Yesterday BuzzFeed News reported the UK only realised “in the last few days” that attempts to “mitigate” the impact of the coronavirus pandemic would not work, and that it needed to shift to a strategy to “suppress” the outbreak, according to a report by a team of experts who have been advising the government.

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The report, published by the Imperial College COVID-19 Response Team on Monday night, found that the strategy previously being pursued by the government, which involved home isolation of suspect cases and their family members but not including restrictions on wider society, could result in as many as two hundred and fifty thousand deaths.

Professor Azra Ghani, chair of infectious diseases epidemiology at Imperial College London, told journalists “We were expecting herd immunity to build. We now realise it’s not possible to cope with that.”

The global impact of COVID-19 has been profound, and the public health threat it represents is the most serious seen in a respiratory virus since the 1918 H1N1 influenza pandemic.

The Imperial College report presents the results of epidemiological modelling which has informed policymaking in the UK and other countries in recent weeks. In the absence of a COVID-19 vaccine, the researchers assessed the potential role of a number of public health measures, called non-pharmaceutical interventions (NPIs), aimed at reducing contact rates in the population and therefore reducing transmission of the virus. They applied a previously published microsimulation model to the UK (specifically Great Britain) and the US.

They conclude that the effectiveness of any one intervention in isolation is likely to be limited, requiring multiple interventions to be combined to have a substantial impact on transmission.

Two fundamental strategies are possible; mitigation or suppression.

Mitigation focuses on slowing but not necessarily stopping epidemic spread, reducing peak healthcare demand while protecting those most at risk of severe disease from infection

Suppression aims to reverse epidemic growth, reducing case numbers to low levels and maintaining that situation indefinitely.

Each policy has major challenges. The researchers found optimal mitigation policies (combining home isolation of suspect cases, home quarantine of those living in the same household as suspect cases, and social distancing of the elderly and others at most risk of severe disease) might reduce peak healthcare demand by two thirds and deaths by half. However, the resulting mitigated epidemic would still likely result in hundreds of thousands of deaths and health systems (most notably intensive care units) being overwhelmed many times over. For countries able to achieve it, this leaves suppression as the preferred policy option.

The researchers show, in the UK and US context, suppression will minimally require a combination of social distancing of the entire population, home isolation of cases and household quarantine of their family members. This may need to be supplemented by school and university closures, though it should be recognised that such closures may have negative impacts on health systems due to increased absenteeism.

The major challenge of suppression is that this type of intensive intervention package, or something equivalently effective at reducing transmission, will need to be maintained until a vaccine becomes available (which will potentially take eighteen months or more), given that the researchers predict transmission will quickly rebound if interventions are relaxed.

The report shows that intermittent social distancing, triggered by trends in disease surveillance, may allow interventions to be relaxed temporarily in relative short time windows, but measures will need to be reintroduced if or when case numbers rebound.

The report says “While experience in China and now South Korea show that suppression is possible in the short term, it remains to be seen whether it is possible long-term, and whether the social and economic costs of the interventions adopted thus far can be reduced.”