Multiple studies indicate that a significant portion of Covid-19 patients are asymptomatic—and that could mean the disease's fatality rate is lower than previously thought.

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Research suggests many patients infected with new coronavirus are asymptomatic

Early research has shown that most people who contract the new coronavirus develop mild cases of Covid-19, the disease caused by the virus, and, in some instances, individuals infected with the virus don't experience any symptoms of Covid-19. However, new studies increasingly are suggesting that the number of people who are infected with the new coronavirus but don't exhibit symptoms of Covid-19 is higher than researchers previously understood. 

For instance, for one study published Wednesday in JAMA Network Open, researchers examined the cases of 78 patients in Wuhan, China, who tested positive for the new coronavirus and found that about 42% of those patients did not exhibit symptoms of Covid-19. According to the study, the asymptomatic patients were more likely to be women and in their 20s, 30s, or early 40s when compared with patients who did show symptoms of Covid-19.

The researchers also found that immune system damage related to Covid-19 was milder in asymptomatic patients than it was in symptomatic patients. The researchers concluded that, in general, asymptomatic patients with Covid-19 experienced less harm than Covid-19 patients who exhibited symptoms of the disease.

Further, the researchers discovered that asymptomatic Covid-19 patients shed the virus for less time, at an average of eight days, than symptomatic patients, who shed the virus for an average of 19 days.

For another study, published in Thorax, researchers looked at the cases of 217 people who were on a cruise ship from Australia to Antarctica and were tested for the new coronavirus. While 59% of those people tested positive for the virus, only 19% of them showed symptoms of Covid-19, according to the study. That means 81% of the people who tested positive for the new coronavirus were asymptomatic, NBC News reports.

And a new Morbidity and Mortality Weekly Report from CDC details a study in which researchers examined patients at a long-term care skilled nursing facility in Los Angeles, where two residents had shown symptoms of Covid-19 and tested positive for the new coronavirus.

After those two cases were identified, all staff and residents at the facility were tested for the virus, and 27 additional people tested positive. Among those people, 18 who tested positive for the new coronavirus were not experiencing symptoms of Covid-19 at the time of testing, including 14 residents of the facility. However, CDC in the report noted that eight of those 14 residents were later classified as presymptomatic, because they developed symptoms of Covid-19 within one to five days after testing—and one of those patients later died.

For another study—led by Nir Menachemi, chair of the health policy and management department at Indiana University's Richard M. Fairbanks School of Public Health—researchers provided more than 4,600 people in Indiana with two tests: one that would tell whether a person was actively infected with the new coronavirus, and one would detect whether a person's blood contained antibodies to the virus, which signals a previous infection. According to NPR's "Shots," preliminary results from the study suggested it was likely that about 3% of the state's population, or about 188,000 people, had been infected with the new coronavirus, which "represented about 11 times more people than conventional selective testing had identified in the state to that point," Menachemi said.

Further, Menachemi and his team found that 45% of those who were infected with the new coronavirus reported not having any symptoms of Covid-19.

What the findings mean for transmission of the new coronavirus—and for Covid-19's fatality rate

Health experts say the new findings are important, because they suggest the new coronavirus could be much more widespread than testing and reporting has shown so far—and because asymptomatic patients still can transmit the new coronavirus.

Menachemi said the number of Americans who have been infected the new coronavirus is likely higher than current counts show, because the current numbers only include people who were tested for the virus. The counts don't "capture the vast number of people out there who might be infected but not seeking medical care," Menachemi said.

Aditya Shah, an infectious disease fellow at the Mayo Clinic, said the latest findings are "very important because, theoretically, you can spread the infection when you're shedding the virus because it's so highly contagious" even if a patient is asymptomatic.

William Schaffner, an infectious disease expert at Vanderbilt University Medical Center, said, "Many people still haven't grasped the notion that asymptomatic people can be so common, and they wonder why it is they have to wear the mask when they're feeling well, or why they have to keep doing this social distancing stuff." He added, "I don't know of a single person, no matter how independently-minded they are, who has any desire to give this virus to anyone else. … But they have to recognize that they could."

Further, experts say incorrect counts of patients who've been infected with the new coronavirus and survived could throw off Covid-19's estimated fatality rate, "Shots" reports. According to "Shots," having a significant amount of asymptomatic Covid-19 patients who survived the disease but aren't counted in official case tallies could mean that Covid-19's actual fatality rate is significantly lower than current estimates suggest.

Caitlin Rivers, an epidemiologist at the Johns Hopkins Center for Health Security, said, "The current best estimates for the infection fatality risk are between 0.5% and 1%," which is dramatically lower than the United States' current calculated Covid-19 case fatality rate of 5.9%.

However, even if Covid-19's fatality rate is below 1%, the disease still poses a significant threat, Rivers said. "That is many times more deadly than seasonal influenza" (Edwards, NBC News, 5/27; Hamilton, "Shots," NPR, 5/28; Dora et. al., Morbidity and Mortality Weekly Report, 5/29).