Sarah Newey Global Health Security Correspondent, in Bangkok

It all starts very quickly. Before health officials have any chance to contain a new strain of influenza, the highly contagious virus has landed in Britain and taken hold.

Over the next two years, wave after wave of infections hit half of the population – 33 million people fall ill and 1.3m are hospitalised. The crisis pushes the health system to the brink and costs the economy £1.9 trillion, while 270m school days are lost.

This scenario – a pandemic caused by a respiratory virus – is the UK Health Security Agency’s worst fear, according to Britain’s first health security assessment, published on Thursday.

“Respiratory infections present the greatest threat to UK health security, with high impact and likelihood, and repeated historical precedence,” the agency warned, estimating that the risk of a “catastrophic” outbreak within the next five years is somewhere between five and 25 per cent.

“Unlike many other infectious diseases, respiratory viruses mutate quickly, this can make existing vaccines less effective and reduce the impact of other interventions,” it said.

A flu pandemic is among 22 detailed in the assessment, which analyses the likelihood and consequences of known health threats over the next five years – including untreatable superbug outbreaks, an influx of disease-ridden invasive mosquitoes, and a plummeting measles vaccination rate.

As well as including sobering but “reasonable worst case scenarios” the report – a health-specific version of the national risk register – plots each threat on a risk matrix.

It said that although a pandemic flu or novel coronaviruses could have “catastrophic” impacts, the most likely events are increased tuberculosis cases linked to migration, extreme heatwaves, and rising rates of tick-borne encephalitis.

It also found antimicrobial resistance has “the potential to exacerbate” many of the threats described. For instance, secondary infections during pandemics may become untreatable, super gonorrhoea and other sexually transmitted infections (STIs) could surge, and outbreaks of healthcare-associated pathogens like invasive candidiasis would prove harder to control.

The report also warned against complacency when tackling lower impact risks. Dengue and Zika, for instance, will not become endemic within the five years as the climate is not yet warm enough for the mosquitoes that carry them. But ignoring the threat now “may lead to a higher impact risk” later.

“Our shared pandemic experience has reinforced the need for a thorough assessment of the range, impact and likelihood of health security risks to the UK population,” wrote in the document’s foreword.

He added that recent outbreaks – including meningitis, hantavirus and mpox – demonstrate “how rapidly health concerns can escalate and the need for effective strategic planning to safeguard the public’s health”.

“Until now, there has been no overarching assessment comparing and contrasting the range of risks that UKHSA needs to consider. The Health Security Risk Assessment fills that gap,” he said.

The agency stressed the report does not map out every hazard, but is intended to help focus the minds of contingency planners across the health system, while scenarios outlined are severe but “plausible manifestations” of a risk – rather than predications.

These scenarios include a fall in vaccination rates among children that leads to 160,000 measles infections within a year; an extended heatwave that results in an additional 2,000 deaths; and a new outbreak of mpox clade 2b that overwhelmed sexual health services.

“It’s useful to have this type of overview of the various types of threats over the next five years and an understanding of where we’re at in terms of impact and how it might fall,” Prof Sridhar, chair of global public health at the University of Edinburgh, told the Telegraph, adding that countries including the United States have long published similar assessments.

But she warned the UK must also be on the lookout for the unexpected, as the only “certainty is that there’s always uncertainty about virtual evolution”. The risk assessment purposefully includes only known threats, and does not evaluate ‘Disease X’ – an as yet unknown pathogen with pandemic potential.

“As much as we can plan for the major risks, and it’s helpful to plan and prepare, we also have to have humility that sometimes we don’t have a crystal ball and simply don’t know what will be the main risk emerging,” Prof Sridhar said.

“We just have to build systems and institutions that can flexibly adapt to the specific outbreak or risk, and platforms which mean medical countermeasures can be rapidly created, approved and manufactured.”

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