Lockdowns and other interventions to reduce virus transmission have saved more than 3 million lives from Covid-19 in 11 European nations, a team of epidemiologists has claimed. Like all modeling, the work rests on assumptions, some of which will certainly be controversial, but the onus is now on those who claim lockdowns were unnecessary to show where the modeling is wrong.

Through March, as nation after nation across Europe attempted to control SARS-CoV-2's spread by closing institutions and issuing stay-at-home orders, everyone was flying almost blind. No one knew just how transmissible the virus was, nor what proportion of infected people would die. Modeling at Imperial College London was influential, particularly in the English-speaking world, in persuading governments to act more strongly than they might otherwise have done. In particular, the Imperial team demonstrated how potentially disastrous the alternative strategy to lockdown, known as the Herd Immunity Strategy, could be if certain plausible assumptions proved true.

Although our ignorance about Covid-19 remains mountainous, the pool of knowledge about the disease has expanded dramatically in the last three months. Members of the same team, have incorporated what has been learned to estimate the effects of the actions taken in 11 Western European nations.

“The rate of transmission has declined from high levels to ones under control in all European countries we study,” said Dr Samir Bhatt of the Imperial College MRC Center for Global Infectious Disease Analysis in a statement. 

In Nature, Bhatt and colleagues estimate 12-15 million people were infected with the virus in 11 European countries by May 4, representing 3.8 percent of the population, ranging from 0.8 percent in Austria to 10 times that in Belgium. This led to 130,000 deaths in the same countries in that period.

Without intervention, however, they conclude, the virus was spreading so rapidly it would have swept through the majority of the populations, and caused an extra 3.1 million deaths.

The authors acknowledge some weaknesses in their model. The nature and timing of government interventions to reduce transmission varied from country to country – and sometimes even within nations, but the modeling pools some of these. “Amidst the ongoing pandemic, we rely on death data that is incomplete, with systematic biases in reporting, and subject to future consolidation,” the authors admit. This could distort estimates of how fast the virus would have spread.

Ultimately, however, an infectious disease either has a reproductive number, or R rate – the average number of people each infected person infects in turn – above or below one. If it remains above one long enough, most of the population will catch it, if it can be kept below one, the disease will fizzle out. Early in the crisis, there was widespread skepticism any European nation could get below one, but the authors calculate all 11 countries have achieved just that.

Unfortunately, because so many interventions were implemented roughly simultaneously, the paper sheds no light on which were most effective.