Boris Johnson has announced a five-tier alert system to rank the threat from coronavirus – although experts said it was not immediately clear how independent or effective the scheme would be.

The current threat level of the pandemic will be categorised on a scale of one to five in different parts of the country, based on assessments by a new “joint biosecurity centre”.

The system is designed to mirror the independent terror alert system, which ranks the threat to the public from “low” to “critical” and helps decide what protective measures are required.

The prime minister said in his broadcast to the nation that the alert system would help the country “avoid going back to square one”. Alert levels, he added, would be determined by the number of cases and the “R” number, or transmission rate, of the virus.

R, or the ‘effective reproduction number’, is a way of rating a disease’s ability to spread. It’s the average number of people on to whom one infected person will pass the virus. For an R of anything above 1, an epidemic will grow exponentially. Anything below 1 and an outbreak will fizzle out – eventually.

At the start of the coronavirus pandemic, the estimated R for coronavirus was between 2 and 3 – higher than the value for seasonal flu, but lower than for measles. That means each person would pass it on to between two and three people on average, before either recovering or dying, and each of those people would pass it on to a further two to three others, causing the total number of cases to snowball over time.

The reproduction number is not fixed, though. It depends on the biology of the virus; people’s behaviour, such as social distancing; and a population’s immunity. 

Hannah Devlin Science correspondent

“In turn, that Covid alert level will tell us how touch we have to be in our social distancing measures. The lower the level, the fewer the measures. The higher the level, the tougher and stricter we will have to be,” Johnson said.

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Terror threat levels are agreed by the Joint Terrorism Analysis Centre, which is independent of ministers but includes representatives of 16 government departments plus police and intelligence agencies.

Further details are expected to be announced when the full lockdown easing plan is presented in detail to parliament on Monday.


No detailed information has yet been released on the specific criteria for setting a level of alert.

A former senior Whitehall security source said the existing terror threat level system was useful because it helped set “a reference point” to shape behaviour across Britain’s wider national security system.

They questioned, however, whether a biosecurity centre would be able to make its assessments independently. “Technocratic measures are being politicised, and ‘blamestorming’ and political manoeuvring seem to be the order of the day at the moment,” they said.

Coronavirus lockdown across Europe

Johnson said the UK was currently under the second highest level of threat, but the situation was improving. “Over the period of the lockdown, we have been in level four, and thanks to your sacrifice we are now in a position to begin to move in steps to level three.”

The new system will apply only to England at first. Scotland has led criticism from the three devolved administrations of Downing Street’s decision to drop the “stay at home” slogan in favour of “stay alert”. No 10, however, said it would seek to work with Scotland, Wales and Northern Ireland to create an integrated approach across the UK.


New Zealand adopted a four-tier coronavirus alert system in late March, in the early stages of the outbreak, so that “people can see and plan for the kinds of restrictions we may be required to put in place”, according to its government.

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Its guidelines cover how people are expected to behave at each level, how the healthcare system should be set up, and what public facilities and businesses are allowed to remain open.

New Zealand, which has suffered relatively mildly from coronavirus compared with the UK and others in Europe, is poised to decide on Monday whether to reduce the threat level from three to two. Any decision has to be signed off by the prime minister, Jacinda Ardern, and her cabinet.



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