The RSV cases England never counted

Ask most people to name the winter viruses that fill our hospital wards and they will reach for two: flu and COVID. Respiratory syncytial virus, or RSV, rarely makes the list. Many still think of it as a childhood illness – something that gives babies a nasty cough and then fades away. That picture is out of date, and our latest study shows just how misleading it has become for adults.

Working with colleagues at Oxford, we looked at every adult who was admitted to hospital for an acute respiratory infection in England during the 2023/24 season (NHS, Hospital Episode Statistics Data, 2024). We found more than 800,000 of them were in people aged 40 and over, showing RSV is not rare in adults. Fewer than 5,000 of those were recorded as RSV. But 79% were coded as 'unspecified', with no virus or bacterium identified at all. RSV is not rare in adults; it is simply not looked for, and therefore not counted. When you correct for that, a much larger – and much more expensive – problem comes into view..

A virus hiding in plain sight

Based on the diagnosis recorded as the main reason for admission, RSV accounted for 4,836 hospital admissions in adults aged 40 and over in 2023/24. That sounds modest. But the vast majority of respiratory admissions in our data (79%) had no identified virus or bacterium; they were coded as 'unspecified'.

This is because RSV testing in adults is neither routine nor standardised in the NHS. Swab tests to check for Flu and COVID are carried out far more often, but RSV, historically seen as a paediatric concern, is less frequently tested for. If you never test for a virus, you never record it – and it never shows up in the statistics that shape policy and funding.

Nearly five times more cases than the records show

To estimate the cases hiding inside that huge ‘unspecified’ group, we used a well-established statistical approach called proportional redistribution, borrowed from how researchers handle ill-defined causes of death. In plain terms: we assumed that the unspecified admissions contain RSV in roughly the same proportion as the admissions where a virus was identified, and we reallocated accordingly.

This showed that RSV admissions in adults 40 and over rose from 4,836 to an estimated 23,407 – almost a five-fold increase. That is our conservative base estimate. Include RSV recorded anywhere in the record, not just as the main diagnosis, and the figure reaches around 25,264. In other words, routine hospital data may be capturing only about one in five adult RSV admissions.

These are estimates, not counts, and they depend on the assumption behind the redistribution. What gives us confidence in the order of magnitude is that independent methods converge on it. A French national study (https://academic.oup.com/ofid/article/12/9/ofaf528/8254054)  using laboratory-linked records found a similar four- to five-fold undercount, and enhanced surveillance across seven English hospitals reached the same conclusion. Three independent methods, applied to different data, produce estimates of a similar scale.

It is not just the over-75s

This is where the findings speak most directly to policy. England launched its first adult RSV vaccination programme in September 2024, initially for people aged 75 to 79, and extended it from April 2026 to those 80 and over. RSV burden rises steeply with age, so focusing on the oldest adults makes sense.

But our data show a substantial burden below that threshold too. Adults aged 40 to 74 accounted for an estimated £22.6 to £26.3 million in RSV-related hospital costs in a single season — a group currently outside the vaccination programme, and one where RSV is least likely to be tested for or coded. Right now, we are largely not testing working-age and early-retirement adults. If we’re not testing, we will not find the true number of cases (https://www.sciencedirect.com/science/article/pii/S0163445326001179).

What it costs

The RSV admissions England actually recorded cost the NHS around £14.2 million in secondary care – that figure comes straight from the coded data. Once we account for the unrecognised cases, our estimate climbs to about £68.5 million in the base scenario and roughly £74 million in the expanded one, of which some £54 million reflects cases that were never coded as RSV at all.

For context, RSV still costs less than flu, which carries a longer-established vaccination programme and higher overall numbers. But RSV follows the same steep age gradient, and a wider RAND Europe analysis put the total annual adult RSV burden – including primary care, community care and lost productivity – at around £320 million. Hospital admissions alone are only part of the story.

What we cannot say

We should be candid about the limits of this work. Our data are aggregated, not patient-level, so we could not follow individuals, adjust for other health conditions, or separate out socioeconomic differences. Our redistribution method rests on an assumption: that among the admissions where no virus was identified, RSV was present about as often as it was among the admissions where doctors did identify a virus. That may not hold equally well in every age group.

Crucially, though, these assumptions almost all push in the same direction. Because RSV is tested for less often than flu or COVID, our approach is more likely to understate the true burden than to overstate it. We would rather present a cautious lower bound than a figure we cannot stand behind.

Why this matters now

England is at a turning point for RSV. A new vaccination programme is rolling out, and to know whether it is working, we first need an honest baseline of the burden it is meant to reduce. Our study, drawn from the last season before the vaccine arrived, offers exactly that – and it suggests the true burden is several times larger than the official record implies.

The practical message is simple. Test more widely for RSV in hospitalised adults, especially in winter; improve how infections are coded in NHS data; and extend surveillance to the 40-to-74 age group, where the virus is currently almost invisible in the data. You cannot manage what you do not measure and for adult RSV in England, we have not been measuring nearly enough.

Truong T, Radin JM, Li L, Ordóñez-Mena JM, Hoang U, Balogh O, Araujo AB, Nicodemo C, de Lusignan S, Madia JE. Burden and economic impact of RSV hospitalisations among English adults, 2023/24. Journal of Infection 2026;93:106792. Open access under CC BY. https://doi.org/10.1016/j.jinf.2026.106792