The US Centers for Disease Control (CDC) increased its best guess on Friday for how many of those infected by coronavirus show no symptoms.
The CDC now estimates 40 percent of COVID-19 positive cases are asymptomatic, up from its previous guess of 35 percent, which US news company CNN reported May 20.
Patients that show no symptoms are referred to as asymptomatic, and have been a particularly thorny issue for public health officials to solve in the fight against COVID-19 as it has proven difficult to identify everyone who has become infected and prevent COVID-19 from spreading.
Efficient contact tracing and isolating are integral to slow the spread of the disease, but asymptomatic carriers are harder to track down and isolate.
“The percent of cases that are asymptomatic, i.e. never experience symptoms, remains uncertain. Longitudinal testing of individuals over time is required to accurately indicate the absence of symptoms for the full period of infectiousness,” the CDC said.
Studies on asymptomatic cases have been difficult to conduct as tracking large sample groups of individuals necessary for proper scientific analysis has been difficult.
One recent study suggested that asymptomatic carriers may struggle to develop the necessary antibodies to prevent reinfection of COVID-19, suggesting that lasting immunity may be unlikely.
The researchers in that study warned authorities that the results mean that caution is needed in easing lockdown measures as immunity may not last as long as hoped – particularly among those that show no symptoms.
“Together, these data might indicate the risks of using COVID-19 ‘immunity passports’ and support the prolongation of public health interventions, including social distancing, hygiene, isolation of high-risk groups and widespread testing,” the researchers wrote.