Human metapneumovirus-associated hospitalisation burden in children aged under 5 years in Scotland between 2017 and 2023: a retrospective analysis

This retrospective analysis of Scottish hospital data from 2017 to 2023 reveals that Human metapneumovirus causes a substantial burden of hospitalizations in children under five, particularly among infants, with incidence rates significantly higher than laboratory-confirmed figures suggest and lower than those for Respiratory Syncytial Virus, despite generally low rates of severe outcomes like intensive care admission or death.

The Gist

The Big Picture: The "Invisible Guest"

Imagine the human body as a busy house. Usually, we worry about the loud, obvious burglars like the Flu or RSV (Respiratory Syncytial Virus). But there is another guest, Human Metapneumovirus (hMPV), who is quieter, sneakier, and often goes unnoticed.

This study is like a detective report from Scotland (2017–2023) trying to figure out how many times this "invisible guest" forced young children (under 5 years old) to check into the hospital.

The Main Findings

1. The "Ghost" in the Hospital
The researchers found that hMPV is a major reason why young kids end up in the hospital. However, because doctors didn't always test for this specific virus (they were often looking for Flu or RSV instead), the official numbers were like looking at a shadow and guessing the size of the object casting it.

• The Analogy: Imagine a party where only 1 out of every 3 guests is wearing a name tag. The researchers realized that for every 1 kid they saw with a name tag (a confirmed test), there were actually 2 or 3 more kids in the room who had the same virus but no name tag. By doing some math (extrapolation), they estimated the true number of sick kids was 2 to 3 times higher than the official test results showed.

2. The "Baby Boom" of Sickness
Who gets hit the hardest? The study found that the 6-month to 1-year-old group is the "sweet spot" for getting sick.

• The Analogy: Think of a baby's immune system like a shield. When they are newborns (0–5 months), they have a "borrowed shield" from their mom that protects them. By the time they are 6 months old, that borrowed shield has rusted away, but their own shield hasn't been forged yet. This leaves them wide open for the virus.

3. The Pandemic "Pause Button"
The study covers the years of the COVID-19 pandemic, which acted like a giant pause button on the virus world.

• 2020/21 (The Quiet Year): Because everyone was wearing masks, staying home, and washing hands to stop COVID, hMPV basically vanished. The hospital numbers dropped to almost zero.
• 2021/22 (The Rebound): As soon as restrictions were lifted, the virus came back with a vengeance. It was like a rubber band that had been stretched tight and then suddenly snapped back. The number of sick kids skyrocketed, reaching the highest point in the study. This is called "immunity debt"—because the kids hadn't been exposed to germs for a while, their bodies weren't ready to fight them off.

4. How Bad Was It?
While the virus caused a lot of hospital visits, it wasn't usually a "killer."

• The Analogy: Most kids who got sick were like a car that broke down on the side of the road. They needed to be towed (hospitalized) and fixed for a few days, but the engine didn't explode.
  • Length of Stay: About 1 in 10 kids stayed in the hospital for 5 days or longer.
  • ICU: Very few (less than 1%) needed the Intensive Care Unit.
  • Deaths: Tragically, a tiny fraction (less than 1%) passed away, but for the vast majority, it was a scary but temporary illness.

5. The "Big Three" Comparison
The researchers also looked at the other two big respiratory viruses: RSV and Influenza A.

• The Analogy: If respiratory viruses were a race, RSV is usually the heavy favorite, causing the most hospitalizations. hMPV is usually the second or third runner. However, during the pandemic years, the race got weird, and hMPV sometimes ran faster than expected because RSV was also behaving strangely.

Why Does This Matter?

The "Missing Puzzle Piece"
Right now, we don't have a vaccine for hMPV. We are currently developing vaccines for RSV and Flu. This study is like a map that says, "Hey, before we build a wall against RSV, we need to know exactly how big the hMPV problem is."

The Takeaway
hMPV is a significant, often overlooked cause of sickness in young children. It causes a lot of stress for families and hospitals, especially in the winter. The pandemic taught us that when we stop the spread of one virus, others can bounce back harder later. To protect kids in the future, we need better testing (so we don't miss the "invisible guests") and eventually, a vaccine to give them their own permanent shield.

In short: hMPV is a sneaky virus that makes a lot of little kids sick in Scotland. It usually isn't fatal, but it fills up hospital beds, and we need to pay more attention to it to keep our little ones healthy.

Technical Summary

1. Problem Statement

Human metapneumovirus (hMPV) is a significant cause of acute lower respiratory infections (ALRIs) in young children, often presenting as bronchiolitis and pneumonia. Despite its clinical impact, there is a lack of robust national burden estimates for hMPV in the UK. This gap exists because:

• Routine testing for hMPV is uncommon compared to Respiratory Syncytial Virus (RSV) and Influenza.
• There are no licensed vaccines or targeted therapies, making clinical management supportive.
• Accurate baseline data is critical for evaluating the potential impact of emerging hMPV vaccine candidates and understanding epidemiological shifts following the introduction of RSV vaccines.
• The COVID-19 pandemic introduced atypical epidemiological patterns (e.g., suppression of respiratory viruses followed by rebounds) that require specific analysis to inform future public health policy.

2. Methodology

Study Design and Setting:

• Type: Retrospective analysis of national hospital and laboratory data.
• Population: Children aged <5 years in Scotland.
• Period: Six annual seasons (July 1, 2017 – June 30, 2023).
• Data Sources:
  • Hospitalizations: Scottish Morbidity Records 01 (SMR01) for inpatient admissions.
  • Laboratory Data: Electronic Communication of Surveillance Scotland (ECOSS) for positive hMPV PCR results.
  • Outcomes: Scottish Intensive Care Society Audit Group (SICSAG) for ICU/HDU admissions; National Records of Scotland (NRS) for mortality.
• Case Definition: RTI hospitalizations (ICD-10 codes) with a positive hMPV test collected 7 days prior to or 3 days post-admission (to account for incubation and avoid nosocomial infection inclusion).

Statistical Approach:

• Extrapolation Strategy: Due to inconsistent testing policies across Scotland, the authors used Lothian (a health board with comprehensive respiratory virus testing via the LUMINEX panel) as a benchmark.
  • They calculated the monthly proportion of hMPV-positive cases among all RTI admissions in Lothian, stratified by age (0–5 months, 6–12 months, 2–4 years).
  • These proportions were applied to all-cause RTI admissions in other Scottish health boards to extrapolate the total hMPV burden.
• Metrics Calculated:
  • Annual hospital incidence (per 100,000 population) for both laboratory-confirmed and extrapolated estimates.
  • Severity indicators: Length of Stay (LOS) ≥5 days, ICU/HDU admission rates, In-hospital Case Fatality Rate (CFR), and 90-day post-discharge CFR.
• Comparators: Similar estimates were generated for RSV and Influenza A for comparative analysis.
• Uncertainty: 95% Confidence Intervals (CIs) were calculated using non-parametric bootstrap resampling.

3. Key Contributions

• First National Extrapolated Estimates: Provides the first comprehensive, extrapolated estimates of hMPV hospital burden in Scottish children <5 years, correcting for significant under-testing outside the Lothian region.
• Pandemic Epidemiology: Offers detailed insights into the "suppression-rebound" phenomenon of hMPV during the 2020/21 and 2021/22 seasons, quantifying the "immunity debt" effect.
• Age-Specific Burden: Identifies the 6–12 month age group as having the highest hospitalization incidence, challenging some global assumptions that the youngest infants (0–5 months) bear the highest burden in high-income settings.
• Severity Baseline: Establishes a baseline for severity (ICU rates and mortality) to serve as a comparator for future vaccine efficacy trials.

4. Key Results

Incidence:

• Total Cases: 1,462 laboratory-confirmed hMPV hospitalizations were identified.
• Extrapolated Burden: The extrapolated incidence ranged from 19 to 537 per 100,000 children <5 years.
• Under-testing: Extrapolated estimates were 2 to 3 times higher than laboratory-confirmed counts, indicating significant under-ascertainment in regions without universal testing.
• Seasonal Trends:
  • 2020/21: Sharp decline (lowest incidence: ~19/100,000) due to COVID-19 non-pharmaceutical interventions (NPIs).
  • 2021/22: Massive rebound (peak incidence: ~537/100,000), the highest recorded in the study period.
  • 2022/23: Incidence remained elevated but lower than the 2021/22 peak.
• Age Distribution:
  • Highest incidence consistently observed in 6 months–1 year (peaking at 1,201/100,000 in 2021/22).
  • In the 2022/23 season, the 2–4 year age group surpassed the 0–5 month group, suggesting a shift in susceptibility patterns post-pandemic.

Severity and Outcomes:

• Length of Stay: ~10% of admissions required a stay ≥5 days. Lothian showed higher rates of prolonged LOS compared to other boards, potentially due to discharge practices rather than disease severity.
• ICU Admissions: Very low rate of 0.34% (95% CI: 0.07–0.7).
• Mortality:
  • In-hospital CFR: 0.14%.
  • 90-day post-discharge CFR: 0.34%.
• Comparison with RSV/Flu: RSV hospital incidence was consistently higher than hMPV (4–8 fold higher in pre-pandemic seasons). Influenza A and hMPV showed variable relative rankings.

5. Significance and Implications

• Public Health Planning: The study confirms that hMPV imposes a substantial burden on pediatric hospital services, particularly for infants aged 6–12 months. This data is vital for resource allocation during respiratory seasons.
• Vaccine Policy: As hMPV vaccine candidates enter clinical trials, this study provides the necessary baseline epidemiology to assess vaccine impact. The data suggests that while hMPV is less severe than RSV (lower ICU/mortality), its high incidence warrants consideration for immunization strategies.
• Surveillance Improvement: The findings highlight the critical need for standardized, universal respiratory virus testing across all health boards to avoid reliance on extrapolation and to accurately monitor disease trends.
• Pandemic Legacy: The data illustrates the long-term epidemiological consequences of NPIs, showing how delayed viral exposure can lead to shifted age distributions and severe rebound seasons, a pattern relevant for future pandemic preparedness.

Limitations Noted:

• Reliance on extrapolation from Lothian introduces potential bias if Lothian's demographics or testing patterns differ significantly from other boards.
• Low event counts for severe outcomes (ICU/Death) in specific subgroups limited the ability to perform stratified severity analysis.
• Incomplete ICD coding for hMPV specifically (only 24% of hMPV cases had specific diagnostic codes) limits the utility of hospital coding data alone.