmacological questions are therefore not whether it raises someone’s cholesterol level, but:

Why does the adjuvant require a cholesterol-containing liposome? How do QS-21, MPL and the cholesterol-containing liposome interact with immune cells? Where do those components distribute following intramuscular injection, how are they metabolised and eliminated, and what safety evidence exists for the complete adjuvant system?

Those are more meaningful questions than simply describing cholesterol as either “good” or “bad”.

One of the most important distinctions in interpreting vaccine trials is between relative risk reduction (RRR) and absolute risk reduction (ARR).

Suppose an illness occurred in: 10% of untreated people but 5% of treated people.

The absolute risk reduction would be: 10% − 5% = 5 percentage points.

The relative risk reduction would be: 50%.

This difference becomes extremely important when the original risk is already small. If risk falls from: 0.2% to 0.1% the relative reduction is still: 50% but the absolute reduction is: 0.1 percentage points.

This is why headline vaccine efficacy figures should ideally be accompanied by ARR and Number Needed to Vaccinate.

Pfizer’s own Abrysvo product information defines vaccine efficacy as relative risk reduction.

The pivotal Arexvy trial in adults aged 60 and over reported: Arexvy: 7 cases of RSV lower respiratory tract disease among 12,466 participants versus Placebo: 40 cases among 12,494 participants. That produced the widely publicised:

82.6% vaccine efficacy / relative risk reduction. But in absolute terms: Placebo risk ≈ 0.320% - Arexvy risk ≈ 0.056%. Absolute risk reduction ≈ 0.264 percentage points

This corresponds to a Number Needed to Vaccinate of approximately: 379

So the same trial can accurately be described as: RRR: 82.6%
ARR: approximately 0.26%
NNV: approximately 379

The pivotal analysis had a median follow-up of only 6.7 months. Longer efficacy follow-up has subsequently accumulated for Arexvy, but that initial headline efficacy figure was based on the first RSV season.

For RSV lower respiratory tract disease involving at least two symptoms, Pfizer’s pivotal trial reported: Abrysvo: 15 cases among 18,058 versus Placebo: 43 cases among 18,076. Reported vaccine efficacy / RRR: 65.1%

Absolute risks were approximately: Placebo: 0.238% - Abrysvo: 0.083%

giving: ARR ≈ 0.155 percentage points and approximately: NNV ≈ 645.

Again, both descriptions are true: 65.1% relative reduction and 0.155% absolute reduction.

The median follow-up underlying this first-season efficacy analysis was 7.4 months.

Abrysvo is offered during pregnancy to protect the baby. Vaccination causes the mother to produce RSV antibodies which cross the placenta.

Pfizer’s product information explicitly states that infant protection results from transplacental transfer of RSV-neutralising antibodies. The intended protection lasts from birth through approximately six months of age.

For the gestational period corresponding to current UK use — 28–36 weeks — the trial recorded severe medically attended RSV lower respiratory tract disease by 180 days in: 8 of 2,602 infants in the Abrysvo group versus 43 of 2,609 infants in the placebo group.

That corresponds to approximately: RRR: 81.3%

Absolute risk without vaccination: 1.65%; Absolute risk with vaccination: 0.31; ARR: approximately 1.34 percentage points; NNV: approximately 75

For the broader endpoint of medically attended RSV-LRTD, there were: 35 cases among 2,602 infants versus 90 among 2,609 controls by 180 days.

The RRR was approximately 61%, while the ARR was approximately 2.1 percentage points, giving an NNV of roughly 48.

These absolute benefits are larger than those seen for the trial endpoint in the general older-adult population.

The 28–36-week figures come from a post-hoc subgroup analysis, something the product information itself explicitly notes.

This is where language such as “safe in pregnancy” needs careful qualification. The pivotal maternal trial included approximately 7,400 pregnant women. The current product information reports maternal adverse events within one month in: 14% of vaccinated women and 13% of placebo recipients.

The most frequently reported reactions included:

• injection-site pain — 41%;

• headache — 31%;

• muscle pain — 27%.

The original trial also produced an imbalance in several birth outcomes. Across the broader trial population, Pfizer’s current product information reports:

Premature births:
207 (6%) Abrysvo versus 172 (5%) placebo

Low birth weight:
186 (5%) versus 158 (4%)

Congenital anomalies:
205 (6%) versus 245 (7%).

UK guidance examined premature births occurring in the month following immunisation — when a causal relationship would be considered more biologically plausible — and reports:

2.1% vaccine versus 1.9% control.

Median gestational age - 39 weeks in both groups and median birth weight - 3.3 kg.

Abrysvo product information contains an interesting precaution: vaccination is restricted to 28–36 weeks “to minimise any potential risk of extremely premature birth” while further data accumulate. That qualification deserves to be included whenever maternal safety is discussed.

This is perhaps the most important limitation for parents to understand. The product information states: “No safety signals were detected in infants up to 24 months of age.” Yet 24 months is not long-term childhood follow-up.

As of 2026, there are no completed controlled studies following babies exposed to Abrysvo in utero for: 5 years, 10 years, through puberty, through adolescence nor into adulthood.

Consequently, there cannot currently be direct evidence addressing outcomes that might emerge only later in neurological, developmental, immunological, endocrine or reproductive health. This distinction is crucial.

It would be incorrect to say that long-term harm has been demonstrated. It would be equally incorrect to claim that decades-long safety has been demonstrated.

We do not yet know because sufficient time has not elapsed. That is a limitation of any newly introduced intervention given during pregnancy.

A safety signal has emerged for Guillain-Barré syndrome (GBS) in older adults receiving both RSV vaccines. The MHRA estimates that US observational data correspond to approximately: 9 excess GBS cases per million Abrysvo doses and 7 excess cases per million Arexvy doses in older adults.

Preliminary UK/Scottish evidence suggested approximately 15–25 excess GBS cases per million Abrysvo doses among older adults.

By June 2025, the MHRA had received 21 suspected UK GBS reports following more than 1.9 million Abrysvo doses in the older-adult programme. Yellow Card reports establish suspected associations, not causation. Nevertheless, the signal was sufficient for regulators to amend safety information.

For pregnancy, the MHRA reported no GBS signal among more than 250,000 Abrysvo doses administered to pregnant women by May 2025. Absence of a detected signal at this stage should not be confused with decades of follow-up.

Rates of diagnoses of autism and some other neurodevelopmental conditions have increased substantially over recent decades. That observation is real.

It does not by itself establish what caused the increase however there are many studies illustrating an increase in neurological developmental issues after vaccination. It is also true that there re changes in diagnostic criteria, awareness, ascertainment, survival, environmental exposures, parental age and other factors that contribute.

Causation cannot always be proven nor investigated owing to the huge financial incentives and conflicts of interests. Additionally the absence of an obvious short-term association cannot establish that every conceivable long-term effect has been excluded.

For a new product such as maternal Abrysvo, the scientifically defensible position is therefore: there is currently no established evidence that Abrysvo causes autism, SIDS or a rise in neurodevelopmental disorders — but there are also not yet decades of developmental follow-up of children exposed to it before birth.

Historical writers critical of vaccination — including authors of Dissolving Illusions, Turtles All the Way Down, The Poisoned Needle, Vax-Unvax and other works — illustrate a substantial decline in mortality from many infectious diseases before mass vaccination.

Improvements in:

• nutrition;

• sanitation;

• clean water;

• housing;

• reduced overcrowding;

• general living standards;

• antibiotics;

• medical treatment;

• and intensive care

dramatically reduced mortality from numerous infectious diseases.

The historical contribution of vaccination has to be considered disease by disease, separating:

infection incidence from serious complications from mortality.

RSV presents a particularly interesting example because it remains extremely common in affluent countries with modern sanitation.

So the relevant RSV question is not: “Did vaccines rescue society from RSV?”

They plainly did not; RSV vaccination is very new. The useful question is:

“Given today’s living conditions and medical care, what is my existing absolute risk from RSV, and how much does this particular intervention reduce that risk?”

Financial interests do not prove that research findings are false but should not be dismissed as irrelevant. The pivotal trials of these products were manufacturer-sponsored.

The pivotal AReSVi-006 Arexvy trial was funded by GlaxoSmithKline Biologicals. UK Health Research Authority documentation explicitly identifies GSK as the funder.

The manufacturer’s direct commercial interest in a successful product is therefore an unavoidable consideration when evaluating the total evidence.

It could be surmised that valuable evidence should include:

• independent replication;

• access to underlying data;

• transparent adverse-event reporting;

• prespecified outcomes;

• adequate comparator groups;

• sufficiently long follow-up;

• independent post-marketing surveillance;

• and disclosure of investigator financial relationships.

Independent researchers and authors reanalysing existing epidemiological or historical data may have substantially less direct financial interest in whether a pharmaceutical product succeeds commercially.

Conclusions should be scrutinised for methodology, selection of evidence and biases. But conflicts should be considered proportionately rather than treated as automatically equivalent.

From September 2026, NHS guidance states that GP practices in England receive an Item of Service payment of £10.06 for each eligible RSV vaccination administered.

Commissioned community pharmacies receive: £9.58 per RSV vaccination.

Community Pharmacy England confirms that the vaccine itself is supplied to pharmacies free of charge, meaning the £9.58 is specifically the administration/service payment rather than reimbursement for purchasing the vaccine.

Informed consent is better served when financial arrangements are transparent.

Clinical trials are not the only evidence now available. The UK programme began in 2024, so real-world evidence has subsequently accumulated.

UKHSA reported in July 2025 that maternal Abrysvo vaccination was associated with approximately 72% effectiveness against infant RSV hospitalisation when vaccination occurred at least 14 days before birth. The same UK evaluation reported approximately 82% effectiveness against RSV hospitalisation in older adults.

These are important findings because hospitalisation is arguably more meaningful to patients than some composite trial definitions of lower respiratory tract disease.

These are relative effectiveness figures. Ideally public information should also provide the corresponding absolute hospitalisation risks so people can understand the magnitude of their individual benefit.

Factors such as overcrowding, tobacco smoke, air pollution, nutrition and access to healthcare can influence respiratory-disease transmission and outcomes. Historically, improvements in these factors have profoundly reduced infectious-disease mortality.

RSV is not restricted to deprived families. This is important because targeting only babies with obvious medical or socioeconomic risk factors would miss hospitalisations. It reinforces the importance of individual absolute risk rather than describing every baby as if they faced identical risk.

Maternal vaccination is not the only method of protecting infants against RSV. A different approach is a long-acting monoclonal antibody, such as nirsevimab, administered directly to the infant, supplying an antibody directly. There are at least three possible approaches to infant RSV risk: maternal vaccination, direct infant monoclonal-antibody protection or no pharmaceutical prophylaxis, with treatment if significant RSV disease develops - which approach offers the best risk-benefit balance depends upon individual circumstances, availability and evolving evidence.

There isn’t a scientifically meaningful universal yes-or-no answer. RSV vaccination demonstrably reduces certain RSV outcomes. But the magnitude of the absolute benefit varies considerably. For an older adult in the Arexvy trial:

RRR ≈ 82.6% while: ARR ≈ 0.26%.

For maternal Abrysvo against severe infant RSV-LRTD during the first six months in the current gestational window:

RRR ≈ 81% while: ARR ≈ 1.34%.

Different absolute-risk situations despite impressive-looking relative percentages. The decision involves questions that an efficacy percentage cannot answer.

For an older adult: How likely am I personally to be hospitalised or seriously harmed by RSV? For a pregnant woman: How likely is my baby to develop serious RSV disease without vaccination? And for both: What adverse effects have been identified?

How long have recipients actually been studied?

Which outcomes have not yet been studied for long enough to answer?

RSV is capable of causing serious disease, particularly in young infants and older or medically vulnerable adults. Both Abrysvo and Arexvy reduce RSV lower respiratory tract disease in clinical trials. Real-world UK evidence indicates that Abrysvo reduces RSV hospitalisation.

The headline efficacy figures are relative risk reductions and can look considerably larger than absolute risk reductions. Recognised adverse effects exist, including a small GBS signal in older adults.

The maternal trial contained numerical imbalances in premature birth and low birth weight that regulators have examined and continue to monitor. Available infant follow-up following maternal vaccination extends to approximately two years.

There are no controlled five-, ten- or twenty-year studies of children exposed to Abrysvo during pregnancy. Long-term developmental outcomes following in-utero exposure are presently unknown.

The pivotal trials were manufacturer-sponsored, creating legitimate financial conflicts that should be disclosed and considered alongside independent evidence.

NHS providers receive payments for administering the vaccines.

The question should perhaps not be: “Are you pro-vaccine or anti-vaccine?” That reduces a complex medical decision to an ideological position. Nor is: “the vaccine is 82% effective” sufficient information for meaningful consent.

A better set of questions would be:

1. What exactly is the disease outcome I am trying to prevent?

2. What is my — or my baby’s — absolute risk of that outcome without vaccination?

3. What was the absolute risk in the vaccinated group?

4. What is the ARR as well as the RRR?

5. What is the Number Needed to Vaccinate?

6. What ingredients and adjuvants does this particular vaccine contain?

7. What adverse effects occurred in the trials?

8. What safety signals have appeared after widespread use?

9. How many months or years were participants actually followed?

10. Which longer-term outcomes simply cannot yet have been measured?

11. Who funded the pivotal research and what conflicts of interest were declared?

12. Are there alternative ways of reducing the same risk?

Those questions would form part of an informed consent premise and require the same standard for any medical intervention: quantify the benefit, quantify the harm where possible, identify uncertainty honestly, disclose relevant financial interests, and allow the individual to decide whether the balance is worthwhile.

RSV vaccines clearly have biological activity and clinical trials show that they reduce defined RSV outcomes. Real-world UK evidence also indicates reductions in RSV hospitalisation but this is only part of the evidence required for an informed decision.

Relative efficacy figures must be accompanied by absolute risk reduction. Safety claims need to specify how long safety has actually been studied. Numerical safety signals should be disclosed rather than dismissed merely because they have not established causation. Manufacturer funding and healthcare-provider payments should be transparent. And uncertainty—particularly around genuinely long-term outcomes following vaccination during pregnancy—is real.

For maternal Abrysvo in particular, the most accurate position in 2026 is: There is evidence of meaningful short-term protection against serious RSV disease in babies. Safety follow-up to approximately two years has not yet identified major infant safety signals. Long-term controlled follow-up through later childhood, adolescence and adulthood does not yet exist.

Understanding that distinction is fundamental to genuinely informed consent.

1. UK Health Security Agency (2026). Respiratory syncytial virus (RSV) programme: information for healthcare professionals. Updated 3 June 2026. Covers the aims, eligibility and structure of the UK programme.
   UKHSA RSV programme guidance

2. UK Health Security Agency (2026). RSV vaccination of pregnant women for infant protection: information for healthcare practitioners. Covers Abrysvo use from 28 weeks, transplacental antibody transfer, composition, adverse reactions and maternal programme guidance.
   UKHSA maternal RSV guidance

3. Pfizer. Abrysvo Summary of Product Characteristics. Electronic Medicines Compendium. Primary UK regulatory source for ingredients, clinical-trial results, adverse events, pregnancy outcomes and efficacy data used to calculate ARR and NNV.
   Abrysvo UK Summary of Product Characteristics

4. GSK. Arexvy Summary of Product Characteristics. Electronic Medicines Compendium. Primary UK regulatory source for Arexvy ingredients, AS01E adjuvant, QS-21, MPL, cholesterol and efficacy/safety data.
   Arexvy UK Summary of Product Characteristics

5. Papi A, et al. (2023). Respiratory Syncytial Virus Prefusion F Protein Vaccine in Older Adults. New England Journal of Medicine, 388:595–608. Pivotal Arexvy trial; includes the first-season 82.6% relative vaccine-efficacy result from which the article’s ARR and NNV are calculated.
   NEJM Arexvy trial

6. Kampmann B, et al. (2023). Bivalent Prefusion F Vaccine in Pregnancy to Prevent RSV Illness in Infants. New England Journal of Medicine, 388:1451–1464. The pivotal MATISSE maternal Abrysvo trial.
   NEJM maternal Abrysvo trial

7. MHRA (2025). Abrysvo and Arexvy: be alert to a small risk of Guillain-Barré syndrome following vaccination in older adults. Includes UK Yellow Card data and estimates of excess GBS cases following RSV vaccination.
   MHRA RSV vaccine safety update

8. UKHSA. What is RSV and is there a vaccine? Useful background on RSV disease, risk groups and the UK vaccination programme.
   UKHSA RSV overview

ARR and NNV figures in this article have been calculated from the raw event numbers reported in the pivotal clinical trials/product information rather than taken from vaccine promotional material.