LONDON (BLOOMBERG) - When the coronavirus swept across Europe this spring, overwhelming hospitals and killing thousands each day, few but the sickest patients could be tested, leaving health officials in the dark about how widely Covid-19 had spread.
Six months on, with the virus surging again, the authorities can point to dramatically expanded testing capacity.
Britain alone carried out some 1.3 million tests in a recent week - a 20-fold increase from early April.
France and Spain, Covid hot spots, have likewise increased their capabilities.
The ramp-up is helping the authorities identify thousands of infected individuals, a key step towards curbing the pathogen without resorting to economically devastating lockdowns.
But cracks are emerging.
The mass testing is straining labs, slowing results and complicating the contact tracing that's crucial to contain the virus. And many with the disease are ignoring rules on self-isolation.
"That warps everything," said Prof James Naismith, a professor of structural biology at Oxford University. "Testing is only useful if it's paired with rigorous contact tracing and isolation."
With infections rising and winter just around the corner, some public health advocates are sounding the alarm that the intense focus on testing is taking attention away from the other measures that are also necessary to fight the pandemic.
Without testing, health officials can't track or isolate the sick, nor fully understand important facets of the virus, such as its lethality.
In March, policymakers caught unprepared for the pandemic were unable to formulate a nimble response. The only way to bring the epidemic to heel was to impose mass lockdowns - to treat everyone as sick - which curbed infections but triggered the worst economic slump in living memory.
That's an outcome leaders across the region have pledged to avoid this time around, though their options are narrowing.
Prime Minister Boris Johnson will convene crisis talks on tackling the resurgent virus on Tuesday, after his top scientific adviser warned the UK is on course for 50,000 new cases a day by mid-October without urgent action. He'll then make a statement to the House of Commons, laying out the next steps in his government's pandemic response.
As infections subsided over the summer, testing capacity kept growing, allowing the authorities to target wider groups - including those returning from vacation or going back to schools or offices.
While the surge in testing identified plenty of mild and even asymptomatic cases, it also brought many laboratories to the limits of their capabilities.
In France, the government made tests free and widely available, recently ratcheting up the total to 1.2 million a week. That's led to "gridlock in access to tests, especially in some large cities where the virus circulates a lot," Health Minister Olivier Veran said at a press conference on Thursday.
Until recently, more than 99 per cent of tests were coming back negative in some nations - a far cry from positive rates of as much as half during the spring.
Nowhere has the turnabout been more pronounced - or the political heat more intense - than the UK, which lagged behind other European countries in testing early on but has now been doing more of it than any of its continental neighbours.
With labs unable to handle the volume, officials in some countries are again focusing testing on those who show symptoms or may have been exposed to the virus. While that approach is in line with World Health Organisation guidelines, it's reviving concern for some that tests are in short supply.
To improve tracing, France, Britain and other countries rolled out smartphone apps designed to notify those who may have been exposed to the virus - but privacy concerns kept many from using them. And, as the number of Covid clusters increases, the job of those working the phones to track down the contacts of infected individuals grows more daunting each day.
Still, if tracers manage to reach some 50 per cent to 80 per cent of contacts - and if those people then adhere to rules on quarantining - those efforts could be enough to prevent another lockdown, estimated Prof Annelies Wilder-Smith, a professor of infectious diseases at the University of Heidelberg's Global Health Institute.
"We are morally and economically obligated not to give up," she said. "We owe it to our citizens."
Isolating the infected, especially those with a mild case, is another challenge. Only one in five people with Covid-19 symptoms in the UK is properly isolating at home, according to a recent report from a government advisory panel. Mr Johnson's office said on Sunday that those in England who refuse an order to self-isolate could be fined as much as £10,000 (S$17,465).
France recently loosened its mandatory period of self-isolation for the infected to seven days from 14 - betting that people will be more likely to obey if the period is shorter.
For its part, Germany has drawn praise during the pandemic for rolling out mass testing early on, imposing a relatively permissive lockdown in March and April and for putting together an effective army of contact tracers. Even so, its case numbers are rising again, heightening anxieties about the autumn and winter.
In the German Alpine resort town of Garmisch-Partenkirchen, the limits of testing recently came into sharp relief. An American woman in her 20s spent several recent nights visiting bars. When showing symptoms a few days later, she was tested and told by officials to isolate, but continued to go out, according to local authorities. Health workers later found more than 30 new infections potentially linked to the woman. Prosecutors from Munich are investigating whether to bring charges.
"That's the whole story right there," Prof Wilder-Smith said. "Isolation must be absolutely strict, you cannot have a mistake."