LONDON • When the coronavirus swept across Europe this spring, overwhelming hospitals and killing thousands each day, only 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 twentyfold increase from early April.

France and Spain, both Covid-19 hot spots, have likewise increased their capabilities.

The ramp-up is helping the authorities to 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 laboratories, slowing results and complicating the contact tracing that is crucial to contain the virus.

And many with the disease are ignoring rules on self-isolation.

"That warps everything," said structural biology professor James Naismith at Oxford University.

"Testing is only useful if it's paired with rigorous contact tracing and isolation."

With infections rising and winter around the corner, some public health advocates are sounding the alarm that the focus on testing is taking attention away from the other measures necessary to fight the pandemic.

In France, the government made tests free and widely available, recently ratcheting up the total to 1.2 million a week.

That has led to "gridlock in access to tests, especially in some large cities where the virus circulates a lot", Health Minister Olivier Veran said last 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 in Britain, 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 prioritising testing for those who show symptoms or may have been exposed to the virus.

While that approach is in line with World Health Organisation guidelines, it is 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-19 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 Dr Annelies Wilder-Smith, a professor of infectious diseases.

"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 Britain is properly isolating at home, according to a recent report from a government advisory panel.

British Prime Minister Boris 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$17,400).

France recently loosened its mandatory period of self-isolation for the infected to seven days from 14 days, betting that people will be more likely to obey if the period is shorter.

Germany has drawn praise during the pandemic for rolling out mass testing early on.

Even so, its case numbers are rising again, heightening anxieties about 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 nights visiting bars.

When she showed symptoms a few days later, she was tested and told by officials to self-isolate, but she continued to go out.

Health workers later found more than 30 new infections potentially linked to the woman.

Prosecutors from Munich are investigating whether to press charges.

"That's the whole story right there," Dr Wilder-Smith said.

"Isolation must be absolutely strict; you cannot have a mistake."

BLOOMBERG