Even asymptomatic people who are infected may be able to spread the virus. But people without symptoms are rarely tested.
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Asymptomatic coronavirus cases seem very common – but those people might only be contagious for half as long, new research suggests
Asymptomatic coronavirus cases appear to be more prevalent than scientists initially thought. A recent study of...Asymptomatic coronavirus cases appear to be more prevalent than scientists initially thought. A recent study of a cruise ship outbreak found that 80% of passengers who tested positive for the virus were asymptomatic. A study from Wuhan, China, also found that asymptomatic patients may only be contagious for half as long as symptomatic patients. Visit Business Insider's homepage for more stories. Asymptomatic transmission is a puzzling element of any infectious disease: People without symptoms are hard to identify, so it's difficult to determine how common these cases are. When it comes to the coronavirus, the US Centers for Disease Control and Prevention (CDC) estimates that around 35% of infections are asymptomatic. But a spate of new research suggests that might be conservative. According to a recent study of a coronavirus outbreak on a cruise ship that left Argentina in mid-March, the majority of passengers who tested positive for the virus — around 80% — showed no symptoms. Another study from Wuhan, China, found that 42% of patients — 33 out of 78 tested — were asymptomatic. Additional research suggests that people with asymptomatic cases, though common, may only be contagious for a short window compared to symptomatic patients. "A lot of this is thought to be settled science, but I think that there are still a lot of questions we have about when asymptomatic transmission occurs and the circumstances that it occurs in," Amesh Adalja, a senior scholar at the Johns Hopkins University Center for Health Security, told Business Insider. Asymptomatic carriers may only be contagious for 8 days A person's ability to transmit the virus depends partly on their viral load: the amount of viral particles they release into the environment. Research indicates that there's little difference in the viral loads between coronavirus patients who show symptoms and those who don't. "A growing body of results shows that people who are asymptomatic appear to have the same viral load as symptomatic cases," Gigi Gronvall, an immunologist and senior scholar at the Johns Hopkins Center for Health Security, told the university. "This suggests that transmission is possible equally from both asymptomatic patients and noticeably sick patients." Coronavirus patients tend to have high viral loads in the throat, nasal cavity, and upper respiratory tract, which makes the virus highly contagious. But the Wuhan study suggests that asymptomatic coronavirus patients don't shed the virus for as much time. On average, the researchers found, symptomatic patients shed for more than twice as long as asymptomatic patients: 19 days compared to eight. No asymptomatic patient in the study shed the virus beyond the 12-day mark — and some only shed for three days. Patients with symptoms, meanwhile, shed for 16 to 24 days. The researchers also found that most asymptomatic patients were women in their 20s, 30s, and 40s. Scientists aren't sure how much asymptomatic carriers fuel transmission Adalja noted that a lot of the initial data on asymptomatic spread came from a choir practice in Washington. On March 3, a person infected with the virus (who wasn't showing symptoms) attended a 2.5-hour choir practice in Skagit County. Most of the attendees subsequently became ill with the coronavirus, leading some local health officials to conclude that the virus had spread through asymptomatic transmission. But the CDC later determined that most of the choir members' exposure occurred during another practice on March 10, when the same infected person was showing symptoms. This might suggest that asymptomatic carriers aren't fueling transmission as much as scientists first thought. "Now that we've seen that incident in the choir did not involve asymptomatic transmission, that's gotten a lot of people thinking," Adalja said. Some scientists still believe, however, that asymptomatic transmission is driving transmission in confined spaces like hospitals, homeless shelters, prisons, and nursing homes. A recent editorial from researchers at the University of California, San Francisco, argued that asymptomatic patients "are playing a major role in the transmission" of the coronavirus. They pointed to an outbreak at a nursing facility in Washington, where 56% of residents tested positive for the virus without showing symptoms. Even though most of those residents developed symptoms within the following week, they likely helped spread the virus when they were asymptomatic. "Symptom-based screening alone failed to detect a high proportion of infectious cases and was not enough to control transmission in this setting," the UCSF researchers wrote. They added that asymptomatic spread was the "Achilles heel" of coronavirus mitigation strategies: Without testing for asymptomatic patients, the US might struggle to contain its outbreak. Hilary Brueck contributed reporting. Join the conversation about this story » NOW WATCH: Pathologists debunk 13 coronavirus myths
The province will let people outside Wuhan leave for the first time since late January. Despite...The province will let people outside Wuhan leave for the first time since late January. Despite the official sign of confidence, many fear the virus is still spreading silently.
One of the biggest challenges in trying to contain the coronavirus is that an unknown number...One of the biggest challenges in trying to contain the coronavirus is that an unknown number of people are spreading it when they don't even seem sick themselves.