The surge in cases after the first diagnosis suggests the virus had been in Italy longer than realised, say researchers.

The first coronavirus infections in Italy date back to January, according to a scientific study presented on Friday, shedding new light on the origins of the outbreak in one of the world’s worst-affected countries.

Italy began testing people after diagnosing its first local patient on February 21 in Codogno, a small town in the wealthy Lombardy region.

Cases and deaths immediately surged, with scientists soon suspecting that the virus had been around, unnoticed, for weeks.

Stefano Merler, of the Bruno Kessler Foundation, told a news conference with Italy’s top health authorities that his institute had looked at the first known cases and drawn clear conclusions from the subsequent pace of contagion.

“We realised that there were a lot of infected people in Lombardy well before February 20, which means the epidemic had started much earlier,” he said.

“In January for sure, but maybe even before. We’ll never know,” he said, adding that he believed the immediate surge in cases suggested the virus was probably brought to Italy by a group of people rather than a single individual.

A separate study based on a sample of cases registered in April said 44.1 percent of infections occurred in nursing homes and another 24.7 percent spread within families. A further 10.8 percent of people caught the virus at hospital and 4.2 percent in the workplace.

Research in Spain, which has the most confirmed coronavirus-linked cases in Europe – above 200,000, and the third-highest death toll worldwide after the US and Italy – also suggested the virus was present in that country long before health officials had realised, at least a month before Madrid imposed a lockdown on the country in mid-March.

Italy was the first major western country to face the viral disease, which originated in China late last year and has spread around the world. Deaths from the epidemic in Italy climbed by 420 on Friday, the smallest daily tally since March 19, the Civil Protection Department said, but the number of new infections rose to 3,021 from 2,646 on Thursday.

Friday’s death toll was down from 464 the day before.

The total of fatalities since the outbreak came to light on February 21 now stands at 25,969, the agency said.

In a bid to prevent the outbreak, Italy had halted air traffic to and from China on January 31, after two Chinese tourists tested positive in Rome. But scientists say it was probably too late.

Another team of Italian scientists has said the coronavirus may have reached Italy from Germany, not directly from China, in the second half of January.