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As part of an international effort to pinpoint the origin of the Covid-19 pandemic and prevent future disease outbreaks, scientists are looking for new clues in frozen blood.

Their efforts span the globe. Organizations including the American Red Cross and the U.S. Centers for Disease Control and Prevention are testing old blood specimens for the presence of antibodies to SARS-CoV-2, the virus that causes Covid-19. Blood banks in the U.S., Europe and other regions are discussing new approaches to testing blood samples for signs of infection.

In a report published March 30, a team of scientists convened by the World Health Organization called for a study of blood specimens from donations collected in Wuhan, China, before the first known outbreak of the new coronavirus in December 2019.

The scientists want to see whether any of the samples, which were frozen and stored, contain SARS-CoV-2 antibodies, which could help reveal how long the virus was circulating in people before the first cases were discovered.

The report also called for the analysis of blood samples collected prior to the pandemic elsewhere in China and in other countries.

Studying blood samples from the past offers a potentially powerful way to determine how and when the pandemic began—a window into an otherwise unseen world. But scientists say how much they learn will depend on the number of samples blood banks have and the level of access to them granted to researchers.

Practices and regulations for collecting and storing blood differ from country to country. Many blood banks store samples only for brief periods, in part because they lack sufficient freezer capacity.

“The historical practice and the reality of limited resources means most blood centers don’t save samples from all their donations," said Michael P. Busch, senior vice president for research and scientific affairs at Vitalant, a network of blood centers and labs, and president-elect of the International Society of Blood Transfusion.

The WHO-led team recommended that the Wuhan Blood Center test for antibodies to SARS-CoV-2 in blood samples collected from adult donors between September and December 2019, and “further back in time until there are two successive months without any evidence of SARS-CoV-2-specific antibodies," according to the report.

The center collects about 200,000 blood donations annually from adults 18 to 60 years of age, the report said. Specimens from the donations are stored for two years, so some blood from 2019 would still be available. The specimens should be compared with blood collected in other regions of China where Covid-19 cases weren’t detected before early 2020, according to the report.

The team is seeking the blood study because other data—rates of flulike illnesses, drug prescriptions and deaths—didn’t reveal earlier cases, said team member Thea Fischer, a Danish public-health specialist. “We cannot exclude that there have been milder cases, and there might have been smaller epidemics that have gone under the radar," she said.

The Wuhan Blood Center has agreed to the study, said Dutch virologist Marion Koopmans, another member of the team. The project needs to be extensive, she said, to be able to detect small pockets of cases that the other surveillance systems didn’t pick up.

The center referred questions to China’s National Health Commission, which didn’t respond to a request for comment.

The U.S. is also scouring blood banks for clues as to how early Covid-19 appeared within its borders, and researchers in Europe have also searched blood samples for antibodies. Dr. Busch said he had reached out to blood centers in China, Brazil, South Africa, Canada and the U.S. to discuss working together to test stored blood samples from 2019 and later.

In late 2019, the American Red Cross and the CDC agreed to create a blood repository collected from donors in various states where mosquito-borne and tick-borne viruses were on the rise. “Then Covid happened," said Dr. Susan Stramer, vice president of scientific affairs at the American Red Cross.

The investigators switched gears, and started testing for early evidence of antibodies for SARS-CoV-2 in the U.S. They found signs of early infections in December 2019 and January 2020.

The first known U.S. case was confirmed by the CDC on Jan. 21, 2020.

To go back earlier than December, when they started collecting blood for the mosquito-borne and tick-borne project, the researchers had to search another blood repository. The investigators found one with blood samples dating to 2016, from donors who felt fine when they gave blood but later reported having a cough, fever, cold or “influenza-like illness." That study is still under way.

Experts are now developing more robust blood-surveillance systems to monitor the spread of Covid-19 and detect early outbreaks of diseases in the future.

The American Red Cross, Vitalant, the CDC and other organizations last year began a study to test blood samples taken around the country for SARS-CoV-2 antibodies. Such testing can help determine the durability of antibodies in people who have had Covid-19 or been vaccinated against it.

The study will continue at least until the end of this year, the researchers said.

The Harvard T.H. Chan School of Public Health has hundreds of thousands of blood-plasma samples from donors around the U.S. The samples date to the beginning of 2020. Michael Mina, an epidemiologist and immunologist at the school, plans to analyze them to figure out how early and widely SARS-CoV-2 was circulating in the U.S. in the first two months of last year, when public-health officials knew about only a few cases.

The samples are part of a “global immunological observatory" he and his team are building—a surveillance system that would allow scientists to detect the spread of viruses in real time by routinely testing blood from donations and medical tests for evidence of infection, Dr. Mina said.

Dr. Mina likened the system to weather forecasting. “We can start to identify perturbations in the same way that we identify tornadoes coming about, or hurricanes," he said.

This story has been published from a wire agency feed without modifications to the text.

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