How COVID Nearly Erased Scotland’s Most Dangerous Meningococcal Strain

Invasive meningococcal disease in Scotland ran on a single strain through most of the COVID-19 pandemic, and four years on, the mix of bacteria causing it still has not returned to its old shape. Group B became the only serogroup causing the disease nationwide, according to a genomic study published in The Lancet Infectious Diseases. Group W and group C, including the hypervirulent lineage behind the UK’s last major vaccine overhaul, nearly vanished.

The strains are drifting back now, but not to where they started. Researchers say Scotland has settled into a new baseline, one that raises fresh questions about whether current vaccine targeting still matches the bacteria actually in circulation.

Group B Becomes the Last Strain Standing

Researchers from Public Health Scotland, the University of Strathclyde and the University of Glasgow sequenced invasive meningococcal disease cases across Scotland before, during and after the pandemic. Before COVID-19 arrived, group B disease circulated alongside group W and group C, with a lineage called clonal complex 11, or cc11, driving much of the W and C activity.

During the pandemic, that changed sharply. Group B became the only cause of invasive meningococcal disease recorded in the country, the study found. Group W and group C cases collapsed, and cc11 declined further than any other lineage tracked.

By 2025 and 2026, all three groups were circulating again. But the paper describes the pattern as trending back toward its pre-pandemic profile from a new baseline, not simply resuming where it left off in early 2020.

The Same Age Groups, a Different Set of Bacteria

One finding did not move. The people most likely to get seriously ill stayed the same the whole way through, even as the strains behind their illness shifted underneath them.

• Infants and young children – immature immune systems leave babies and toddlers exposed before routine vaccination is complete.
• Teenagers and young adults – close social contact, including in university halls of residence, drives the highest rates of bacterial carriage in the nose and throat.
• Adults over 65 – waning immunity raises the risk of severe disease later in life, a pattern echoed in international reviews of post pandemic case data.

That stability matters for public health planners. It means the pandemic reshuffled which bacteria cause disease without touching who is vulnerable to it, so vaccination priorities by age group did not need to be rebuilt from scratch.

Clonal Complex 11 Has Caused Trouble Before

The lineage that declined furthest during the pandemic is not a stranger to UK public health teams. Starting in the 2009-10 epidemiological year, a hypervirulent group W strain within clonal complex 11, believed to have originated in South America, drove a year-on-year rise in cases across the UK.

By 2015, the increase was severe enough that health officials replaced the adolescent meningococcal C conjugate booster with a quadrivalent vaccine, MenACWY, which protects against groups A, C, W and Y. The targeted vaccination of teenagers after the group W surge began in August 2015, prioritizing 17 and 18 year old school leavers before expanding to the wider 13-18 age group. Coverage in eligible school cohorts reached 84% by 2018.

That history is what makes the new Scottish data notable. The exact lineage that forced a nationwide vaccine overhaul a decade ago nearly disappeared on its own during the pandemic, for reasons that had nothing to do with vaccination policy and everything to do with reduced social contact.

Halls of Residence and the Carriage Engine

Meningococcal bacteria live harmlessly in the nose and throat of a share of the population at any given time, a state doctors call carriage. Most carriers never get sick. But carriage is also how the bacteria spread, and teenagers moving into shared student housing have long been one of the biggest amplifiers.

A study of UK first-year students documented a rise in group W carriage among first year students tied to the same clonal complex 11 lineage behind the 2015 vaccine change, with rates climbing during the first weeks after students moved into halls of residence. Campus closures and remote learning during the pandemic interrupted exactly that kind of mixing.

The picture is not uniform everywhere, though. One study of Australian university students found carriage rates rose after containment measures took hold, from 3.7% in February and March of 2020 to 6.8% that August and September, possibly because students confined together in residence halls had more close contact with each other, not less.

Epidemiologists describe the broader pattern as an immunity debt: reduced circulation of the bacteria during lockdowns, combined with disrupted routine vaccine uptake, may have left parts of the population less primed once normal mixing resumed. Separate English surveillance data point the same direction, recording a substantial rise in group B disease among older teenagers and young adults after final restrictions lifted in July 2021 and students returned to full-time education that November.

Four Years On, a Baseline Nobody Has Mapped Yet

Four years after Scotland’s restrictions eased, the strain mix has not simply snapped back. The study frames it as a genuinely new baseline, and two things about that baseline remain open.

What We Know

• Group B, group W and group C are all circulating in Scotland again, and the highest-risk age groups have not changed.
• Clonal complex 11 fell furthest of any lineage during the pandemic and genomic sequencing caught the shift as it happened.

What Is Unconfirmed

• Whether the new strain mix will stabilize where it is now or keep moving toward something else entirely.
• Whether current MenB and MenACWY targeting still lines up with the post pandemic picture, or needs adjusting.

The stakes are not hypothetical. A recent literature and surveillance review documented a rebound in cases among older adults across several countries once pandemic restrictions lifted. And in England, health officials separately logged a group W increase tied to pilgrimage travel to Saudi Arabia between January 2024 and June 2025, a reminder that the lineage Scotland saw retreat at home is still moving through travelers elsewhere.

Why Does Genomic Surveillance Matter So Much Now?

Continuous genomic sequencing lets health agencies see a strain shift within months rather than waiting years for case numbers to climb high enough to raise alarms, which is roughly how long it took officials to act on the 2009-2015 group W surge.

Meningococcal disease is rare, but it can be very serious and its patterns can change quickly.

Dr. Shaun Keegan, a research fellow at the University of Strathclyde’s Institute of Pharmacy and Biomedical Sciences and a co-author of the study, said the finding shows why constant tracking matters. “It helps us understand which strains are circulating, how that changes over time, and how public health and vaccine strategies can respond,” Keegan said.

The research, led by Niall Ahmad alongside Keegan and colleagues, arrives as UK health teams renew attention on MenB vaccines for groups at increased risk. Public Health Scotland is folding the genomic data directly into ongoing vaccine cost-effectiveness reviews already underway, rather than waiting for case numbers to force the question the way they did a decade ago.

Frequently Asked Questions

What Is the Difference Between Meningitis and Meningococcal Septicaemia?

Meningitis is inflammation of the membranes surrounding the brain and spinal cord, while septicaemia is blood poisoning that happens when the same bacteria enter the bloodstream. Meningococcal disease can cause either or both together, and septicaemia without the classic rash is often more dangerous because it is easier to miss early.

What Are the Warning Signs of Meningococcal Disease?

Symptoms can include high fever, a stiff neck, severe headache, sensitivity to light, confusion, cold hands and feet, and a rash that does not fade when a glass is pressed against it. Symptoms can worsen within hours, which is why UK health guidance treats suspected cases as an emergency requiring immediate medical attention.

Who Is Eligible for the MenACWY Vaccine in the UK?

The vaccine is routinely offered to teenagers in school year 9 or 10 and to new university entrants up to age 25 who were not previously vaccinated, reflecting the higher carriage rates seen among students living in close quarters such as halls of residence.

How Does Meningococcal Bacteria Spread Between People?

The bacteria spread through close, prolonged contact such as coughing, kissing or sharing living space, typically traveling in respiratory droplets from someone carrying the bacteria harmlessly in the nose or throat. Most carriers never become ill themselves.

Does the MenB Vaccine Protect Against Other Meningococcal Groups?

Real-world surveillance in England found that the MenB vaccine, known clinically as 4CMenB and sold as Bexsero, also provided some real world protection against group W disease, though it is not licensed or recommended as a substitute for MenACWY vaccination against groups C, W or Y.

Disclaimer: This article reports on published research for general interest and is not medical advice; anyone concerned about meningococcal symptoms should seek immediate care from a doctor or emergency services, and vaccine eligibility details are accurate as of publication.