An avalanche of new research suggests that many more people have had the coronavirus without any symptoms, fueling hopes that it will turn out to be far less deadly than originally feared.
While that is clearly good news, it also means that it is impossible to know who may be contagious around you. That complicates decisions about returning to work, school, and normal life.
In the past week, reports of silent infections come from a homeless shelter in Boston, a United States Navy aircraft carrier, pregnant women in a New York hospital, several European countries, and California.
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The head of the US Centers for Disease Control and Prevention (CDC). USA It says that 25 percent of infected people may have no symptoms. Deputy Chairman of the Joint Chiefs of Staff, General John Hyten, believes it can be as high as 60pc to 70pc among military personnel.
None of these numbers can be trusted because they are based on faulty and inadequate evidence, said Dr. Michael Mina of the Harvard School of Public Health.
However, collectively, they suggest that "we've been out of place for a huge number, huge" to estimate total infections, he said.
Worldwide, more than 2.3 million infections and more than 160,000 deaths have been confirmed. The virus has caused almost unprecedented economic and social damage since its existence was reported in early January.
According to known cases, health officials have said that the virus generally causes a mild or moderate flu-like illness. Now the evidence is mounting that a considerable number of people may have no symptoms.
Scientists in Iceland examined six percent of its population to see how many infections had previously gone undetected, and found that about 0.7 percent tested positive. So did 13% of a group at higher risk due to recent travel or exposure to someone sick.
Aboard the aircraft carrier USS Theodore Roosevelt, where a crew member died from the virus, "the approximate numbers are 40 percent symptomatic," said Vice Admiral Phillip Sawyer, deputy commander of naval operations. The ratio may change if more symptoms develop later, he warned.
In New York, a hospital examined all pregnant women who were due to deliver over a two-week period. Almost 14 percent of those who came without symptoms of coronavirus had it. Of the 33 positive cases, 29 had no symptoms when evaluated, although some developed them later.
Previously, tests on passengers and crew of the Diamond Princess cruise ship found that almost half of those who tested positive had no symptoms at the time. The researchers estimate that 18 percent of infected people never developed any.
These studies used tests to look for fragments of the virus from throat and nose swabs, which may overlook cases. Someone can test negative one day if there is not much virus to detect and then positive the next.
Symptoms may also not appear when someone is tested, but appear later. A Japanese study found that more than half of those who had no symptoms when they tested positive later felt sick.
Better answers may come from more recent tests that check the blood for antibodies, substances that the immune system produces to fight the virus. But the precision of these is also to be determined.
On Friday, researchers reported the results of antibody tests on 3,300 people in Santa Clara County, California: They claimed that between 1.5 and 2.8 percent had been infected. That would mean between 48,000 and 81,000 cases in the county, more than 50 times the number that has been confirmed.
The work has not been formally published or reviewed, but some scientists were quick to question it. Participants were recruited through Facebook ads, which would appeal to many people who are probably positive and have had symptoms and want to know if the coronavirus was the reason. Some neighborhoods also had far more participants than others, and "hotspots" within the county may have made infections seem more common than elsewhere.
Ships, maternity wards, and individual counties also do not provide data that can be used to generalize about what is happening elsewhere. And many of the figures come from snapshots, not from research on large populations over time.
Antibody tests in particular should be done "in an unbiased approach" on groups of people who are representative of geographic, social, racial and other conditions, Mina said.
CDC and other groups plan such studies, and could guide public health advice on how to return to normal life for people in certain areas.
If infections are more widespread than previously understood, more people may have developed some level of immunity to the virus. That could stifle the spread through what's called collective immunity, but scientists caution that there is still much to learn about whether mild diseases confer immunity and how long it can last.
It will probably be months before enough reliable tests are done to answer those questions and others, including the spread of infections and the true death rate from the virus, which has only been estimated so far.
"If everyone has seen the virus before, then maybe they can relax in that neighborhood" and ease social estrangement, Mina said. "We're still not close to where we should be" on antibody testing to do that, he said.