Most people with COVID-19 develop symptoms, study finds


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.”

A new study from Switzerland found that four out of five people with COVID-19 develop symptoms. (Photo: Getty Images)
A new study from Switzerland found that four out of five people with COVID-19 develop symptoms. (Photo: Getty Images)

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.

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.

For the latest coronavirus news and updates, follow along at According to experts, people over 60 and those who are immunocompromised continue to be the most at risk. If you have questions, please reference the CDC’s and WHO’s resource guides.

Read more from Yahoo Life

Want lifestyle and wellness news delivered to your inbox? Sign up here for Yahoo Life’s newsletter.