Suggestions that the novel coronavirus was circulating in Europe months before China officially confirmed the first case in Wuhan have been revived by a new study of blood samples collected in Italy as early as October 2019.

Cancer researchers at Istituto Nazionale dei Tumori in Milan reported in a paper published on Monday that renewed tests of pre-pandemic blood samples at two laboratories were found to have antibodies normally associated with the COVID-19 infection.

"The results of this retesting suggest that what we previously reported in asymptomatic patients is a plausible signal of early circulation of the virus in Italy," Giovanni Apolone, one of the researchers, told the Financial Times.

"If this is confirmed, this would explain the explosion of symptomatic cases observed in Italy. SARS-CoV-2, or an earlier version, circulated silently, under the surface."

Before the pandemic started, Italian researchers had screened 959 individuals for lung cancer, and last year they retested those samples looking for coronavirus-linked antibodies. They claimed they had found traces of COVID-19 infection in some samples.

The World Health Organization asked for further testing to be carried out, and the samples were sent to the VisMederi laboratory in Siena, Italy, and to a facility at Erasmus University in the Netherlands, affiliated with the WHO.

Positive readings

Three samples from the new tests were found by both Erasmus and VisMederi to have positive readings for the COVID-19-linked antibody IgM, according to the Financial Times.

Lab researchers retested 29 of the Italian samples, including some positive and some negative. They also retested 29 control cases from 2018.

The paper has been published as a preprint on the science website medRxiv and has not yet been peer-reviewed. Marion Koopmans, head of virology at Erasmus University, said the new research may not be conclusive.

She said results from the new study were "interesting", but according to the university's strict criteria, none of the samples provided conclusive proof of prior COVID-19 infection.

"We use a rather stringent threshold and cannot rule out that some of the observed reactivity is real," she was quoted as saying by the FT.

"However, for confirmation of earlier circulation, we would recommend studies of patients with unexplained illness for virological confirmation. That does not mean it is impossible … Just that you would like to see other pieces of evidence."

The study stated that none of the samples contained "high enough levels of each of the three types of antibodies" that Erasmus requires to be considered proof of infection.

Gabriella Sozzi, one of the Italian researchers, said that in nine samples that VisMederi said were positive for infection, "levels of IgM antibodies were below the cutoff point set by Erasmus".

The virus may have been "less aggressive or contagious" in the pre-pandemic period, Sozzi told the FT, adding that this made it "necessary to use highly sensitive tests despite the risk of finding 'false positive' cases".

The FT reported that the WHO had said it was "grateful" to scientists trying to advance the understanding of the origins of COVID-19. It added that "it was not part of the laboratory analysis" and that the results "highlighted the challenge of conducting antibody tests on samples from 2019".