An estimated 45 percent of SARS-CoV-2 infections could be void of symptoms, creating the potential for infected individuals to silently spread the dangerous pathogen as many parts of the world are beginning to reopen after lockdown, according to a new analysis.

"Our estimate of 40 to 45 percent asymptomatic means that, if you’re unlucky enough to get infected, the probability is almost a flip of a coin on whether you’re going to have symptoms,” said behavioral scientist Daniel Oran in a statement.

Researchers from the Scripps Institute examined data from 16 cohorts of Covid-19 patients, including those from nursing home residents, cruise ships, and prison inmates. Though not all of the datasets were published in a peer-reviewed journal at the time of the study, the researchers say that all information must be considered to mitigate the spread of the virus.

Many of those infected with Covid-19 are “invisible,” according to the findings published in the Annals of Internal Medicine. These “silent” patients may be exacerbating efforts to stop the disease from spreading. Overall, it’s estimated that between 40 and 45 percent of people who have become infected will never have symptoms.

“What virtually all of them had in common was that a very large proportion of infected individuals had no symptoms,” said Oran. “Among more than 3,000 prison inmates in four states who tested positive for the coronavirus, the figure was astronomical: 96 percent asymptomatic.” 

However, the authors note being asymptomatic does not mean that person is not impacted or harmed by the infection. One caveat is that some infected individuals may have clinical abnormalities in their lungs. It is not clear how common these may be or what the health implications might be, but two series of lung CT scans have seen the round glass opacities distinctive of Covid-19 infection.

The researchers also found that asymptomatic individuals may be able to transmit the virus for longer periods of time than previously thought, stretching beyond the 14-day quarantine period many governments have implemented. Though it is unclear how infectious a person might be during this timeframe.

The findings confirm previous work that has found similar results. A study published in the New England Journal of Medicine in April found that 43 percent of those infected in Iceland and Italy with the virus was shown to be asymptomatic. Earlier this year, 60 percent of crewmembers aboard the US Aircraft Carrier Theodore Roosevelt who tested positive for the virus did not present symptoms. In May, another study published found that at least 8-in-10 cruise ship passengers who tested positive for Covid-19 went without symptoms.

“The silent spread of the virus makes it all the more challenging to control,” says Eric Topol, MD, founder and director of the Scripps Research Translational Institute and professor of Molecular Medicine at Scripps Research. “Our review really highlights the importance of testing. It’s clear that with such a high asymptomatic rate, we need to cast a very wide net, otherwise the virus will continue to evade us.”   

Authorities, health officials, and the public alike need to pay more attention to people who contract the virus but have no symptoms by making them more visible through testing and digital surveillance tools, the researchers conclude. Wearing a mask can serve as the first line of defense.