Many have had Covid-19 with no symptoms – reports

A FLOOD of new research suggests that far more people have had the coronavirus without any symptoms, fuelling hope that it will turn out to be much less lethal than originally feared.

While that’s clearly good news, it also means it’s impossible to know who around you may be contagious. That complicates decisions about returning to work, school and normal life.

In the past week, reports of silent infections have come from a homeless shelter in Boston, a US Navy aircraft carrier, pregnant women at a New York hospital, several European countries and California.

The head of the US Centers for Disease Control and Prevention says 25% of infected people might not have symptoms. The vice-chairman of the Joint Chiefs of Staff, General John Hyten, thinks it may be as high as 60% to 70% among military personnel. None of these numbers can be fully trusted because they’re based on flawed and inadequate testing, said Dr Michael Mina of Harvard’s School of Public Health.

Collectively, though, they suggest “we have just been off the mark by huge, huge numbers” for estimating total infections, he said.

Worldwide, more than 2.3 million infections and more than 160 000 deaths have been confirmed. The virus has caused nearly unprecedented economic and social harm since its existence was reported in early January.

Based on known cases, health officials have said the virus usually causes mild or moderate flu-like illness. Now evidence is growing that a substantial number of people may have no symptoms at all. Scientists in Iceland screened 6% of its population to see how many had previously undetected infections and found that about 0.7% tested positive. So did 13% of a group at higher risk because of recent travel or exposure to someone sick.

Aboard the aircraft carrier USS Theodore Roosevelt, where one crew member died from the virus, “the rough numbers are that 40% are symptomatic”, said Vice Admiral Phillip Sawyer, deputy commander of naval operations. The ratio may change if more develop symptoms later, he warned.

In New York, a hospital tested all pregnant women coming in to deliver over a two-week period. Nearly 14% of those who arrived with no symptoms of coronavirus turned out to have it. Of the 33 positive cases, 29 had no symptoms when tested, although some developed them later.

Previously, tests on passengers and crew from the Diamond Princess cruise ship found nearly half who tested positive had no symptoms at the time. Researchers estimate that 18% of infected people never developed any.

These studies used tests that look for bits of the virus from throat and nose swabs. Someone can test negative one day if there’s not much virus to detect and then positive the next.

Better answers may come from newer tests that check blood for antibodies, substances the immune system makes to fight the virus. But the accuracy of these, too, is still to be determined.

Last week, researchers reported results from antibody tests on 3 300 people in California’s Santa Clara county: Between 1.5% and 2.8% have been infected, they claimed. That would mean 48 000 to 81 000 cases in the county – more than 50 times the number that have been confirmed.