Mild to asymptomatic cases of coronavirus were a large driver of the rapid spread of the coronavirus outbreak in Wuhan, China, researchers at Columbia University’s Mailman School of Public Health say.
In many European countries, laboratory tests are reserved for those vulnerable to the illness and health workers with severe symptoms.
But as the European Union closes its borders and some countries confine people to their homes, the World Health Organization (WHO) has pushed governments to keep testing.
“You cannot fight a fire blindfolded. And we can't fight this pandemic if we don't know who is infected,” said Dr Tedros Adhanom Ghebreyesus, WHO's director-general.
Prior to Wuhan’s January 23 lockdown, 86 per cent of coronavirus infections were undocumented, researchers at Columbia's Mailman School said in a study published in Science.
Those undocumented cases were the source of over two-thirds of confirmed cases.
“The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited or no symptoms who went undetected,” co-author Jeffrey Shaman, a professor of environmental health sciences at Columbia University’s Mailman School of Public Health said.
Mild or asymptomatic infections can “expose a far greater portion of the population to virus than would otherwise occur,” the study authors explain.
Lothar Wieler, head of Germany’s Robert-Koch-Institute, said on Tuesday that they estimate that just half of those infected show symptoms of the virus.
The public health experts at the Mailman School say that the epidemic changed when China began implementing control measures including travel restrictions, self quarantine and rapid testing.
Coronavirus testing by the numbers in Europe
Italy has the most cases in Europe and has so far carried out more than 148,000 tests. The president of the German Robert Koch Institute said Germany had carried out 160,000 tests. Wieler said they would work to increase their capacity for testing.
Spain did not provide the number of tests carried out, but newspaper El Pais said it was around 30,000 since the beginning of the crisis. The Ministry of Health announced on Wednesday that they would work on carrying out more rapid tests on anyone who has symptoms, following guidance from WHO.
France listed the number of tests through March 10 before its public health body stopped releasing the data. Public Health France told Euronews that the country carries out at least 1,200 tests a day out of a capacity of 2,500 a day.
That would mean, based on the data available since February 24, there have been between 24,600 and 35,000 tests carried out in France. The first data point in late February states that just 24 people were tested.
The French public health authorities said that in an epidemic situation, the policy is to focus only on testing serious cases.
NHS England said that more than 50,000 people have been tested and the country plans to do more testing soon. Austria, which has more than 1,400 confirmed cases has tested just under 12,000 people.
Many of these numbers are still significantly lower than the hundreds of thousands of tests carried out in South Korea and China.
South Korea has carried out 286,716 tests to date and has the capacity to carry out more than 10,000 tests per day. China said back in February that it had carried out over 300,000 tests.
Back in February, China even began including clinical diagnosis by CT scan instead of laboratory tests as a means of confirming cases.
The World Health Organization (WHO), meanwhile, has pushed governments to conduct more testing.
"It's very important in epidemic response to understand your problem because without understanding your problem it's very difficult to fix it,” Dr Mike Ryan, the director of WHO's health emergencies programme said in early March.
“We've seen this; China's numbers went up very, very quickly because they started to look for cases; the same happened in Korea when Korea started to do active surveillance,” he added.
Many European countries are reserve testing for people with severe symptoms, healthcare workers and vulnerable populations.
France only tests people with symptoms among vulnerable populations and health workers, the public health department said. They also test people who work in retirement homes or with those vulnerable to the illness if they have symptoms.
A doctor can make a clinical diagnosis for those who cannot access a test, public health authorities said, but those numbers are not part of the confirmed cases statistics released by the government.
Those confirmed clinically are told to self-isolate regardless.
Spain tests people with acute respiratory infection who are hospitalised and people with any respiratory infection in healthcare professions or other essential services.
Others vulnerable to the illness may be tested with respiratory symptoms regardless of severity, Spain’s health ministry said.
In Germany, there are drive-in coronavirus testing stations. If your doctor says you need the test you can drive there, get the swab and talk to the staff through your car window, like in South Korea.
Testing is not recommended for asymptomatic individuals in Germany, the Robert-Koch-Institute said. People who have symptoms and have been in contact with a confirmed case or who have symptoms and have travelled to a risk area or affected area of Germany are tested.
Patients who have viral pneumonia without another diagnosis are also tested for coronavirus, the institute said.
People with mild symptoms who travelled can be tested in Finland but asymptomatic people are not tested.
Virologist Illaria Capua wrote in Milan's newspaper Corriere della Sera the EU needs to harmonise their methods of detecting coronavirus cases. She asked if all countries test everyone who has died to see if they had coronavirus.
Giovanni Maga, a virologist at the CNR Institute of Molecular Genetics, said epidemiologists agree that the recorded cases "are at least a third or a fifth of the real ones."
Test and isolate: the most effective way to break the chain of infection
Susanna Esposito, President of the World Association for Infectious Diseases and Immunological Disorders (WAidid) told Euronews that the key is to test, early diagnosis, isolation and treatment.
Hong Kong and South Korea had effective test policies, she said. Hong Kong has few cases because anyone with respiratory symptoms is tested and isolated.
“In Italy, asymptomatic people who were in contact with positive cases sometimes have not been tested,” said Esposito who is also a pediatric doctor at the University of Parma, one of the hardest hit areas of Italy.
She added that there are hospitals where they can diagnose cases in four hours and start therapy.
But others do not start treatment until the test comes back, days later, even if clinical diagnosis indicates COVID-19 disease.