A new meta-analysis from PLOS Medicine is casting doubt on the theory that 50 percent of those with COVID-19 are asymptomatic, showing that four out of five with the virus display symptoms.
Led by researchers at the University of Bern in Switzerland, the paper analyzed dozens of studies from March through early June of individuals who tested positive for COVID-19 through a PCR test. A total of 79 studies were reviewed for data on asymptomatic spread, encompassing 6,616 individuals. Of these, just 20 percent (1,287 individuals) remained asymptomatic during follow-up, meaning that four out of five individuals ultimately showed symptoms of the virus.
The study distinguished between pre-symptomatic individuals, those who show no symptoms at the time of a positive test, and asymptomatic individuals, those who never show symptoms. It theorized that many pre-symptomatic individuals may be “wrongly classified” as asymptomatic when in reality they simply develop symptoms later on.
Overall, the study concludes that the majority of those who contract COVID-19 will have symptoms.
“The findings of this living systematic review suggest that most people who become infected with SARS-CoV-2 will not remain asymptomatic throughout the course of the infection,” the authors conclude. “The contribution of pre-symptomatic and asymptomatic infections to overall SARS-CoV-2 transmission means that combination prevention measures, with enhanced hand hygiene, masks, testing tracing, and isolation strategies and social distancing, will continue to be needed.”
The authors note that the data has limitations, including that the studies may have “selection bias” — meaning that the sample isn’t reflective of the population. In an interview with U.S. News and World Report, Dr. Len Horovitz, a pulmonologist at Lenox Hill in New York City, said that the studies may have missed, for example, individuals who — without any signs of the virus — didn’t get a test. “If you’re under-testing people, you’re not going to uncover the asymptomatic people,” said Horovitz.
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Vice President Mike Pence bumps elbows with Washington Gov. Jay Inslee, left, during a news conference, Thursday, March 5, 2020, at Camp Murray in Washington state. Pence was in Washington to discuss the state’s efforts to fight the spread of the COVID-19 coronavirus, and officials have been avoiding shaking hands to prevent the spread of germs. (AP Photo/Ted S. Warren)
In this image from video, provided by the California National Guard, airmen with the 129th Rescue Wing drop virus testing kits down to the Grand Princess cruise ship off the coast of California Thursday, March 5, 2020. Scrambling to keep the coronavirus at bay, officials ordered a cruise ship with 3,500 people aboard to stay back from the California coast Thursday until passengers and crew can be tested, after a traveler from its previous voyage died of the disease and at least two others became infected. The California National Guard 129th Rescue Wing lowered test kits onto the 951-foot (290-meter) Grand Princess by rope as the vessel lay at anchor off Northern California, and authorities said the results would be available on Friday. Princess Cruise Lines said fewer than 100 people aboard had been identified for testing. (California National Guard via AP)
A woman wears a face mask to protect against the COVID-19 (Coronavirus) after arriving at the LAX airport in Los Angeles, California on March 5, 2020. – California has declared a state of emergency as the number of novel coronavirus cases and deaths in the US continues to climb. (Photo by Mark RALSTON / AFP) (Photo by MARK RALSTON/AFP via Getty Images)
Empty containers for travel hand sanitizer are seen at a store following warnings about COVID-19 in Kirkland, Washington on March 5, 2020. – The US reported its first case of the disease in January and its first death on February 29 — both in the state of Washington in the country’s Pacific Northwest. Since then the toll has risen to 11 and the virus has spread to at least 14 states, infecting more than 180 people, according to an AFP tally. On Thursday, Washington state officials announced a jump in cases, from 39 to 70. Ten of the 11 deaths have been reported there, with the other in California. (Photo by Jason Redmond / AFP) (Photo by JASON REDMOND/AFP via Getty Images)
Mike Weatherill, holds a photo of his mom Louise Weatherill, 85, a resident of Life Care Center who died, speaks during a press conference held by family of residents of the nursing home, where some patients have died from COVID-19, in Kirkland, Washington on March 5, 2020. – The US reported its first case of the disease in January and its first death on February 29 — both in the state of Washington in the country’s Pacific Northwest. Since then the toll has risen to 11 and the virus has spread to at least 14 states, infecting more than 180 people, according to an AFP tally. On March 5, Washington state officials announced a jump in cases, from 39 to 70. Ten of the 11 deaths have been reported there, with the other in California. (Photo by Jason Redmond / AFP) (Photo by JASON REDMOND/AFP via Getty Images)
DENVER, CO – MARCH 5: Gov. Jared Polis speaks during a press conference to address the first confirmed case of Coronavirus in Colorado at the Colorado Department of Public Health and Environment on Thursday, March 5, 2020. Gov. Jared Polis said the first confirmed individual is a man in his 30s, who traveled to Italy last month and was recreating in Colorado. Polis added that the man’s girlfriend, who was traveling with him has been quarantined. A second case was reported during the press conference, but no information was provided by Polis or the officials flanking him. (Photo by AAron Ontiveroz/MediaNews Group/The Denver Post via Getty Images)
Adilisha Patrom, owner of a co-working and event space next to Gallaudet University, organizes face masks, hand sanitizer and other supplies inside her pop up shop on Thursday, March 5, 2020, in Washington. Inside her storefront, she displays different face mask models and hand sanitizer bottles alongside information from the Centers for Disease Control and Prevention.(AP Photo/Nathan Ellgren)
In this March 4, 2020 photo, two women wear masks as they ride a subway train, in New York. Two more cases of the new coronavirus have been confirmed in New York City, raising New York state’s total to 13, Mayor Bill de Blasio said Thursday. (AP Photo/Richard Drew)
A security guard wears gloves while holding a basketball during halftime of an NBA basketball game between the Los Angeles Clippers and Houston Rockets Thursday, March 5, 2020, in Houston. The NBA has told players to avoid high-fiving fans and strangers and avoid taking any item for autographs, the league’s latest response in its ongoing monitoring of the coronavirus crisis. (AP Photo/David J. Phillip)
Vice President Mike Pence, left, bumps elbows with Maj. Gen. Bret Daugherty, right, Adjutant General of Washington State, before a tour of the Washington State Emergency Operations Center, Thursday, March 5, 2020 at Camp Murray in Washington state to discuss the COVID-19 coronavirus. Officials are avoiding shaking hands as a precaution against the virus. (AP Photo/Ted S. Warren)
Golden State Warriors fan Brandon Lai wears a mask because of the coronavirus outbreak before attending an NBA basketball game between the Warriors and the Toronto Raptors in San Francisco, Thursday, March 5, 2020. (AP Photo/Jeff Chiu)
A police officer wears a face mask inside El Alto airport in Bolivia, Wednesday, Feb. 26, 2020. With Brazil reporting the first case of the COVID-19 virus, neighboring countries and other nations around Latin America are attempting to block the possible arrival of the virus. (AP Photo/Juan Karita)
Passengers wear face masks to protect against the COVID-19 (Coronavirus) after arriving at the LAX airport in Los Angeles, California on March 5, 2020. – California has declared a state of emergency as the number of novel coronavirus cases and deaths in the US continues to climb. (Photo by Mark RALSTON / AFP) (Photo by MARK RALSTON/AFP via Getty Images)
Foreign diplomats wearing face masks attend at a briefing by South Korean Foreign Minister Kang Kyung-wha on the situation of the COVID-19 outbreak in Korea, at the foreign ministry in Seoul Friday, March 6, 2020. (Jung Yeon-je /Pool Photo via AP)
People wearing face masks walk through a subway station in Beijing, Friday, March 6, 2020. North Korea said Friday it has released several hundred foreigners from a quarantine imposed as part of its vigilant prevention efforts to avoid an outbreak of the coronavirus that has spread around the world. (AP Photo/Mark Schiefelbein)
A worker produces hand sanitizer at the Companhia Nacional do Álcool (CNA) factory in Piracicaba, Brazil, Tuesday, March 3, 2020. Last week the factory added a second shift of workers to produce more hand sanitizer, and while the CNA was never an exporter, it’s considering that by adding a third shift. One week ago, Brazil confirmed Latin America’s first case of the new coronavirus. (AP Photo/Andre Penner)
A Child is seen wearing a mask inside an MTR Train on February 22, 2020 in Hong Kong, China. the coronavirus or Covid-19 which originated from Wuhan China has infected over 77,000 and killed 2361 worldwide to date. (Photo by Vernon Yuen/NurPhoto via Getty Images)
People wear masks to help guard against the Coronavirus on a street in downtown Tehran, Iran, Sunday, Feb. 23, 2020. Iran’s health ministry raised Sunday the death toll from the new virus to 8 people in the country, amid concerns that clusters there, as well as in Italy and South Korea, could signal a serious new stage in its global spread. (AP Photo/Ebrahim Noroozi)
People wear masks to help guard against the Coronavirus in downtown Tehran, Iran, Sunday, Feb. 23, 2020. On Sunday Iran’s health ministry raised the death toll from the new virus to 8 people in the country, amid concerns that clusters there, as well as in Italy and South Korea, could signal a serious new stage in its global spread. (AP Photo/Ebrahim Noroozi)
This Saturday, Feb. 22, 2020, photo released by Xinhua News Agency, shows medical workers in protective suits at a coronavirus detection lab in Wuhan in central China’s Hubei Province. The fresh national figures for the disease that emerged in China in December came as the number of viral infections soared mostly in and around the southeastern city of Daegu, where they were linked to a local church and a hospital. (Cheng Min/Xinhua via AP)
Mary Cahill, left, leaves a news conference where officials discussed the proposal for housing coronavirus patients at the Fairview Development Center in Costa Mesa, Calif., Saturday, Feb. 22, 2020. A court temporarily blocked the U.S. government from sending up to 50 people infected with a new virus from China to the Southern California city for quarantine after local officials argued that the plan lacked details about how the community would be protected from the outbreak. (Mindy Schauer/The Orange County Register via AP)
Personnel carry new beds inside the hospital of Codogno, near Lodi in Northern Italy, Friday, Feb. 21,2020. Health officials reported the country’s first cases of contagion of COVID-19 in people who had not been in China. The hospital in Codogno is one of the hospitals – along with specialized Sacco Hospital in Milan – which is hosting the infected persons and the people that were in contact with them and are being isolated. (AP Photo/Luca Bruno)
Una persona toma fotografías cerca del crucero Diamond Princess en cuarentena, en el puerto de Yokohama, cerca de Tokio, el viernes 21 de febrero de 2020. (AP Foto/Eugene Hoshiko)
A medic worker disinfects a room in the sanatorium after the quarantine period has ended in Bogandinsky in the Tyumen region, about 2150 kilometers (1344 miles) east of Moscow, Russia, Friday, Feb. 21, 2020. 144 people who were evacuated from the epicenter of the CONVID-19 coronavirus outbreak in Wuhan, China, were released from a 14-day quarantine in western Siberia. None of them tested positive for the virus. (AP Photo/Maxim Slutsky)
Un trabajador vestido con un traje de protección rocía desinfectante como precaución contra un coronavirus en una estación de metro de Seúl, Corea del Sur, el 21 de febrero de 2020. (AP Foto/Ahn Young-joon)
Workers wearing protective gears help clean each other’s suits after disinfecting as a precaution against the coronavirus at a subway station in Seoul, South Korea, Friday, Feb. 21, 2020. South Korea on Friday declared a “special management zone” around a southeastern city where a surging viral outbreak, largely linked to a church in Daegu, threatens to overwhelm the region’s health system. (AP Photo/Ahn Young-joon)
Un autobús con pasajeros del buque crucero en cuarentena Diamond Princess sale del puerto de Yokohama, cerca de Tokio, Japón, jueves 20 de febrero de 2020. Dos pasajeros ancianos han muerto, siendo las primeras víctimas fatales del nuevo virus en el buque. (AP Foto/Eugene Hoshiko)
China’s Foreign Minister Wang Yi speaks during a joint press conference of the Co-Chairs Special ASEAN-China Foreign Ministers’ meeting on the Novel Coronavirus Pneumonia in Vientiane, Laos, Thursday, Feb. 20, 2020. (AP Photo/Sakchai Lalit)
En esta fotografía publicada por la agencia noticiosa Xinhua, un farmacéutico procesa gránulos de la medicina tradicional china en el hospital provincial de Gansu especializado en este tipo de medicina, en Lanzhou, en la provincia noroccidental china de Gansu, el jueves 20 de febrero de 2020. La medicina tradicional china se ha utilizado ampliamente para combatir la enfermedad causada por el nuevo coronavirus. (Fan Peishen/Xinhua vía AP)
En esta foto distribuida por la embajada de Ucrania en China, una pasajera ucraniana hace una señal de victoria en el bus del aeropuerto al avión chárter que evacuará a unas 70 afectadas por el brote de un nuevo virus. Entre los evacuados hay 45 ucranianos y 27 personas de Argentina, República Dominicana, Costa Rica, Panamá, Israel y Ecuador. (AP Foto/Servicio de Prensa del Ministerio del Exterior de Ucrania)
Clinical support technician Douglas Condie extracts viruses from swab samples as analytic work continues to examine the genetic structure of a virus in the coronavirus testing laboratory at Glasgow Royal Infirmary, Glasgow, Scotland, Wednesday Feb. 19, 2020.The COVID-19 virus originally centred in China now has multiple cases confirmed worldwide. (Jane Barlow/PA via AP)
In this Monday, Feb. 17, 2020, photo released by Xinhua News Agency, patients infected with the coronavirus take rest at a temporary hospital converted from Wuhan Sports Center in Wuhan in central China’s Hubei Province. China reported thousands new virus cases and more deaths in its update Tuesday on a disease outbreak that has caused milder illness in most people, an assessment that promoted guarded optimism from global health authorities. (Xiao Yijiu/Xinhua via AP)
In this Monday, Feb. 17, 2020, photo, volunteers make the handmade cotton masks in Hong Kong. Volunteers from a Hong Kong theater group are turning their backstage skills to helping out against the new coronavirus, sewing reusable protective masks for those who can’t access or afford them. Jo Ngai, a drama lover and founder of the Nonsensemakers, along with her friend Jessie Han, have turned a theater rehearsal room into a temporary handmade mask factory. (AP Photo/Kin Cheung)
This undated electron microscope image made available by the U.S. National Institutes of Health in February 2020 shows the Novel Coronavirus SARS-CoV-2. Also known as 2019-nCoV, the virus causes COVID-19. The sample was isolated from a patient in the U.S. (NIAID-RML via AP)
In this Feb. 16, 2020, photo, runners, some wearing masks, compete in a Kumamoto castle marathon in Kumamoto city, western Japan. Organizers of the Tokyo Marathon set for March 1, 2020 are drastically reducing the number of participants out of fear of the spread of the coronavirus from China. The general public is essentially being barred from the race. (Kyodo News via AP)
En esta imagen del 17 de febrero de 2020, autobuses con pasajeros del crucero en cuarentena Diamond Princess salen de un puerto en Yokohama, cerca de Tokio. (AP Foto/Jae C. Hong, Archivo)
Family members of Pakistani students studying in Wuhan, China, hold a demonstration calling for the evacuation of their relatives after the Chinese city was badly hit by the coronavirus, in Karachi, Pakistan, Sunday, Feb. 16, 2020. (AP Photo/Fareed Khan)
A group of quarantined passengers exercise on the Diamond Princess cruise ship Saturday, Feb. 15, 2020, in Yokohama, near Tokyo. A viral outbreak that began in China has infected more than 67,000 people globally. The World Health Organization has named the illness COVID-19, referring to its origin late last year and the coronavirus that causes it. (AP Photo/Jae C. Hong)
SEOUL, SOUTH KOREA – FEBRUARY 23: In this handout image provided by South Korean Presidential Blue House, South Korean President Moon Jae-in speaks during a meeting about coronavirus (COVID-19) at a government complex on February 23, 2020 in Seoul, South Korea. South Korea reported 169 new cases of the coronavirus (COVID-19) bringing the total number of infections in the nation to 602, with the potentially fatal illness spreading fast across the country. (Photo by South Korean Presidential Blue House via Getty Images)
Japan’s Prime Minister Shinzo Abe (L) attends a meeting at the new COVID-19 coronavirus infectious disease control headquarters at the prime minister’s office in Tokyo on February 23, 2020. – Japan on February 23 confirmed the first case of a former Japanese passenger of a coronavirus-stricken cruise ship testing positive for the disease after initially receiving a clean bill of health. (Photo by STR / JIJI PRESS / AFP) / JAPAN OUT (Photo by STR/JIJI PRESS/AFP via Getty Images)
TAIPEI, TAIWAN – 2020/02/23: Commuters in the Taipei MRT system all wearing surgical mask as the 2019 Novel coronavirus that originated in China is spreading to other countries worldwide. Taiwan authorities fear a serious pandemic and people are advised to wear surgical masks especially when confined in small spaces such as public transports. (Photo by Alberto Buzzola/LightRocket via Getty Images)
WUHAN, Feb. 22, 2020– A staff member handles nucleic acid testing samples at a novel coronavirus detection lab in Wuhan, central China’s Hubei Province, Feb. 22, 2020. Since its operation on Feb. 6, the novel coronavirus detection lab named “Huoyan”, jointly constructed by the local government and other institutions, has tested tens of thousands of nucleic acid testingsamples. For days, including the “Huoyan” lab, Wuhan’s 40 NAT institutions together, have been able to test 14,000 NAT samples per day, zeroing the number of daily NAT from all possible coronavirus patients. (Photo by Cheng Min/Xinhua via Getty) (Xinhua/Cheng Min via Getty Images)
BEIRUT, LEBANON – FEBRUARY 22: Students wear masks to protect themselves from coronavirus as a precaution in Beirut, Lebanon on February 22, 2020. Lebanese Health Minister Dr. Hamad Hassan made a statement on first coronavirus case at the Rafik Hariri University Hospital. (Photo by Hussam Chbaro/Anadolu Agency via Getty Images)
A Lebanese nurse wearing a protective mask works at a ward where the first case ofcoronavirus in the country is being treated, at the Rafik Hariri University Hospital in the southern outskirts of the capital Beirut, on February 22, 2020. – Lebanon confirmed on February 21, the first case of the novel coronavirus, found in a 45-year-old Lebanese woman who had travelled from the holy city of Qom in Iran, while two other cases were being investigated. (Photo by ANWAR AMRO / AFP) (Photo by ANWAR AMRO/AFP via Getty Images)
SOUTHAMPTON, ENGLAND – FEBRUARY 22: An LED screen shows Southampton’s support against the coronavirus during the Premier League match between Southampton FC and Aston Villa at St Mary’s Stadium on February 22, 2020 in Southampton, United Kingdom. (Photo by Alex Broadway/Getty Images)Up Next
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Dr. Angela Rasmussen, a leading virologist at Columbia University, says the results show how muddy the waters remain between asymptomatic and pre-symptomatic cases. “This does not surprise me, because reports of ‘asymptomatic’ confirmed cases are difficult to distinguish from pre-symptomatic cases if the patients are lost to clinical follow-up,” Rasmussen tells Yahoo Life. “Many people who are asymptomatic at the time of testing are then sent home to isolate. If they develop mild disease that doesn’t require them to seek further clinical care, they may be listed as asymptomatic even though they eventually develop symptoms.”
Rasmussen adds that even when follow-up occurs, the less common symptoms of COVID-19 may go unnoticed. “Asymptomatic disease relies on self-reporting symptoms, which is notoriously unreliable,” says Rasmussen. “We already know that COVID-19 can be associated with a diverse array of symptoms beyond fever or respiratory disease. If a patient develops a symptom that they do not think is associated with COVID-19, such as diarrhea or anosmia (loss of sense of smell), then they might not report that if asked.”
As of now, more data is needed to get a clearer picture of how many with COVID-19 may be asymptomatic, but Rasmussen says that a key takeaway, for now, is that even those who appear to be healthy may simply be in the early stages of the virus. “The important thing is that pre-symptomatic transmission is a major driver of community spread,” says Rasmussen. “Whether a patient develops symptoms or not, they are contagious before they are aware that they are sick, and this means they can inadvertently spread the virus to others.”
As far as public health guidelines, Rasmussen says this “shouldn’t have any impact” on how individuals are staying safe. Wearing a mask, frequently washing your hands and maintaining social distancing remain the three most important ways to prevent the virus. For more information about how to stay safe, visit the CDC.
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